The Design of Radar Corner Reflectors
for the Australian Geophysical Observing
System
A single design suitable for InSAR deformation monitoring
and SAR calibration at multiple microwave frequency bands
GEOSCIENCE AUSTRALIA
RECORD 2015/03
Garthwaite, M. C., Nancarrow, S., Hislop, A., Thankappan, M., Dawson, J. H., Lawrie, S.
Department of Industry and Science
Minister for Industry and Science: The Hon Ian Macfarlane MP
Parliamentary Secretary: The Hon Karen Andrews MP
Secretary: Ms Glenys Beauchamp PSM
Geoscience Australia
Chief Executive Officer: Dr Chris Pigram
This paper is published with the permission of the CEO, Geoscience Australia
© Commonwealth of Australia (Geoscience Australia) 2015
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ISSN 2201-702X (PDF)
ISBN 978-1-925124-57-6 (PDF)
GeoCat 82751
Bibliographic reference: Garthwaite, M. C., Nancarrow, S., Hislop, A., Thankappan, M.,
Dawson, J. H., Lawrie, S. 2015. The Design of Radar Corner Reflectors for the Australian Geophysical
Observing System: a single design suitable for InSAR deformation monitoring and SAR calibration at
multiple microwave frequency bands. Record 2015/03. Geoscience Australia, Canberra.
http://dx.doi.org/10.11636/Record.2015.003
Contents
Executive Summary.................................................................................................................................. v
1 Introduction ............................................................................................................................................1
1.1 Australian Geophysical Observing System......................................................................................1
1.2 Interferometric Synthetic Aperture Radar (InSAR) ..........................................................................2
1.3 Radar reflectors ...............................................................................................................................3
1.3.1 Deformation studies ...................................................................................................................3
1.3.2 SAR calibration ...........................................................................................................................4
1.4 Orbiting SAR sensors ......................................................................................................................4
2 Design considerations ...........................................................................................................................7
2.1 Brightness requirements ..................................................................................................................7
2.1.1 Clutter .........................................................................................................................................8
2.1.2 Radiometric calibration ...............................................................................................................8
2.1.3 Deformation studies ...................................................................................................................9
2.2 Choice of target ..............................................................................................................................13
2.2.1 Trihedral plate shape ................................................................................................................14
2.3 Size of target ..................................................................................................................................16
2.4 Manufacturing tolerances...............................................................................................................18
2.4.1 Plate material ...........................................................................................................................21
2.4.2 Mesh perforation ......................................................................................................................22
2.5 Other design features ....................................................................................................................24
2.6 Observed distortions of CR prototypes ..........................................................................................25
2.6.1 Mesh-perforated CR .................................................................................................................25
2.6.2 Larger solid sheet CR ...............................................................................................................28
3 Radar Signature Characterisation .......................................................................................................29
3.1 Experimental procedure .................................................................................................................29
3.2 RCS characterisation results .........................................................................................................32
3.2.1 RCS profiles .............................................................................................................................32
3.2.2 Peak RCS measurements ........................................................................................................33
4 Field Testing ........................................................................................................................................38
4.1 Description of test site ....................................................................................................................38
4.1.1 Site selection ............................................................................................................................38
4.1.2 Installation ................................................................................................................................39
4.1.3 CR site positions ......................................................................................................................40
4.2 SAR acquisitions ............................................................................................................................43
4.3 Field orientation strategy................................................................................................................44
4.3.1 Intentional misalignment ...........................................................................................................45
4.3.2 Field alignment methods ..........................................................................................................47
4.3.3 Absolute accuracy of azimuth measurements .........................................................................48
4.3.4 Field measurement accuracy ...................................................................................................49
4.4 Processing methodology................................................................................................................50
4.5 Results ...........................................................................................................................................52
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
iii
4.5.1 CR impulse responses in SAR imagery ...................................................................................52
4.5.2 Clutter .......................................................................................................................................57
4.5.3 RCS ..........................................................................................................................................60
4.5.4 LOS height error .......................................................................................................................62
4.5.5 Impact of alignment errors........................................................................................................62
4.5.6 Impact of flooding .....................................................................................................................66
5 Conclusions and Recommendations ...................................................................................................68
Acknowledgements ................................................................................................................................70
References .............................................................................................................................................71
.............................................................................................................................................74
iv
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Executive Summary
As part of the AuScope Australian Geophysical Observing System initiative, Geoscience Australia
constructed a new regional-scale geodetic network that includes an array of radar corner reflectors.
The purpose of the new geodetic network is to monitor crustal deformation by combining spatially
dense but temporally sparse deformation maps derived from the Interferometric Synthetic Aperture
Radar (InSAR) technique and temporally dense but spatially sparse point measurements from Global
Navigation Satellite System (GNSS) networks. The radar corner reflector array is also designed to
support calibration and validation of Synthetic Aperture Radar (SAR) products from orbiting satellites.
This GA Record outlines the prototyping exercises undertaken to determine the most appropriate
design of radar corner reflector that can exploit SAR acquisitions at X-, C- and L-band radar
frequencies. A set of 18 corner reflector prototypes were manufactured that had different sizes and
plate finishes. These prototypes had their radar signatures characterised in experiments conducted at
the Defence Science and Technology Organisation ground radar reflection range in St Kilda, South
Australia. Following this, the prototypes were temporarily deployed between December 2013 and May
2014 at a grazing property in Gunning, New South Wales. During this deployment the radar response
of the corner reflectors was tested in SAR images from the TerraSAR-X, COSMO-SkyMed,
RADARSAT-2 and RISAT-1 satellites. As a result of these experiments, a triangular trihedral corner
reflector design with an inner leg dimension of 1.5 metres and powder-coated plate finish was chosen
for permanent deployment in the new array. Fifteen of the prototypes and 25 new 1.5 metre corner
reflectors were fully installed in the new array in the northern Surat Basin, Queensland, by 21
November 2014.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
v
vi
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
1 Introduction
1.1 Australian Geophysical Observing System
In 2010 the Australian Government invested $23 million to develop an Australian Geophysical
Observing System (AGOS) infrastructure through funding from the Education Investment Fund (EIF)
Round 3. Administered by AuScope Ltd, the purpose of AGOS is to enable collection of new baseline
data including surface geospatial and subsurface imaging and monitoring data, to provide an
understanding of the physical state of the accessible crust of the Australian continent. Geoscience
Australia (GA) has been responsible for implementing the AGOS geospatial observatory, which
features:

A geodetic survey mark network, including co-located radar corner reflectors, to enable the
precise measurement of crustal deformation at a regional scale using Interferometric Synthetic
Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) techniques;

Four high precision continuously operating reference site (CORS) GNSS monuments installed
at Mitchell (Queensland), King Island (Tasmania), Blinman (South Australia) and at the
Murchison Radio Observatory, Boolardy (Western Australia). These supplement 101 more
CORS funded by a National Collaborative Research Infrastructure Strategy (NCRIS) award to
AuScope;

A robotic GNSS antenna calibration facility; the only one of its kind in the southern hemisphere;

A deployable pool of GNSS instruments for episodic campaign surveys in Australia. This
includes 80 GNSS instruments, 10 ionospheric receivers and 3 Real Time Kinematic (RTK) kits;

An open-access repository of Synthetic Aperture Radar (SAR) data acquired by the ERS
satellites (operated by the European Space Agency, ESA) over the Australian territory, suitable
for InSAR analysis to detect ground surface deformation.
The above geospatial infrastructure will enable combination of multiple geodetic techniques to yield
spatial and temporal estimates of multi-scale surface deformation with millimetre level precision and
centimetre-level accuracy. With an increasing societal demand for improved positioning for geospatial
applications, there is a need for such spatial and temporal accuracy in the Australian coordinate
system. The Intergovernmental Committee on Surveying and Mapping (ICSM) is currently undertaking
research towards the implementation of a future Australian geodetic datum to replace the Geocentric
Datum of Australia 1994 (GDA94), which is anticipated to incorporate time-variable coordinates.
In this GA Record we describe the design and evaluation of prototype radar corner reflectors primarily
suitable for crustal deformation studies using InSAR, but also suitable for calibration of orbiting SAR
sensors. The design of AGOS corner reflectors has previously been discussed in Garthwaite et al.
[2013b] and preliminary results from the radar signal characterisation were presented in Thankappan
et al. [2013]. As a result of this prototyping exercise, a network of 40 corner reflectors has been
installed in the northern Surat Basin in Queensland [Garthwaite et al., 2015].
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
1
1.2 Interferometric Synthetic Aperture Radar (InSAR)
InSAR is a technique that can identify movements of the Earth’s surface at the millimetre to centimetre
scale and with high spatial resolution. Observations of surface movement made using InSAR can be
used to detect, measure, and monitor crustal changes associated with geophysical processes such as
tectonic activity (earthquakes, inter- and post-seismic deformations), volcanic eruptions and
landslides. Ground subsidence or uplift caused by anthropogenic influences such as groundwater or
hydrocarbon extraction or CO2 injection can also be identified with InSAR. When combined with
ground-based geodetic monitoring techniques, such as GNSS, InSAR can be used to infill the gaps
within traditional sparse geodetic point networks and potentially capture deformation anomalies of
small spatial extent that would be missed with a network comprised of discrete point samples.
InSAR uses two or more SAR images of the same area to identify surface movements and their
evolution through time. Remote sensing satellites that collect SAR imagery transmit pulses of
microwave energy to the Earth’s surface and record the amount of backscattered energy. The use of
microwave energy provides an all-weather capability because of its low sensitivity to clouds and rain.
SAR images contain information on the Earth’s surface in the form of the magnitude (intensity) and
phase components of the backscattered radar signal. The intensity image records information on the
terrain slope and surface roughness, while the phase image provides information about the distance
between the satellite and the Earth’s surface.
Differential InSAR uses the phase component of two SAR images from the same area acquired at
different times. If the distance between the ground and satellite changes between the two acquisitions
due to surface movement, a phase shift will occur. These phase shifts are mapped out spatially in the
interferogram (Figure 1.1).
By performing a linear least squares inversion on a network of multiple interferograms formed from
many SAR acquisitions, and with careful treatment of the various noise signal components, velocity
and time-series maps spanning the period of SAR data coverage can be generated [Berardino et al.,
2002]. A velocity map gives the surface movement for each image pixel averaged over the total
observation period whereas the time-series shows the history of surface positions for a pixel at each
acquisition time. The former is useful for mapping geophysical processes that are steady through time,
for example the pattern of tectonic deformation at a crustal fault zone [Garthwaite et al., 2013a]. The
latter is useful for detecting geophysical processes that vary considerably through time and cause
fluctuations in the direction of surface movement, for example due to hydraulic head changes in a
confined aquifer system [Chaussard et al., 2014; Reeves et al., 2014].
The interested reader is referred to the following review papers about the InSAR methodology:
[Bürgmann et al., 2000; Rosen et al., 2000; Simons and Rosen, 2007]. Furthermore, Hooper et al.
[2012] give a review of recent advances in InSAR time series analysis for measuring crustal
deformation.
2
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 1.1: Cartoon depiction of the InSAR methodology. Two SAR images of the same area are acquired at
different times. If the surface moves between the two acquisitions a phase shift occurs and an interferogram maps
this phase difference.
1.3 Radar reflectors
A radar reflector is a passive device that reflects incoming electromagnetic energy directly back to the
source of that energy. There are many different types of reflector being used including: spheres,
cylinders, dihedrals, trihedrals, flat plates, top hats and bruderhedrals. A trihedral radar reflector is
often known as a “corner reflector” (henceforth abbreviated as CR) because the reflection is facilitated
by the bouncing of the incident radar wave from three mutually orthogonal plates. The orthogonality of
all three plates ensures that the CR resembles the corner of a cube (and hence the name “corner
reflector”), with one baseplate and two ‘vertical’ plates. Trihedral CR have been used for many years
as a target suitable for calibration of SAR images. They are also gaining widespread popularity as
targets suitable for accurately measuring ground deformation using InSAR.
1.3.1 Deformation studies
The Persistent/Permanent Scatterer InSAR technique (PSInSAR) [Ferretti et al., 2001; Hooper et al.,
2004; Kampes, 2006] is becoming increasingly popular in geophysical studies of ground movements
induced by wide-ranging natural and anthropogenic phenomena. One significant advantage of the
PSInSAR technique over conventional Differential InSAR techniques is that the effects of temporal
and spatial decorrelation are less significant. This is because PSInSAR makes use of surface
scatterers that have a strong and stable backscatter response in SAR imagery over long time periods
and different viewing geometries. This means that the cumulative signal response of other weaker
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
3
scatterers within the resolution cell is effectively ignored in the PSInSAR technique, and decorrelation
is less of an issue.
The distribution of persistent scatterers can be dense in urban areas where there are many man-made
angular structures and corners to reflect energy back to the satellite, however in non-urban areas their
distribution may be sparse. CR are being increasingly deployed to monitor deformation worldwide in
order to artificially provide strong backscatter responses in regions where natural persistent scatterers
are sparse or non-existent [e.g. Fu et al., 2010; Hanssen, 2001; Ketelaar, 2009; Li et al., 2012; Qin et
al., 2013; Singleton et al., 2014; Strozzi et al., 2013]. Previous validation experiments have found that
artificial reflectors can yield displacement estimates from InSAR analysis with sub-millimetre accuracy
in both vertical and east-west directions [Ferretti et al., 2007].
1.3.2 SAR calibration
SAR data used for quantitative temporal and/or spatial analysis requires calibration to ensure that
observed pixel values of amplitude and phase can be related to the geophysical parameters of interest
[Freeman, 1992]. Furthermore, if SAR images from different sensors are absolutely calibrated they
can (in principle) be directly compared. The process of radiometric calibration of SAR images involves
comparison of the backscattered radar reflectivity signal from a ground resolution element containing a
calibration target of known signal response, such as a CR [Gray et al., 1990]. CR are considered to be
reliable targets for SAR calibration because the magnitude of the returned signal is large relative to the
size of the target, their signal response is insensitive to errors in alignment (unlike dihedral reflectors),
and they are relatively cheap to manufacture and maintain (unlike transponders). If the geodetic
location of a deployed CR is accurately known then it can be used for geometrical calibration of SAR
products as long as it is visible above the background signal level in the SAR imagery.
1.4 Orbiting SAR sensors
Current and future SAR satellites of interest are given in Table 1.1. SAR sensors are typically the
payload on a low earth orbiting satellite, with sun-synchronous orbit. As such, for each point on the
ground there will be one ascending pass and one descending pass where that point is imaged during
one orbital cycle. The satellite flight direction is skewed with respect to the Earth reference frame such
that the ground projection of the flight path vector at the equator has an azimuth of ~108 degrees on
descending passes and ~352 degrees on ascending passes.
Typically, the SAR sensor line of sight on each satellite is orthogonal to, and right looking with respect
to the flight direction vector, though some missions have an additional left-looking capability. During
operation the SAR sensor illuminates a swath of finite width, which is typically of the order of 100 km
on the ground (depicted in Figure 1.1). Any ground target within that swath will be imaged, but the
incidence angle of the illuminating radar will vary according to the vector from SAR sensor to the
ground target. Therefore when deploying a CR the expected imaging geometries from different orbital
locations for that particular ground position must be determined and one geometry chosen.
The SAR sensors operate within the microwave range of the spectrum, typically at X-band, C-band, or
L-band with a narrow bandwidth on the order of 100 MHz (Table 1.2). Each SAR sensor has the
capability to operate in different imaging modes (Table 1.3). Generally, each imaging mode will tradeoff the pixel resolution against overall spatial coverage such that fine pixel resolution is achieved only
for small scene areas and wide area coverage is only achieved with coarse pixel resolution.
4
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Table 1.1: Current (January 2015) and future orbiting SAR sensors
Mission
Band
Satellite
Commenced
Agency
Country
Orbital
revisit
RADARSAT-2
C
-
2007
CSA/MDA
Canada
24
TerraSAR-X
X
-
2008
DLR / Airbus
Germany
11
TanDEM-X
X
-
2010
DLR / Airbus
Germany
11
PAZ
X
-
2015*
HISDESAT /
Airbus
Spain
11
TerraSAR-X NG
X
-
2018*
DLR / Airbus
Germany
11
COSMO-SkyMed
X
1/2/3/4
2007 / 2007 /
2008 / 2010
ASI
Italy
16
RISAT-1
C
-
2012
ISRO
India
25
KOMPSAT-5
X
-
2013
KARI
Korea
28
Sentinel-1
C
A/B
April 2014 /
2016*
ESA
Europe
12 (6)
ALOS-2
L
-
May 2014
JAXA
Japan
14
SAOCOM-1
L
A/B
2015* / 2016*
CONAE
Argentina
16 (8)
NOVASAR-S
S
-
2015*
SSTL
UK
14
RADARSAT
Constellation
C
1/2/3
2018*
CSA/MDA
Canada
12 (4)
NISAR
S&L
-
2020*
NASA/ISRO
USA/India
12
Tandem-L
L
-
2020*
DLR
Germany
8
* Anticipated launch
Table 1.2: Frequencies and wavelengths of microwave bands commonly used in orbiting SAR sensors..
L-band
S-band
C-band
X-band
Frequency (GHz)
1.270
2.500
5.400
9.650
Wavelength (m)
0.236
0.120
0.056
0.031
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
5
Table 1.3: Pixel resolutions of current and some future SAR sensors for different imaging modes. Ground range is
calculated at the middle of the incidence angle range.
Incidence angle
TerraSAR-X
COSMO-SkyMed
RISAT-1
KOMPSAT-5
Sentinel-1
ALOS-2
RADARSAT
Constellation
6
Ground
range
resolution
(m)
Ground range
resolution
area (m2)
Beam mode
Near
range
Ultra-Fine
20.0
54.0
2.8
1.0
2.7
Multi-Look Fine / Wide MultiLook Fine / Extra-Fine
22.0
50.0
4.6
1.8
8.4
Fine / Wide-Fine
20.0
50.0
7.7
3.0
23.0
Standard
20.0
52.0
7.7
5.3
40.7
Wide
20.0
45.0
7.7
7.3
55.9
Stripmap
19.7
45.5
3.3
2.6
8.7
Stripmap (dual pol)
19.9
45.4
6.6
2.6
17.5
ScanSAR (4 beam)
19.7
45.5
18.5
2.6
48.8
ScanSAR (6 beam)
15.6
49.0
40.0
5.0
198.2
Himage (Stripmap)
20.0
60.0
3.0
2.2
6.7
Wideregion (ScanSAR)
20.0
60.0
16.0
5.2
83.1
Hugeregion (ScanSAR)
20.0
60.0
30.0
14.8
445.3
PingPong
20.0
60.0
15.0
11.1
167.0
Fine Res Stripmap (FRS)
12.0
55.0
3.0
1.7
5.0
Medium Res ScanSAR
(MRS)
12.0
55.0
24.0
13.8
331.2
Coarse Res ScanSAR
(CRS)
12.0
55.0
48.0
27.6
1324.6
Standard
20.0
45.0
3.0
3.0
9.0
Wide Swath
20.0
45.0
20.0
20.0
400.0
Stripmap
20.0
47.0
5.0
3.8
18.9
Interferometric Wide Swath
25.0
46.0
20.0
4.0
80.7
Extra Wide Swath
20.0
47.0
40.0
15.1
603.7
Ultra-Fine
8.0
70.0
3.0
3.0
9.0
High-sensitive
8.0
70.0
6.0
6.0
36.0
Fine
8.0
70.0
10.0
10.0
100.0
ScanSAR
8.0
70.0
100.0
100.0
10000.0
Very High Res
19.0
53.0
3.0
3.0
9.0
High Res
19.0
53.0
5.0
5.0
25.0
Medium Res 16m
19.0
53.0
16.0
16.0
256.0
Medium Res 30m
19.0
53.0
30.0
30.0
900.0
Medium Res 50m
19.0
53.0
50.0
50.0
2500.0
Low Res
19.0
53.0
100.0
100.0
10000.0
Mission
RADARSAT-2
Azimuth
resolution
Far
(m)
range
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
2 Design considerations
2.1 Brightness requirements
Williams [2011a] gives a reasoned narrative on the brightness requirements of CR for SAR calibration
and deformation studies. We summarise the key points here and then discuss the implications for
choice of target type.
The radar cross section (henceforth abbreviated as RCS) is a measure of the size of a target as seen
by the imaging radar. Mathematically it is the ratio of the energy reflected by the target relative to the
energy incident on the target:
𝜎 = lim 4𝜋𝑅2
𝑅→∞
|𝐸𝑠 |2
|𝐸𝑖 |2
where 𝑅 is the range from the target, and 𝐸𝑠 and 𝐸𝑖 are the scattered and incident electrical field
strength in W/m2 [Knott, 2006]. The limit imposed here removes the range dependence of the relation
because at infinity the target is illuminated by a planar wavefront. In this definition of the target RCS it
is assumed that incident energy is scattered isotropically in every direction, such as would be the
response of a metal sphere placed in the wavefront. Therefore the RCS is often referred to as being
equivalent to that of a sphere with a certain projected area that would scatter energy with the same
intensity as the target [Knott, 2006]. In reality the RCS of a target is usually anisotropic, and depends
on the illumination angle, radar frequency and polarisation. For this reason Döring and Schwerdt
[2013] prefer to term the measurement quantity with respect to the equivalent sphere as the
‘equivalent RCS’. However, for simplicity we refer herein simply to the RCS.
The unit of RCS is m2, though values are often given in terms of decibels:
𝜎(𝑑𝐵𝑚2 ) = 10 log10 (𝜎(𝑚2 ))
The backscatter “Sigma Nought” is the conventional measure of brightness of a distributed target in a
SAR image. It is the RCS in decibels normalised by the illuminated area 𝐴 [Freeman, 1992]:
𝜎0 =
⟨𝜎𝑛 ⟩
𝐴
where 𝜎𝑛 is the 𝑛th RCS value and angle brackets indicate an ensemble average. The illuminated
area is:
𝐴=
𝑝𝑟 𝑝𝑎
sin 𝜃
where 𝜃 is the local incidence angle (that takes in to account the local terrain) and 𝑝𝑎 and 𝑝𝑟 are the
azimuth and slant range pixel spacings respectively. Using these relations, the approximate RCS of
any point target in a SAR image can be estimated. Conversely, in absolute radiometric calibration the
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
7
response of point targets with known RCS are analysed to determine a calibration factor that can be
applied to the SAR image to retrieve Sigma Nought values. Following calibration in this way, Sigma
Nought values can in principle be compared to those in SAR images from other sensors. However, in
practice there can still be differences in Sigma Nought levels between SAR sensors [e.g. Pettinato et
al., 2013]
To be of use for SAR calibration or as a stable phase target for temporal InSAR analyses the target
must be visible in the SAR image above the background signal level (the ‘clutter’). The typically used
measure of target visibility in a SAR image is the Signal-to-Clutter Ratio [SCR; Freeman, 1992]:
𝑆𝐶𝑅 =
𝜎𝑇
𝜎𝑇
=
⟨𝜎𝐶 ⟩
⟨𝜎 0 ⟩𝐴
where 𝜎𝑇 is the point target RCS, ⟨𝜎𝐶 ⟩ is the ensemble average of clutter RCS in the vicinity of the
point target.
2.1.1 Clutter
The magnitude of clutter depends on terrain type, vegetation density, soil moisture, radar wavelength,
incidence angle, polarisation and SAR resolution. AGOS CRs are generally to be sited on flat
cultivated terrain with low vegetation density, although rocky terrain may be encountered that typically
has a relatively high backscatter coefficient. With reference to typical clutter levels for different land
cover types and dependent on incidence angle [Skolnik, 1970], it is not likely that AGOS CRs will
experience clutter levels at C-band greater than -10 dB, and more likely to be within the range -12 dB
to -14 dB. Clutter levels at C- and X-bands should be broadly similar because the small difference in
frequency means that attenuation rates in vegetation will be similar, though the expected magnitude of
clutter at X-band for equivalent incidence angle and land cover could be up to 3 dB greater than Cband [Williams, 2011a]. Measurements of tussocky grassland from an airborne L-band SAR at VV
polarisation vary between about -15 dB to -20 dB, whereas bare soil reaches the lower end of this
range [Dong, 2003]. Therefore for AGOS CR sites with low vegetation levels we might expect clutter of
about -16dB as an upper bound at L-band.
2.1.2 Radiometric calibration
The effectiveness of using CR for radiometric calibration of SAR images is influenced by the following
target characteristics: magnitude of RCS, pattern of RCS, physical size of CR, stability of RCS and
sensitivity of RCS to external influences [Sarabandi and Tsen-Chieh, 1996]. Generally, the SCR
should be at least 20 dB (and perhaps more like 30 dB) in order to minimise errors in computation of
the calibration factor but also the signal should not be saturated [Curlander and McDonough, 1991;
Freeman, 1992].
High resolution beam modes at all radar bands (Table 1.3) have a ground resolution area of less than
10 m2. The background pixel brightness should therefore be -2 dBm2 at C-band (assuming -12dB
clutter) and about 0 dBm 2 at X-band (assuming -10 dB clutter) for high resolution systems. For
accurate calibration of high resolution beam modes we would therefore require a target with an RCS
not less than ~30 dBm2 at X-band and ~28 dBm2 at C-band.
Coarser resolution beam modes at C- and L-band (Table 1.3) have a ground range resolution between
40 and 100 m2. For these beams the pixel brightness should therefore be between 4 and 8 dBm 2 at C8
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
band (assuming -12 dB clutter). A suitable target for calibration of coarser beam modes at C-band
would have an RCS of 34-38 dBm2. At L-band this should also be 34 dBm 2 for the coarser resolution
mode of ALOS-2.
2.1.3 Deformation studies
The complex radar observation at each pixel is the coherent sum of the response from many
distributed scatterers within that pixel. Deformation studies using differential InSAR techniques exploit
the phase component of the complex radar signal. Pixels containing distributed scatterers that are uncorrelated and no single scatterer dominates, the pixel is unlikely to remain correlated for long periods
of time. This can often lead to large areas within differential interferograms where the phase is
incoherent, especially if there is a large temporal or spatial baseline between SAR image pairs
[Hanssen, 2001; Kampes, 2006].
Figure 2.1: The backscattered signal from the pixel is a complex sum of each scatterer within the pixel
represented here by the vector 𝑧. A dominant scatterer is represented by the vector 𝑆 = 𝜎𝑇 and the complex sum
of the background clutter 𝐶 = ⟨𝜎𝐶 ⟩. The angle subtended by 𝑧 and 𝑆 is the phase error due to the clutter 𝜑𝑒𝑟𝑟
[redrawn from Adam et al., 2004].
The PSInSAR technique [e.g. Ferretti et al., 2001] only exploits those pixels within which there is a
dominant scatterer exhibiting long-term stable phase characteristics (Figure 2.1). The phase
component from the dominant scatterer depends on the range from the target to the SAR sensor
whereas the phase due to the background clutter is essentially random [Kampes, 2006]. The temporal
phase stability for dominant scatterers has been demonstrated with a simulation by Dawson [2008]
(Figure 2.2).
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
9
Figure 2.2: Simulation results of phase distribution within a pixel for the signal model given in Figure 2.1. A 1000
simulations were made each with 100 random uncorrelated scatterers (top row). The process was then repeated
with the inclusion of a single dominant scatterer (bottom row). a) and d) show the distribution of 100 scatterers
within the pixel for one model simulation. b) and e) show the complex observations for 1000 random simulations.
c) and f) show the wrapped phase signal in the interval ±𝜋 for the 1000 model simulations. The phase component
𝐼𝑚𝑎𝑔
of the complex signal (𝜑) is equal to 𝑡𝑎𝑛−1 (
). Reproduced from Dawson [2008].
𝑅𝑒𝑎𝑙
Assuming that the detected response from a single pixel contains un-correlated signal from the
distributed background scatterers within, the probability density function for the phase error 𝜑𝑒𝑟𝑟 of a
point scatterer due to the influence of clutter is [Adam et al., 2004]:
𝑝𝑑𝑓(𝜑) =
√𝑆𝐶𝑅 ∙ |cos(𝜑)|
√𝜋
∙ 𝑒𝑥𝑝 −𝑆𝐶𝑅∙𝑠𝑖𝑛
2 (𝜑)
This function implies that the phase error magnitude is determined by the point target SCR. As the
SCR of a point target increases, the width of the probability density function of the phase error narrows
since the impact of the clutter is reduced (Figure 2.3).
The estimated effective phase error in radians drawn from the probability density function (Figure 2.3)
is [Adam et al., 2004]:
𝜑𝑒𝑟𝑟 =
1
√2 ∙ 𝑆𝐶𝑅
Therefore the expected phase error can be estimated a-priori using the measured point target SCR
from the SAR intensity image [Adam et al., 2004; Ketelaar et al., 2004]. An alternative method for
estimating the phase error a-priori is the amplitude dispersion method first described by Ferretti et al.
[2001]. Through a simulation exercise, Adam et al. [2004] find that the SCR is a more effective
10
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
estimator of phase error than the amplitude dispersion for SCR greater than 9dB. Below this threshold,
an optimistic bias occurs in both methods, with the amplitude dispersion being more optimistic.
Ketelaar et al. [2004] find that both these methods are only applicable for targets with SCR > 9dB
since phase residuals are only approximately normally distributed when the phase error magnitude is
less than 0.25 radians.
Figure 2.3: Probability density function of the phase error in a point scatterer observation for different values of
SCR. The function is only valid for SCR > 4.8 dB and within the range ±𝜋/2 radians [after Adam et al., 2004].
The a-priori phase error (standard deviation) can be converted to a height error in the SAR sensor line
of sight (LOS; i.e. a slant-distance error) using the radar wavelength 𝜆:
herr =
φerr ∙ 𝜆
4π
We refer to this as a ‘LOS height error’ since for most cases the vertical component has more
influence on the LOS vector than the horizontal component. This is the case whenever the SAR
incidence angle is less than 45 degrees. If we arbitrarily choose a tenth of a millimetre as the desired
LOS height error magnitude arising from phase noise alone we would require point target SCR values
exceeding 25 dB, 38 dB and 63 dB at X-, C-, and L-bands respectively (Figure 2.4). A tenth of a
millimetre seems reasonable given that there are other sources of noise (including atmospheric
propagation errors, orbital errors, topographic errors, processing noise, instrument noise and
unwrapping errors) that contribute to the overall measured differential phase signal.
Based on the background resolution cell brightness calculated in §2.1.2 and the identified SCR
requirements, the target must therefore have an RCS of at least 25 dBm 2 for high resolution X-band
SAR imagery, 36 dBm2 for high resolution C-band SAR imagery, between 42-46 dBm2 for low
resolution C-band SAR imagery and 67 dBm 2 for low resolution L-band SAR imagery.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
11
The target brightness for different radar frequencies and purposes are summarised in Figure 2.5. Now
that we understand the brightness requirements we can begin to look at different target designs.
Figure 2.4: Line of sight height error herr as a function of SCR for the radar frequencies of interest.
Figure 2.5: Summary of target brightness requirements at different radar frequencies for SAR calibration and
InSAR applications.
12
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
2.2 Choice of target
The RCS of a radar reflector depends on the target size and radar frequency of illumination
(Figure 2.6). Equations for the maximum RCS for different reflector types are given in Table 2.1.
Figure 2.6: Theoretical relationship between peak RCS, target size and radar frequency. The plotted values of
RCS are calculated as 10 ∙ 𝑙𝑜𝑔(𝑎4 ⁄𝜆2 ), where 𝑎 is the inner leg dimension (size) of the target (see Figure 2.7a).
This calculation neglects the target-specific absolute magnitude factor. Absolute values of RCS for specific targets
can be obtained by converting values from this plot into the linear domain and multiplying by the magnitude
factors given in Table 2.1 for common reflector targets.
Flat plate and dihedral reflectors are simple structures that have a relatively high RCS. However both
suffer from a very narrow scattering pattern that means that alignment must be extremely accurate
(better than 1 degree). For example, a square aluminium flat plate exhibits a 15 dB reduction in RCS
for ~3 degrees of azimuth rotation at X-band [Drake and Hatty, 2013]. Although a dihedral reflector is
useful for calibrating cross-polarisation returns, the necessary alignment accuracy makes them and
flat plates less useful for general deployment in the landscape. Theoretical calculations indicate that a
triangular trihedral has a 3 dB beamwidth of approximately 40 degrees (Figure 2.7b; Curlander and
McDonough [1991]; Doerry and Brock [2009]), meaning that with an alignment error of 20 degrees the
RCS loss is 3 dB from the peak value. This property makes trihedral CR (in general) much more
forgiving of alignment inaccuracies compared to other reflector designs.
The boresight of a trihedral CR is the vector along which the maximum radar cross section exists. The
boresight vector emanates from the CR apex and points out to space and the largest reflected signal
magnitude will be measured along this vector. Therefore the boresight must be oriented as close as
possible to the radar source, in this case an orbiting SAR satellite. From physical optics the boresight
vector for a trihedral CR is oriented half way (θ = 45 degrees) between the two vertical plates, and
1
elevated 𝛹 = 𝑡𝑎𝑛−1 ( ) = 35.26 degrees from the baseplate (Figure 2.7a).
√2
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
13
Figure 2.7: a) Viewing geometry of a trihedral CR at boresight. b) The RCS response of a triangular trihedral CR
as a function of azimuth (θ) and elevation (ψ) angle relative to the peak RCS. 𝑎 is known as the inner leg
dimension. After Doerry and Brock [2009].
2.2.1 Trihedral plate shape
The plate shape of a trihedral CR impacts on the magnitude of the RCS (Table 2.1). The most
common plate shape of trihedral CR used in SAR studies is triangular, but square, pentagonal and
‘circular’ (i.e. quarter circle segment) plates have also been used [e.g. Qin et al., 2013; Sarabandi and
Tsen-Chieh, 1996].
For a given inner leg dimension, the brightest trihedral plate shape is the square, followed by the
quarter-circle and then the triangle. The triangular trihedral is the least bright of all the mentioned
reflector designs yet it is traditionally the most popular design. Square plates will not be as structurally
rigid as a triangle, impacting on the longevity of the CR and the ability for inter-plate orthogonality to
be maintained (see §2.4). Although the quarter-circle offers a compromise between brightness and
rigidity, it requires a more complicated process to cut the panels, and would therefore be more costly
to manufacture. Furthermore, the triangular trihedral requires less material to manufacture than either
the square or quarter-circle. For very low radar frequencies (i.e. L-band and lower) the size of
triangular trihedral becomes so large that it may distort under its own weight, thus reducing the RCS.
As an example, NASA JPL designed a 4.8 m triangular trihedral CR for P-band and L-band calibration.
A stress analysis on this 238 kg structure found that the baseplate would be displaced by 12 mm and
an angular deflection of 0.005 degrees was estimated when a 2G vertical load was imparted on the
structure [Chau et al., 2011]. For low radar frequencies, other plate shapes may be a realistic
consideration in order to reduce the physical size of the reflector required to achieve a specific level of
RCS.
14
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Table 2.1: Theoretical maximum RCS 𝜎𝑇 for commonly used reflector designs in SAR studies. 𝑎 is the inner leg
dimension of the target (see Figure 2.7a). The RCS magnitude factor is for converting values given in Figure 2.6
to absolute maximum RCS for each target type.
Target
Example
Triangular Trihedral
Maximum Theoretical
RCS (dBm2)
RCS magnitude
factor
𝜎𝑇 =
4𝜋𝑎4
3𝜆2
4.19
𝜎𝑇 =
4𝜋𝑎4
𝜆2
12.57
0.507𝜋 3 𝑎4
𝜆2
15.92
8𝜋𝑎4
𝜆2
25.13
12𝜋𝑎4
𝜆2
37.70
Image credit: Geoscience Australia
Flat square plate
Image credit: Drake and Hatty [2013]
Circular Trihedral
𝜎𝑇 =
Image credit: www.radartutorial.eu
Dihedral
𝜎𝑇 =
Image credit: Ferretti et al. [2007]
Square Trihedral
𝜎𝑇 =
Image credit: Qin et al. [2013]
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
15
Figure 2.8: Aperture of a triangular trihedral CR when the reflector is normal to the radar wavefront. The triple
bounce mechanism only occurs from the hexagonal ‘effective area’ shaded white. Redrawn from Knott [2006].
Most incoming radar rays that are incident on the trihedral CR are reflected off the three orthogonal
faces before being reflected back in the direction they came. This triple-bounce mechanism is
responsible for the wide beamwidth of the trihedral CR [Knott, 2006]. The triangular trihedral aperture
contains portions at the tips of the aperture where only double bounces occur (Figure 2.8). This ‘ineffective’ area, constituting a third of the overall area [Knott, 2006], could adversely affect the overall
RCS by the potential introduction of coherent interactions of double-bounce reflections from the tips
with the ground plane [Sarabandi and Tsen-Chieh, 1996]. Self-illuminating CR, such as pentagonal or
square trihedrals, do not suffer this problem since all reflections are triple bounce mechanism from
within the reflector aperture. Pentagonal-plated CRs that remove the in-effective area (shaded grey in
Figure 2.8) have been used as a SAR target by others (e.g. W. Albright, Alaska SAR Facility, Pers.
Comm. 2014), but the size and shape of the effective area changes with viewing angle and may not
remain hexagonal [Knott, 2006]. As a result, any marginal misalignment of the pentagonal CR would
lead to a loss of RCS compared to the triangular trihedral in the same situation.
Given these considerations, we chose the triangular trihedral as the design for AGOS CR
manufacture.
2.3 Size of target
As seen in Figure 2.6, the RCS for a given radar frequency is dependent on the reflector size. In
Table 2.2 we give the maximum RCS (at boresight) for triangular trihedral CR for the specific centre
frequencies of SAR sensors of interest. S-band is included here since the upcoming NOVASAR
mission will feature an S-band SAR sensor and the future NISAR NASA-IRSO joint SAR mission will
include both an L-band and S-band SAR sensor.
The brightness requirements identified in Figure 2.5 result in non-overlapping triangular trihedral CR
sizes therefore it is not possible to find a single size that can satisfy the requirements of all radar
frequencies. This would be true even for other trihedral CR designs. Consequently a compromise
must be made if only one size is to be used. When making this compromise the likelihood of signal
saturation must be considered. A very bright target could saturate the signal since the SAR receiver
16
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
uses an analogue-to-digital converter with a fixed number of bits to encode the signal as a digital
number. A bright signal may be outside of the dynamic range of this encoding.
Table 2.2: Maximum RCS in dBm2 at boresight for shape-independent reflectors (SIR; corresponding to RCS
values given in Figure 2.6) and triangular trihedral CR (TTCR) at different frequency bands.
L-band
S-band
C-band
X-band
Inner leg
dimension (m)
SIR
TTCR
SIR
TTCR
SIR
TTCR
SIR
TTCR
0.5
0.49
6.71
6.38
12.60
13.06
19.29
18.11
24.33
1.0
12.53
18.75
18.42
24.64
25.11
31.33
30.15
36.37
1.5
19.58
25.80
25.46
31.68
32.15
38.37
37.19
43.41
2.0
24.57
30.80
30.46
36.68
37.15
43.37
42.19
48.41
2.5
28.45
34.67
34.33
40.55
41.02
47.24
46.07
52.29
3.0
31.62
37.84
37.50
43.72
44.19
50.41
49.23
55.45
Döring et al. [2007] report that DLR use 1.5 m and 3.0 m triangular trihedral CR for calibrating the
TerraSAR-X sensor without saturation. Buck [2002] reports that ESA used transponders with an RCS
of 62.5 dBm2 (equivalent to a 5.8 m triangular trihedral CR) for calibration of the Envisat ASAR C-band
instrument. Furthermore, a transponder of 70 dBm 2 (equivalent to a 9.3 m triangular trihedral CR) has
been designed for Sentinel-1 calibration [Snoeij et al., 2010]. At L-band, JAXA use 3.0 m triangular
trihedrals as a standard calibrator for both ALOS-PALSAR and ALOS-2 PALSAR-2 (M. Shimada,
JAXA, Pers. Comm. 2014). Correspondingly a triangular trihedral CR size of less than 3.0 m is not
likely to present any saturation issues for the SAR sensors mentioned, and probably not for other SAR
sensors at these radar frequencies.
The compromise should therefore be made at X-band, since it is better for a target to be too bright but
still visible rather than too dark and not visible. We chose to manufacture prototype CRs at 1.0 m, 1.5
m, 2.0 m, and 2.5 m in order to further analyse the signal magnitude from the different radar
frequencies. As indicated in Figure 2.9, this range of sizes spans the full requirements of both high
and low resolution C-band SAR imaging modes, is within the demonstrated limit of saturation for Xband sensors and grazes on the brightness required for L-band calibration.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
17
Figure 2.9: Theoretical relationship between peak RCS, target size and radar frequency for a triangular trihedral
CR. The plotted values of RCS are calculated according to the formula in Table 2.1. Thick horizontal black lines
indicate the size of triangular trihedral CR required to meet the brightness requirements summarised in Figure 2.5
at specific radar frequencies. Black crosses mark the targets used by others for SAR calibration as discussed in
the text. Dashed vertical red lines indicate the size of triangular trihedral CR manufactured for prototyping.
2.4 Manufacturing tolerances
There are four factors that can act to reduce the RCS of a CR at boresight compared to the theoretical
value: misalignment of the reflector, inter-plate orthogonality, plate curvature and surface irregularities
[Döring et al., 2007]. Here we discuss the latter three which must be addressed in the manufacture
process.
The inter-plate orthogonality is the extent to which the plates form 90° angles at their common edges.
It is the most important tolerance to observe and maintain because the reflector RCS decreases
rapidly as the angle departs from 90°. Robertson [1947] conducted a series of physical experiments to
measure the RCS profile of trihedral CR when the inter-plate angles are varied from 90°. When only
the angle between the two vertical plates is varied, the azimuth profile flattens. Furthermore, the peak
RCS is less, with the reduction being more severe when the inter-plate angle is less than 90°.
Robertson [1947] also found that the RCS reduction effect of inter-plate angle is more severe as the
size of CR increases and the radar wavelength decreases.
Using a modelling approach that combines theory from geometrical and physical optics, Sarabandi
and Tsen-Chieh [1996] find that for distorted triangular, square and pentagonal CR the loss of RCS
compared to an undistorted CR is 0.2-1 dB for an angular deviation of ±1° and 1.3-2.8 dB for ±2°.
Again the losses are more severe when the inter-plate angle is acute rather than obtuse. These
results suggest that it is very important that 90° angular relationships between the intersecting plates
are maintained not only during manufacture but also through transportation and installation.
Zink and Kietzmann [1995] express the loss (𝐿) of RCS due to an inter-plate orthogonality error ε as:
18
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
𝐿𝑑𝐵 = 60 log10 (
(𝜋⁄2)2 𝑐𝑜𝑠(𝑎𝜆 𝑠𝑖𝑛(2𝜀))
2)
(𝜋⁄2)2 − (𝜋𝑎𝜆 𝑠𝑖𝑛(2𝜀))
where 𝑎𝜆 = (1⁄2𝜆)√𝐴𝑒𝑓𝑓 ⁄0.87, 𝜆 is the radar wavelength and 𝐴𝑒𝑓𝑓 is the CR effective area. We
evaluate this function for X-band (which is the most lossy) and plot the results in Figure 2.10. Even a
modest angular error of 0.5 degrees can yield a 9.2 dB loss at X-band for a 2.5 m trihedral CR. The
loss at C-band for the 2.5 m CR is 2.5 dB and at L-band only 0.1 dB.
Figure 2.10: Reduction of RCS caused by inter-plate orthogonality error for trihedral CR plates of different size at
X-band. The values agree closely with those tabulated by Zink and Kietzmann [1995].
Plate curvature is the deformation of the plate from a perfectly flat plane along its entire length such as
a gradual warp across the plate. The loss (𝐿) of RCS was given by Zink and Kietzmann [1995]:
𝐿𝑑𝐵 = 60 log10 (𝐶 2 + 𝑆 2 )
1
1
where 𝐶 = ∫0 𝑐𝑜𝑠(𝛽𝑥 2 )𝑑𝑥 , 𝑆 = ∫0 𝑠𝑖𝑛(𝛽𝑥 2 )𝑑𝑥, 𝛽 = 4𝜋√2𝑠⁄(𝑙 2 𝜆), 𝑠 is the plate curvature deviation and 𝑙
is the CR inner leg dimension. In general, the effect of plate curvature on RCS is wavelength and
target-size dependent. We evaluate this function for X-band (which is the most lossy) and plot the
results in Figure 2.11. The calculations show that the RCS reduction is more severe for smaller
trihedral reflectors. An RCS loss exceeding 10 dB could result from a 5 mm plate curvature in a 1.0 m
trihedral CR.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
19
Figure 2.11: Reduction of RCS caused by plate curvature deviation on trihedral CR plates of different size at Xband. The values agree closely with those tabulated by Zink and Kietzmann [1995].
Plate surface irregularities are the presence of any small scale deformation from perfect flatness at
any given location across the plate. Introduction of fasteners such as pop rivets or retaining bolts on
any of the flat reflecting surfaces could affect the performance of the CR. Zink and Kietzmann [1995]
give the loss of RCS due to surface irregularities as:
𝑑 2
𝐿𝑑𝐵 = −1028.72 ( )
𝜆
where 𝑑 is the RMS surface deviation across the plate. This relationship shows that the RCS reduction
due to surface irregularities is wavelength but not target size dependent (Figure 2.12). A surface
feature of 1 mm wavelength could introduce a 1 dB loss at X-band.
For the manufacture of triangular trihedral CR to be used as calibration targets for the TerraSAR-X
SAR sensor, the German Aerospace Center (DLR) specify the following tight tolerances [Döring et al.,
2007]: Inter-plate orthogonality ≤ 0.2°; Plate curvature ≤ 0.75 mm; Surface irregularities ≤ 0.5 mm.
With these tolerances and the above relationships, the RCS of the DLR CR should be accurate to
better than 1 dB. Although we adopted these tolerances as a guide for our manufacture of prototype
CR, we recognised these were challenging to achieve within the available budget for each CR.
Tolerances observed in manufacture should be preserved during handling. To aid this, prototype CR
panels were manufactured from 6 mm-thick aluminium sheet (4 mm for mesh-perforated CR). To
minimize potential plate curvature, 4 mm L-section aluminium angle was affixed to the backside of
each plate across the hypotenuse edge and one non-hypotenuse edge. Insert studs installed flush
with the reflecting surface were used to attach each angle such that no surface irregularity was
knowingly introduced.
20
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 2.12: Relationship between RCS loss and plate surface irregularities. The values agree closely with those
tabulated by Zink and Kietzmann [1995].
2.4.1 Plate material
Aluminium is commonly used for the construction of CR flat plates. Although aluminium is generally
more costly than steel, it does not suffer as badly from corrosion and is relatively lightweight.
Alucobond® was also considered as a potential material for the manufacture of the flat plates.
Alucobond® is a light composite material consisting of two aluminium sheets sandwiching a
Polyethylene core. The advantage of using such a material compared to aluminium is the relatively
light weight. However the dielectric performance of such a material needed to be considered.
Drake and Hatty [2013] performed RCS measurements in an anechoic chamber of 300 mm square
samples of aluminium (1.6 mm thick) and Alucobond® (4 mm thick). They found that at C-band the
samples had identical RCS (to within 0.02 of a dB) and at X-band the Alucobond® had a marginally
greater RCS (0.25 dB)
In the end the prototype CR panels were manufactured exclusively from aluminium since there was
some concern over the long term structural stability of Alucobond®, particularly its ability to remain flat
over long deployment time periods and under daily heating and cooling cycles.
To ensure the longevity of the aluminium CR we considered the use of a thin (several microns)
powder-coat layer to cover all exposed aluminium. Although such a thin layer should not impact
seriously on the dielectric properties of the aluminium sheet, this was an unknown and we decided to
produce prototypes with a powder-coat finish to compare against those with a plain metal finish. The
powder-coat finish should also help make the panel less susceptible to oxidisation under the
conditions they may be exposed to during longer term deployments. Three prototype panel sets were
powder coated white by the manufacturer.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
21
2.4.2 Mesh perforation
Mesh perforating the panels has the benefit of allowing quick drainage during heavy rainfall, to relieve
some of the force applied to the CR structure by wind and also promoting self-cleaning of dust and
other wind-blown deposits. We therefore considered this design feature for mass manufacture of
AGOS CR by manufacturing prototype CR with mesh perforation to compare against solid plate CR.
Perforating the CR plates could reduce the RCS at certain radar frequencies. In order to not seriously
affect the RCS of the CR the hole diameter must be less than one sixth of the radar wavelength (C.
Anderson, DSTO, Pers. Comm. 2012). The maximum size of perforation for our CR design is therefore
5mm, dictated by our shortest radar wavelength of interest (X-band at ~31mm).
To determine the effect of perforation spacing on overall RCS, Drake and Hatty [2013] performed RCS
measurements in an anechoic chamber of four 300 mm square samples of 1.6 mm thick aluminium
sheet. One sample had no perforations whilst the other three had perforations of 5 mm diameter in a
square pattern with varying hole centre spacings (10 mm, 13 mm and 18 mm). The mean of two RCS
measurements made in HH and VV polarisation respectively are given in Table 2.3.
Table 2.3: Results of RCS measurement of aluminium sheet samples with varying mesh perforation spacings in a
square pattern. Values given are in decibels and are relative to an identical sample with no mesh perforation.
Mesh perforations are 5mm diameter. Open area is the non-material area (filled by air) expressed as a
percentage of the total area.
Spacing (mm) Open Area (%) C-band X-band
10
19.6
-0.260
-1.245
13
11.6
-0.200
-0.615
18
6.1
-0.200
-0.630
At C-band, all perforated samples exhibit a reduction of ~0.2 decibels compared to the sample with no
perforations. As expected, the result at X-band is more severe since the hole diameter is closer to the
radar wavelength. It appears that the spacing between holes does have an influence, with a greater
reduction in RCS when the percentage open area (that is the area filled by air rather than panel
material) increases. The number of samples tested does not enable a conclusive relationship between
spacing and RCS reduction to be deduced. Nevertheless, the results in Table 2.3 indicate that an
open area between about 10-15% would be desirable to trade-off the disadvantage of RCS reduction,
particularly at X-band, against the advantages of mesh perforation mentioned above.
The final mesh design used an equilateral triangular grid with 5 mm holes and 12 mm hole spacings,
giving a 15.7% open area (Figure 2.13). Narrow gussets of un-punched material were left along the
edges where bolts and insert studs were to be installed to the framework through the panels. The
equilateral triangular grid has a pitch of 60 degrees between adjacent rows and gives a uniform
spacing between holes, a greater open area for a given hole spacing (Figure 2.14), and the total
number of holes required is less to achieve the same open area, compared with a square grid. This
latter point has advantages in the production process, where a turret punch was used to physically
remove material from the sheet to create the hole. This process can result in distortions that introduce
plate surface irregularities or even an adverse plate curvature that will affect the RCS performance of
the manufactured CR as discussed in §2.4. Therefore even a marginal reduction in the number of
punches to be made to the sheet is advantageous.
22
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 2.13: Detail of the equilateral triangular mesh perforation used for prototype mesh CR with annotated
dimensions. This mesh has an open area of 15.7%.
Figure 2.14: Mesh perforation spacing versus percentage open area (i.e. air void) for square and triangular
meshes with holes of 5 mm diameter.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
23
2.5 Other design features
During the design process we gave due consideration to the remote locations that the CR would be
deployed in and with minimal installer assistance. As such the CR assembly was designed to be
modular to enable easy dismantling and reassembly, easier transportation and to ensure the angular
integrity of the CR is not compromised when reassembled.
The CR ground mount was designed so that when the CR was deployed within the 0-40 degree
baseplate elevation range it would not be visible at boresight. This way the ground mount
infrastructure cannot impact on the RCS response of the CR.
When designing a CR for permanent deployment in the landscape it is important to consider all the
SAR sensor imaging geometries (and modes) that may image that CR during its life time. Also, the CR
should be easy to re-orient in the field with minimal effort or tools. In our design, the CR can be rotated
360 degrees in azimuth and the baseplate elevated between 0 and 40 degrees (with respect to the
horizontal ground surface). These orientation parameters were designed to be suitable for all current
and planned orbiting SAR sensors (Table 1.1) such that any location where a CR is deployed will fall
in an imaging swath of each SAR sensor at some occasion during its orbital cycle.
When deployed in the landscape the trihedral CR design will accumulate precipitation unless holes are
present to allow drainage. We included a single 2 cm diameter hole at the apex of the CR to allow
water drainage. This was perceived to be adequate when the CR is deployed at any non-zero
baseplate elevation angle. Snow deposits are harder to clear from within a CR, but in the warm
Australian climates this should not be an issue. However, for polar deployment sites, a ‘radome’
(membrane made of non-metallic material) can be used to cover the open aperture of the reflector to
prevent snow accumulation. Since the non-metallic material is virtually invisible to radar the CR would
still perform with minimal reduction to the RCS. Bird et al. [1993] conducted an experiment with
reflector radomes manufactured from Goretex fabric and found they only contributed a loss of 0.4 dB
in C-band ERS-1 SAR imagery. A more expensive solution has been used by DLR on their robotic
SAR calibration CR (B. Döring, DLR, Pers. Comm. 2014). These CR are programmed to orient
themselves shortly before an overpass and afterward move back to an upside-down stowed position
that minimises the potential for precipitation accumulation or wind damage.
Finally we manufactured 18 prototype CR for performance analysis. There were 6 designs (Table 2.4)
and we manufactured 3 of each design in order to test the consistency of the manufacturing process.
Design drawings for the four different sizes manufactured are shown in Figure 2.15, Figure 2.16,
Figure 2.17, and Figure 2.18.
Table 2.4: The different prototype triangular trihedral CR designs, and costs excluding ground mounting stand.
CR size (m)
Panel finish
Cost of 3 CR panels (AUD, excl. GST)
1.0
Solid Al sheet
$844.65
1.5
Solid Al sheet
$1,723.80
1.5
Powder-coated solid Al sheet
$1,988.10
1.5
Mesh-perforated Al sheet
$4,204.65
2.0
Solid Al sheet
$2,823.75
2.5
Solid Al sheet
$3,510.15
24
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
The additional size of the larger 2.0 m and 2.5 m CR panels meant that they could not be
manufactured from a standard 1,200 mm x 2,400 mm sheet of aluminium. Larger and not so readily
available 6,100 mm x 1,830 mm x 6 mm aluminium sheets had to be sourced for their manufacture.
Some of the smaller CRs were included in the sheet cut-out design to minimise the wastage
associated with cutting the larger panels from these sheets. Given the increased area of each
reflective surface, a more substantial support frame was designed to help the panels hold their shape.
These were manufactured from 6 mm x 50 mm extruded aluminium angles that were bolted together
in strategic places and fixed to the panels by insert studs. Three supports were run from a central
location on the main support brace on the hypotenuse, one from the centre of the hypotenuse to the
boresight, and two from the centre of the hypotenuse to the centre of the other two edges of the panel.
2.6 Observed distortions of CR prototypes
Geometrical distortions of some CR panels were observed. The amount of distortion was also
proportional to the CR size (i.e. the larger the CR the greater the distortion issues). The strict design
criteria required for the CR, in particular no visible frame or supports at boresight through the 0° - 40°
deployment elevation angles, meant that support structures and frames could not be fitted in their
optimal positions or edges (in an engineering sense) to minimise distortion in the most efficient
manner. As a result, compromises had to be made during the design process.
Investigations were made on the angular accuracy and strength of thicker 6 mm x 50 mm extruded
aluminium angle as an alternative for the 4 mm angle used on the smaller panels. It was discovered
that the consistency of an accurate 90° in the 6 mm angle was much better and that it had less flex
along the longer hypotenuse sections. A full set of 6 mm x 50 mm extruded aluminium angles were
manufactured and retro-fitted to one of the prototype 1.5 m CR during the Gunning field deployment
(§4). Each new angle section was checked for angular and longitudinal accuracy before it was
installed. The introduction of this better quality stock material reduced the deformation in each sheet
significantly. Bulging of approximately 5 mm amplitude was reduced to approximately 1-2 mm
amplitude on the prototype CR. The positive results of this testing prompted a change in the design
drawings for the larger manufacture run of CR for AGOS deployment. A central support angle that was
parallel with the hypotenuse support angle was also introduced on the two vertical panels in the
revised design drawings in a further attempt to reduce distortion.
2.6.1 Mesh-perforated CR
The three 1.5 m CR with mesh perforated panels were manufactured from thinner 4 mm aluminium
sheet because this was the maximum thickness that could be penetrated by the available turret punch.
While the perforated panels were lighter and had better draining and self-cleaning properties, the
physical punching process introduced additional distortion to the panels. The individual holes had
slightly raised edges around their circumference that affected the surface flatness. The manufacturer
reduced this by using an industrial orbital sander across the entire sheet which reduced the distortion
but produced a rougher surface finish that affected the panel’s self-cleaning properties (i.e. dust
adhered to the surface more readily). The punching process also appeared to stretch the material and
release internal stresses that distorted the panels at varying levels. In particular the leading edges of
each panel had forces applied to them that tended to dip the entire edge outwards from the reflective
surfaces being measured (Figure 2.19). This distortion was not consistent with some panels only
having minor deflections while others were over 5 mm.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
25
Figure 2.15: Design drawings of prototype 1.0 m CR with ground mounting stand and pre-fabricated concrete
slab. The CR is depicted with a baseplate elevation of zero degrees. Dimensions in millimetres.
Figure 2.16: Design drawings of prototype 1.5 m CR with ground mounting stand and pre-fabricated concrete
slab. The CR is depicted with a baseplate elevation of zero degrees. Dimensions in millimetres.
26
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 2.17: Design drawings of prototype 2.0 m CR with ground mounting stand and pre-fabricated concrete
slab. The CR is depicted with a baseplate elevation of zero degrees. Dimensions in millimetres.
Figure 2.18: Design drawings of prototype 2.5 m CR with ground mounting stand and pre-fabricated concrete
slab. The CR is depicted with a baseplate elevation of zero degrees. Dimensions in millimetres.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
27
2.6.2 Larger solid sheet CR
When assembled the larger 2.0 m and 2.5 m CR (particularly the 2.5 m CR) displayed visible distortion
along the leading edges of the two vertical panels. The longer spans and physical weight of the
reflector panels seemed to make them more susceptible to gravitational forces. This effect only
increased when they were tilted back. The panels bellied outwards under their own weight up to 10
mm in the centre of the hypotenuse-edge. The distortion could temporarily be manually pushed back
into position. While the problem was discussed at length, no attempt was made to remediate this
distortion mainly because it was thought it would be unlikely that these large CR sizes would be mass
produced for the main AGOS deployment and the amount of effort required to remediate the problem
in the timeframe available was not practical.
Figure 2.19: Example of distortion in a 1.5 m mesh-perforated panel observed during radar signature
characterisation. Level is approximately 50 mm tall.
28
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
3 Radar Signature Characterisation
In order for the AGOS CR to be considered a useful SAR calibration target, the radar signature for
each CR must be characterised since the actual RCS of each may differ somewhat from the
theoretical values. The Radar Signatures Group of the Commonwealth Defence Science and
Technology Organisation (DSTO) were tasked with determining the Radar Cross Section (RCS) of the
prototype CRs. The trials were conducted over 9 days between 17-27 June 2013. The results of this
exercise are summarised here but are fully documented in Drake and Hatty [2013].
3.1 Experimental procedure
Figure 3.1: Aerial view of the St Kilda ground reflection range taken at the time of the GA reflector characterisation
exercise (20 June 2013). The turntable is situated in the middle of a flat dolomite field, 180 m from the radar
antenna and control room situated on the edge of the range.
DSTO advised that the prototype triangular trihedral CRs were too large to be effectively measured in
their anechoic chamber (indoor controlled measurement environment). Therefore the DSTO outdoor
ground reflection range at St Kilda (South Australia) was used for the RCS measurements
(Figure 3.1). This facility consists of a large laser-levelled area covered in crushed Dolomite. It is
equipped with a 2-tonne rated turntable for spinning targets in the horizontal plane, and an
instrumentation radar connected to height-adjustable dual-polarisation wideband antennas. For each
CR, RCS calibration was performed using the substitution method, whereby a small trihedral calibrator
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
29
whose RCS was accurately known a-priori was measured at the beginning and end of the set of
reflector measurements under the same experimental conditions. Drake and Hatty [2013] report a
good agreement (better than 1 dB at C-band) between these measurements and post-trial
measurements in the anechoic chamber on the known calibrator and with simulated results from a
computational electromagnetics model.
For an RCS measurement accurate to 1 dB (decibels) it is required that the target is in the far-field
such that the illuminating radar wavefront is planar across the entire aperture (face) of the target.
Although ideally the target should be at an infinite distance from the radar source to be uniformly
illuminated, we must use a finite far-field distance for practical reasons. The generally accepted farfield distance criterion is given by [Knott, 2006]:
𝑅 ≥
2𝑥 2
𝜆
Where 𝑅 is the far-field range, 𝑥 is the largest dimension of the target in the plane normal to the
illuminating wavefront (see Figure 2.8) and 𝜆 is the radar wavelength (with all dimensions in metres).
This far-field range is for the full aperture of the trihedral reflector, but only around two thirds of the
aperture actually contributes to the strong triple bounce echo when the aperture is normal to the radar
wavefront [Knott, 2006]. The effective area of this hexagonal region is shown in Figure 2.8 and the
largest dimension of the target is now 2𝑥/3.
Table 3.1 gives the far-field range for the prototype CR sizes at different radar frequencies after taking
the effective area into account. Since the distance between the radar antenna and the centre of the
turntable is 180 m the ground reflection range was found to be only suitable for 1.0 m and 1.5 m CR at
X- and C-bands. Therefore the larger 2.0 m and 2.5 m reflector prototypes did not have their RCS
characterised. Although L-band characterisation was of interest, measurements could not be made
due to the lack of appropriate calibrators for use in the substitution method.
Table 3.1: Calculated Far-Field Ranges (FFR) for different sized reflectors at X-, C- and L-band radar frequencies.
The largest dimension for the full and effective apertures are illustrated in Figure 2.8.
Full aperture
Inner leg
dimension
(m)
30
Effective aperture
Largest
dimension
(m)
FFR Xband (m)
FFR Cband (m)
FFR Lband (m)
Largest
dimension
(m)
FFR Xband (m)
1.0
1.41
128.67
72.00
16.77
0.94
57.19
32.00
7.45
1.5
2.12
289.50
162.00
37.73
1.41
128.67
72.00
16.77
2.0
2.83
514.67
288.00
67.07
1.89
228.74
128.00
29.81
2.5
3.54
804.17
450.00
104.79
2.36
357.41
200.00
46.57
FFR CFFR Lband (m) band (m)
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 3.2: Schematic of the experimental setup for CR RCS measurements at the St Kilda ground reflection
range.The vertical axis shown that bisects the reflector corner apex also bisects the centre of the turntable used
to rotate the reflector in azimuth.
The RCS of the three 1.0 m CR and nine 1.5 m prototype CR were determined at X- and C-band. For
each band, a 1 GHz bandwidth was used, and horizontal (HH) and vertical (VV) polarisations during
separate sweeps. Results presented here are for the discrete frequencies of 9.65 (X-band) and 5.4
GHz (C-band). Figure 3.2 shows the experimental setup and the two path propagation technique used
for the RCS measurements. The height of the radar antennae was adjusted so that the direct wave
and ground reflected wave interfere constructively at the target. This required a different antenna
height of 1.34 m for X-band and 0.82 m for C-band measurements.
Each reflector in turn was attached to a turntable-mounted stand that was designed in such a way that
the reflector corner apex remained in the turntable centre whilst being rotated 180 degrees in azimuth
through the incident radar wavefront (Figure 3.3). An RCS measurement was sampled at 1 degree
azimuth intervals as the reflector was rotated between [-90 90] degrees, with zero degrees
corresponding to the boresight alignment (i.e. the aperture view shown in Figure 2.8). For all
reflectors, the reflector corner apex was held at a height of 3.2 m above the turntable. To minimise
reflections from the stand infrastructure, panels of radar absorbing material (RAM) were used to cover
the stand. Measurements were made before and after the addition of RAM and these showed that the
RAM effectively minimised the radar signature of the stand infrastructure.
The measurement procedure at each frequency and polarisation was conducted for an ‘azimuth cut’
where the leading edge of the nominated ‘base plate’ is parallel to the ground surface and an
‘elevation cut’ where the leading edge is perpendicular to the ground, and on the right hand side of the
reflector when viewed from the front (Figure 3.4). A plumb-bob was used to ensure verticality of the
reflector aperture and for azimuth cut measurements a digital level was used to ensure the reflector
baseplate dipped at the correct angle such that the boresight was parallel with the ground. These
measurements ensured that the reflector aperture was normal to the incident radar wavefront when
the boresight was at a rotation azimuth of zero.
All the combinations of alignment, frequency and polarisation gave a set of eight measurement
sweeps for each CR that were all made before the next CR was mounted on the stand. Typically, the
full set of eight measurements for two CRs could be made in a working day. All eight measurements
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
31
on a particular CR were made on the same calendar day ensuring that as far as practically possible
the atmospheric conditions were ‘constant’ for all measurements.
Figure 3.3: Turntable-mounted stand covered in RAM (left) and with a mesh-perforated 1.5 m reflector attached
(right).The circular mounting plate enabled rotation of the reflector through 90 degrees between azimuth cut and
elevation cut measurements.
Figure 3.4: An aperture view of an aluminium 1.5 m CR aligned for an azimuth cut measurement (left) and an
elevation cut measurement (right). Between azimuth cut and elevation cut measurements the reflector was
rotated 90 degrees anti-clockwise.
3.2 RCS characterisation results
3.2.1 RCS profiles
All prototype CRs measured displayed the radar signature characteristics expected of a triangular
trihedral when measured in the azimuth cut (Figure 3.5) and elevation cut (Figure 3.6). The peak RCS
in both cuts is found when the CR aperture is parallel to the incident radar wavefront and
perpendicular to the radar line of sight vector, which corresponded to a turntable rotation azimuth of
zero degrees. This orientation is equivalent to the boresight vector (Figure 2.7).
In the azimuth cut measurements, the RCS pattern is symmetrical about the boresight. A gradual
reduction in RCS from triple bounce reflections occurs as the orientation increases to approximately
±35 degrees. These “Batman ears” [e.g. Knott, 2006] are spikes in RCS located at ±35-40 degrees,
32
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
which are caused by dihedral double bounces as the third face providing the triple bounce mechanism
rotates out of the radar line of sight. At orientations greater than about 40 degrees there are no
coherent double or triple bounces coming from within the reflector aperture and the measured RCS
correspondingly drops and shows no coherent pattern.
In the elevation cut measurements, the RCS pattern is non-symmetrical due to the different angles
subtended above and below the boresight vector (see Figure 2.7). A double-bounce dihedral
secondary peak in RCS occurs at approximately 35 degrees and a tertiary RCS peak of dihedral origin
at approximately -55 degrees. Outside of the range [-55, 35] degrees the RCS again drops to a low
background level without a coherent pattern.
Generally there is good agreement of RCS profiles of individual CR within type groups in the region of
triple bounce reflections (as indicated by a tight error envelope in Figure 3.5 and Figure 3.6). The
exception to this is the 1.0 m CR type group at X-band.
3.2.2 Peak RCS measurements
Table 3.2: Mean and standard 1-sigma error of RCS measurements in dBm2 from all reflectors in a type group
from all four measurement combinations (azimuth and elevation cuts; HH and VV polarisations; total of 12
measurements each, except for ‘All 1.5m’ which has a total of 36 measurements). The RCS value corresponds to
the measurement at zero degrees azimuth rotation, which we assume, based on theory, to be the peak RCS of
each CR. Also given is the mean difference from theoretical RCS values for each CR size and radar frequency
(theoretical minus measured value).
X-band
All 1.0m
All SM
All SP
All MM
All 1.5m
C-band
Mean
Std. Error
Mean
Std. Error
Measured
33.187
0.483
29.696
0.249
Diff. w/ Theory
3.206
0.483
1.639
0.249
Measured
38.178
0.263
36.271
0.149
Diff. w/ Theory
5.259
0.263
2.108
0.149
Measured
38.125
0.131
36.414
0.145
Diff. w/ Theory
5.313
0.131
1.964
0.145
Measured
39.161
0.082
36.488
0.036
Diff. w/ Theory
4.277
0.082
1.890
0.036
Measured
38.488
0.221
36.391
0.121
Diff. w/ Theory
4.950
0.221
1.987
0.121
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
33
Figure 3.5: RCS profiles versus turntable rotation azimuth from the ‘azimuth cut’ measurements grouped by
reflector type (rows; 1.0m plain sheet, 1.5m plain sheet, 1.5m mesh perforated and 1.5m powder-coated) and by
radar frequency (columns; X- and C-band, blue and red line respectively). Each plot shows a mean profile in the
solid coloured line calculated from the 6 sample combinations between the three reflectors in the type group and
two radar polarisations (HH or VV). A standard 2-sigma error envelope is plotted as a grey polygon for each mean
RCS profile. Curves are sampled at 1 degree intervals.
34
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 3.6: RCS profiles versus turntable rotation azimuth from the ‘elevation cut’ measurements grouped by
reflector type (rows; 1.0m plain sheet, 1.5m plain sheet, 1.5m mesh perforated and 1.5m powder-coated) and by
radar frequency (columns; X- and C-band, blue and red line respectively). Each plot shows a mean profile in the
solid coloured line calculated from the 6 sample combinations between the three reflectors in the type group and
two radar polarisations (HH or VV). A standard 2-sigma error envelope is plotted as a grey polygon for each mean
RCS profile. Curves are sampled at 1 degree intervals.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
35
Figure 3.7: Mean differences (theoretical minus measured value) in RCS measurements for all twelve 1.0m and
1.5m prototype CR. Means for each frequency are calculated from the four measurement combinations (HH and
VV polarisation, and azimuth and elevation cut measurements). The RCS value corresponds to the measurement
at zero degrees azimuth rotation, which we assume from theory to be the peak RCS of each CR. Blue points are
X-band and red points are C-band measurements. Error bars indicate the standard 1-sigma error.
For SAR calibration purposes it is important that the RCS of the CR at boresight is accurately known.
In this section we give the peak RCS measured for each of the twelve CR measured during the DSTO
characterisation exercise, at both X- and C-bands.
In Table 3.2 the eight RCS measurements at each frequency, polarisation and cut combination at
boresight (zero degrees azimuth rotation) is summarised for each reflector. More comprehensive
tables with calculated mean peak RCS and standard errors on measurements for each CR and for
each type of measurement are given in Appendix A. In Figure 3.7 the mean differences in RCS are
plotted for each reflector individually at both X- and C-band.
In general, the results show that the RCS at C-band of the prototype 1.5 m CRs is 2.0 ± 0.3 dBm2 less
than theory whereas the 1.0 m CRs are around 1.6 +0.6/-0.3 dBm2 less than theory. At X-band the RCS
of individual CR is more variable, ranging between 5.0 +1.5/-1.0 dBm2 less than theory for 1.5 m CRs and
3.2 ± 1.0 dBm2 less than theory for 1.0 m CRs. At both bands, the measured RCS differences are
statistically significant since they are much greater than the standard errors on the measurements. At
the longer radar wavelengths not measured in this radar signal characterisation (e.g. S-, L- and Pband) it is likely that the differences from theoretical RCS will be smaller and therefore less significant.
The results indicate that departures of the CR from perfect inter-plate orthogonality and plate flatness
are less tolerated at shorter radar wavelengths. Anecdotally we can also confirm that RCS-loss is
proportional to CR size, and we would therefore expect the loss for the un-tested 2.0 m and 2.5 m
CRs to be proportionally larger than the smaller 1.0 m and 1.5 m CRs tested here.
The standard error values given in Table 3.2 and Figure 3.7 indicate the variability of measurements
within each type-group of CR. Generally there is good consistency within and across the 1.5 m typegroups, but this is not the case for the 1.0 m CRs, particularly at X-band. Some difference between
36
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
individual CRs can be attributed to the different atmospheric conditions on the day of measurements.
As mentioned previously, the RCS measurements of the known calibrator made at the beginning and
end of the trials agreed to within 1 dB at C-band. Furthermore, a set of measurements of CR 4 were
made on 18 June 2013 and 26 June 2013 and these agreed to within 0.5 dB at C-band. To a lesser
degree there may also be differences within measurement sets of a particular CR attributable to
atmospheric changes during the set of 8 measurements (e.g. potentially those CR in Figure 3.7 with
large error bars), though unfortunately this is not possible to quantify.
To achieve a CR with an RCS much closer to theoretical values would require much greater
adherence to the tolerances outlined in §2.4. This would only be possible with a much greater
manufacturing cost. Regardless, the departures from theoretical RCS should not have any negative
impact on the application of the CRs. For use as SAR calibration targets it is only important that the
actual RCS is accurately known. For use as a deformation target it is only important that the phase
response remains stable. As discussed in §2.1.3, the phase stability can be linked to the SCR and this
is more dependent on the clutter level in the imagery rather than the absolute RCS of the target.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
37
4 Field Testing
4.1 Description of test site
All 18 CR prototypes were deployed in paddocks belonging to a sheep grazing company at a property
near Gunning, NSW, approximately 55 km north of Canberra (Figure 4.1). The installation of all 18 CR
was completed by 12 December 2013 and all CR were removed by 16 May 2014. During this period
24 SAR image acquisitions were made using the TerraSAR-X, COSMO-SkyMed, RADARSAT-2 and
RISAT-1 missions (Table 4.2). An acquisition was also made of the Gunning CR array by KOMPSAT-5
on 26 March 2015, though we do not analyse that SAR image here.
Figure 4.1: a) Overview map showing the town of Gunning, NSW to the north of Canberra. Grey shaded region is
the Australian Capital Territory. b) Shaded relief map of the grazing property paddocks (outlined in red) situated to
the south-west of Gunning that were available for temporary CR deployment.
4.1.1 Site selection
Several factors were considered when choosing sites for CR deployment:
38
1.
The flatness of the surrounding land. The country within which the available paddocks
are situated is quite hilly (Figure 4.1). Using a 1-arcsecond digital elevation model (DEM)
derived from the SRTM mission [Farr et al., 2007] we calculated a slope map (Figure 4.4a).
Using this as a guide we chose candidate sites where the local slope was less than 10
degrees (i.e. a gradient of less than 17.6%), and usually less than 5 degrees.
2.
Perceived sources of radar clutter in the vicinity. Sites were chosen as much as
possible that were a good distance away from perceived sources of clutter (e.g. trees or
dense vegetation, rock outcrops, farm buildings and infrastructure), though it was not
always possible to achieve this due to the nature of the land available.
3.
Distance from metallic boundary fences. Since horizontally polarised SAR images were
to be acquired, it was recognised that metallic boundary fences oriented perpendicular to
the radar LOS (and parallel to the satellite flight direction) would introduce a high
magnitude response in the imagery (for example see Figure 4.11). Generally the flight
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
azimuth of the SAR missions used here is ~194.4 ± 2 degrees for descending passes and
therefore fences oriented between 185 and 205 degrees were specifically avoided.
Generally, sites were chosen in the middle of paddocks at least 50 metres away from the
nearest boundary fence.
4.
Overlap of adjacent CR responses. A further consideration when choosing site locations
for CR deployment was to ensure that the side-lobe response of adjacently sited CR would
not overlap. Although the spatial extent of side lobe ringing was expected to be low for 1.0
m and 1.5 m CR, the larger 2.0 m and 2.5 m CR could introduce side lobes extending over
large distances. In a desktop study, it was ensured that for each CR no other CR site
intersected the geographically-projected azimuths of potential side-lobes (TerraSAR-X
orbital geometry was used). Generally, the baselines between all CR sites are greater than
200 m (except for the baseline between site 4 and 5, which was 186 m).
4.1.2 Installation
Figure 4.2: Photos of CRs installed at Gunning. Clockwise from top left: 1.0 m CR at site 7; 1.5 m meshperforated CR at site 6; 2.5 m CR at site 18; 2.0 m CR at site 17.
Each CR and stand was mounted on a pre-fabricated square concrete slab as shown in Figure 4.2.
The slabs were sized appropriately for each CR such that expected wind loadings would not topple the
CR when installed. The slabs were pre-fabricated off-site and lifted on to a sand bed such that they
were approximately level. Three brass screw-pins were installed into drilled holes in the slab with
epoxy mortar and the CR stand was bolted to these such that the base triangle was confirmed level
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
39
with a spirit level. Following removal of the slabs at the end of the exercise it was confirmed that the
sand beds had remained intact and therefore slabs would have remained largely level throughout the
period of deployment.
Installation of 1.0 m and 1.5 m CR was manageable with two people. The increased size and weight of
the 2.5 m CR panels meant that a crew of three was required in addition to the use of a truck-mounted
crane to lift the two vertical panels in to position. The 2.0 m CR panels were more manageable and
could be handled by three people but in most cases they were manipulated into position using the
crane.
4.1.3 CR site positions
The positions of each CR site were surveyed using Real Time Kinematic (RTK) equipment. An RTK
base station was set up on a new survey mark established at a high point on the property such that
the radio transmission from the base station antenna would be detectable at all the CR sites. A roving
antenna was then used to pick up the position of each CR (Figure 4.3). The survey mark on each CR
was an indentation added to the centre of the azimuth adjustment pivot bolt, which remains fixed
regardless of CR orientation. For 1.0 m and 1.5 m CR it is possible to pick up this survey mark with
reflector panels installed by tilting forward the CR assembly (Figure 4.3), however this is not possible
for larger 2.0 m and 2.5 m CR due to the extra weight and surveying with a staff-mounted antenna as
used here can only occur with reflector panels removed. Following the end of the prototype
experiment the surveying exercise was repeated before CR were fully dismantled.
The positions of each CR within the array are shown in Figure 4.4 and annotated in Table 4.1.
Figure 4.3: (left) RTK base station set up at the new survey mark. (right) RTK survey to establish the position of
the 1.0 m CR at site 7 prior to installation of the reflector panels. Even with reflector panels installed the assembly
can be tilted forward as in this photo for survey occupation.
40
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 4.4: Map of the deployed CR sites at the Gunning test site between December 2013 and May 2014. a)
Slope map derived from ~30 m SRTM DEM used to aid site selection. b) Landsat-8 RGB-composite optical image
acquired on 15 January 2014. The optical image has a 30 m pixel resolution. The crossed-circle symbol indicates
the position of the survey mark established for the RTK base station (Figure 4.3).
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
41
Table 4.1: Surveyed locations of CR sites and survey mark (SM) at beginning and end of CR prototyping exercise
at Gunning. Coordinates are given in Map Grid of Australia 1994 (MGA94; zone 55) and heights are referenced to
the GRS80 ellipsoid.
Date of survey 22/11/13 & 5/12/13
Date of survey 15/05/2014
Comparison
Easting (m) Northing (m) Height (m) ΔE (m) ΔN (m)
ΔH (m)
670.593
704319.339
6147500.946
670.597
-0.018
0.005
-0.004
6147706.312
659.850
704556.834
6147706.309
659.851
-0.012
0.003
-0.001
705069.224
6147177.416
636.752
705069.226
6147177.415
636.772
-0.002
0.001
-0.020
4
704931.434
6147030.189
640.856
704931.457
6147030.184
640.869
-0.023
0.005
-0.013
5
704863.639
6146856.294
641.857
704863.667
6146856.286
641.850
-0.028
0.008
0.007
6
704890.155
6146633.079
640.745
704890.184
6146633.081
640.733
-0.029
-0.002
0.012
7
705205.622
6146173.630
647.911
705205.648
6146173.626
647.911
-0.026
0.004
0.000
8
705227.615
6145767.027
632.899
705227.621
6145767.023
632.886
-0.006
0.004
0.013
9
704815.631
6145811.789
638.122
704815.645
6145811.792
638.131
-0.014
-0.003
-0.009
10
704877.970
6145472.737
644.563
704877.970
6145472.735
644.539
0.000
0.002
0.024
11
703957.713
6145567.093
659.018
703957.714
6145567.077
659.023
-0.001
0.016
-0.005
12
703576.580
6145528.694
666.212
703576.601
6145528.690
666.226
-0.021
0.004
-0.014
13
703205.750
6145796.819
671.504
703205.780
6145796.815
671.508
-0.030
0.004
-0.004
14
702976.245
6145427.012
685.007
702976.269
6145427.022
684.996
-0.024
-0.010
0.011
15
702652.281
6144488.778
674.239
702652.297
6144488.783
674.232
-0.016
-0.005
0.007
16
702493.140
6144057.711
678.500
702493.136
6144057.693
678.506
0.004
0.018
-0.006
17
702153.329
6144255.690
700.931
702152.057
6144255.414
700.133
1.272
0.276
0.798
18
701645.405
6144285.531
731.308
701645.446
6144285.522
731.322
-0.041
0.009
-0.014
Site
Easting (m) Northing (m) Height (m)
SM
705469.553
6146007.124
654.400
1
704319.321
6147500.951
2
704556.822
3
The size, type and unique ID of CRs installed at each site is given in Table 4.4.
The differences between the positions from the two surveys are at the centimetre level, which is within
the expected accuracy of the RTK method. Therefore we assume that any movement of the CR over
the period of the deployment is sub-centimetre in magnitude. The exception to this is site 17 which
shows a movement of over a metre. This is due to the second survey being made after dismantling the
CR stand in this case.
42
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
4.2 SAR acquisitions
Twenty-four SAR acquisitions were made from descending passes of the TerraSAR-X, COSMOSkyMed, RADARSAT-2 and RISAT-1 satellites (Table 4.2). The imaging modes and acquisition
parameters used for each SAR sensor are detailed in Table 4.3.
Table 4.2: SAR acquisitions of the Gunning reflector array
Acquisition #
Date (UTC)
Time (UTC)
SAR
sensor
CR Alignment notes
1
15/11/2013
19:27:59
TSX
Pre-deployment
2
7/12/2013
19:27:59
TSX
Average; only 1.0m and 1.5m reflectors
3
11/12/2013
7:14:35
CSK-1
Average; only 1.0m and 1.5m reflectors
4
14/12/2013
19:18:48
RSAT-2
Average alignment
5
27/12/2013
7:14:31
CSK-1
Average alignment
6
29/12/2013
19:27:58
TSX
Average alignment
7
7/01/2014
19:18:47
RSAT-2
Average alignment
8
9/01/2014
19:27:57
TSX
Average alignment
9
12/01/2014
7:14:23
CSK-1
Average alignment
10
20/01/2014
19:27:58
TSX
Aligned for TSX
11
28/01/2014
7:14:18
CSK-1
Aligned for CSK
12
31/01/2014
19:27:57
TSX
Aligned for RSAT-2
13
31/01/2014
19:18:49
RSAT-2
Aligned for RSAT-2
14
3/02/2014
19:28:11
RISAT-1
Aligned for RISAT-1
15
11/02/2014
19:27:56
TSX
Aligned for TSX
16
13/02/2014
7:14:12
CSK-1
Aligned for CSK
17
22/02/2014
19:27:56
TSX
Aligned for TSX but with mis-alignment
18
24/02/2014
19:18:44
RSAT-2
Aligned for RSAT-2 but with mis-alignment
19
28/02/2014
19:27:59
RISAT-1
Aligned for RISAT-1
20
5/03/2014
19:27:57
TSX
Aligned for RISAT-1
21
25/03/2014
7:14:00
CSK-2
Aligned for CSK but with mis-alignment
22
10/04/2014
7:13:59
CSK-2
Aligned for CSK but with mis-alignment
23
14/04/2014
7:13:57
CSK-4
Aligned for CSK but with mis-alignment
24
18/04/2014
7:13:57
CSK-1
Aligned for CSK but with mis-alignment
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
43
Table 4.3: SAR imaging modes used for the acquisitions of the Gunning reflector array listed in Table 4.2.
TerraSAR-X
COSMO-SkyMed
RADARSAT-2
RISAT-1
StripMap
HIMAGE
Fine
FRS-1
Product
SSC
SCS_B
SLC
SLC
Beam
009
05
F21
85
Polarisation
HH
HH
HH
HH+HV
0.031 (X)
0.031 (X)
0.055 (C)
0.056 (C)
Range pixel size (m)
0.9
1.2
4.7
1.8
Azimuth pixel size (m)
1.9
2.1
5.1
2.4
Image Mode
Radar wavelength (m)
(Frequency Band)
Notes
gain attenuation 10dB
Calibration-2 lookup table
4.3 Field orientation strategy
[Williams, 2011b] describes a methodology for aligning CR targets in the field, which we summarise
here. At Gunning, CRs were deployed for descending passes of the four satellite missions.
The Line of Sight (LOS) vector of the SAR sensor with respect to a fixed ground location can be
considered to be fixed over intermediate time periods; it will vary over time due to orbit creep and
satellite manoeuvres conducted to maintain a nominal orbit. At worst this unknown component of the
orbital baseline may be up to a kilometre in magnitude but this is very small compared to the range
vector joining the CR and the satellite (hundreds of kilometres), and should correspondingly introduce
only a small elevation alignment error. As an example, a 1 km deviation from the nominal orbit and a
typical TerraSAR-X satellite-to-ground range of ~600 km gives an elevation alignment error of ~0.1
degrees.
For our CR orientation strategy we assume that the LOS vector for all SAR sensors is perpendicular to
the travel direction of the satellite platform. This may not be strictly true for all SAR sensors since
some may use beam steering to introduce a ‘squint’ to the imaging geometry (known as Doppler
steering). However it does significantly simplify the calculation required to determine the orientation of
the CR boresight so it coincides with the SAR sensor LOS. Fortuitously under this assumption and
due to the perpendicularity of the LOS vector with respect to the travel direction, the LOS vector
coincides with the time when the range vector between CR and SAR satellite is at a minimum. At this
time, the azimuth and elevation of the vector pointing from the ground to the satellite can be
calculated. We used the Systems Tool Kit (STK) software to calculate CR orientations for different
satellite passes (Figure 4.5). Orientations were calculated on a desktop machine before travelling to
the field to re-orient the CRs. Over the trial period of ~3 months, 11 field visits were made to perform
CR alignment re-orientations.
44
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 4.5: Example of an STK calculation for a TerraSAR-X pass. The boresight azimuth and elevation is taken
at the time of the minimum range denoted by the vertical black line.
When a CR is deployed permanently in the landscape, maybe in a remote location, it could be a long
time before it is re-visited and a re-orientation is possible. Therefore a compromise orientation must be
chosen that maximises the visibility for all SAR sensors of interest. Our strategy is to point each CR in
an orientation computed as the average of the different SAR sensor boresights. Fortunately, for a
particular choice of ascending or descending passes, the flight path (and therefore nominal
perpendicular LOS) only varies by about 1 degree for the SAR sensors used at Gunning (Figure 4.6).
The range of incidence angles is greater though, being about 3 degrees for the SAR sensors used at
Gunning.
4.3.1 Intentional misalignment
For 7 acquisitions (marked in Table 4.2), some of the CR were aligned with a known misalignment
from the calculated boresight orientation (Figure 4.7). Misalignments of 10 or 20 degrees magnitude
were used. The purpose was to see if the drop off in RCS measured from the imagery tallied with the
known reduction from the DSTO measurements. Table 4.4 gives the misalignment angles that were
added to the boresight orientations for each CR in the array. These CR-specific misalignments were
consistent for the 7 acquisitions.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
45
Figure 4.6: Boresight alignments (azimuth and elevation) for all 18 CR when deployed at Gunning calculated for
each SAR satellite and an average orientation.
Figure 4.7: Distribution of misalignment angles for 7 SAR acquisitions at Gunning.
46
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Table 4.4: Misalignment angles for azimuth and elevation added to CR boresight orientations for 7 acquisitions at
Gunning.
Site
CR size (m)
CR finish
CR ID
Azimuth misalignment
(degrees)
Elevation misalignment
(degrees)
1
1.5
Mesh
012
10.0
0.0
2
2.5
Metallic
016
0.0
0.0
3
2.0
Metallic
015
20.0
0.0
4
1.5
Powder
008
20.0
0.0
5
1.5
Metallic
004
0.0
-20.0
6
1.5
Mesh
011
0.0
10.0
7
1.0
Metallic
003
0.0
0.0
8
1.5
Metallic
006
0.0
20.0
9
1.5
Powder
007
0.0
-10.0
10
1.5
Mesh
010
10.0
10.0
11
1.0
Metallic
001
20.0
0.0
12
1.5
Powder
009
20.0
20.0
13
2.0
Metallic
013
0.0
0.0
14
2.5
Metallic
017
20.0
0.0
15
1.0
Metallic
002
0.0
20.0
16
1.5
Metallic
005
0.0
0.0
17
2.0
Metallic
014
0.0
20.0
18
2.5
Metallic
018
0.0
20.0
4.3.2 Field alignment methods
The calculated orientation of the boresight must be converted to physical quantities relating to the CR.
If the elevation angle is given with the ground level being zero, then subtracting the quantity 35.26
degrees from the elevation gives the elevation angle of the reflector baseplate (see §2.2). The azimuth
angle pointing towards the satellite is easy to measure in the field using a sighting compass.
Figure 4.8 shows the set up used to orient each reflector. Firstly the azimuth of the reflector is
adjusted. This is facilitated by threading a plumb-bob through the CR apex (where there is a hole for
water drainage) and run across the baseplate. Each reflector baseplate has a groove sawn halfway
along the hypotenuse edge, and the plumb line is fed through this groove. By lining up the vertically
hanging plumb-bob with the intersection line of the two vertical plates at the back of the reflector, a
sighting compass is used to find the correct azimuth ensuring that calculated azimuths are manually
corrected for local magnetic declination. The sighting compass has a precision of around 0.5 degrees
though the accuracy is affected considerably by the individual observer (see §4.3.4).
Once the azimuth is set, the CR is tilted forward or back to the correct baseplate elevation. A digital
level (that was calibrated on level workshop machinery at GA) is placed on the baseplate and parallel
to the plumb line such that the elevation corresponds to the intersection of the boresight vector and
the baseplate. The precision of the digital level used is 0.05 degree (1-sigma).
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
47
Figure 4.8: (left) CR at site 5 with plumb bob and digital level in place for re-orientation. (right) A sighting compass
being used to set the CR azimuth at site 10.
Figure 4.9: Results of the absolute azimuth alignment accuracy experiment. The crosses represent the mean of
the absolute difference of independent azimuth measurements made from 4 different pairs of ground stakes by 5
observers with a sighting compass, and with RTK equipment. Error bars give the 2-sigma standard error (95%
confidence interval). The dashed black line gives the mean of all 20 measurements made and the grey polygon
indicates the 2-sigma standard error (95% confidence interval).
4.3.3 Absolute accuracy of azimuth measurements
We conducted an experiment to investigate the absolute accuracy of our azimuth alignment methods
and assess the impacts of the differing eyesight of individual observers. The alignment of four pairs of
ground stakes (positioned ~7 m apart) was measured independently by 5 observers using the sighting
compass and RTK equipment. RTK has a typical positional uncertainty of 0.02 m which could
introduce an error of ~0.4 degrees to RTK-determined azimuth measurements. The absolute
difference between measurements from both techniques was taken after both were converted to true
48
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
north (Figure 4.9). From these results and considering the level of error in the RTK measurements we
determine that the accuracy of the azimuth alignment method is ±2.63 degrees around the true
azimuth (based on the square-root sum of squared errors of 2.6 degrees maximum statistical observer
error and 0.4 degrees RTK error). RCS calibration results for the CR prototypes measured (§3)
indicate that this level of alignment error would introduce a reduction in the RCS of about 0.1 dBm2.
4.3.4 Field measurement accuracy
To investigate the repeatability and relative accuracy of our field alignment methods we analysed
measurements of the azimuth and elevation made on the CR directly before re-alignment during nine
re-alignment visits. On each re-alignment visit two observers made measurements of the azimuth of
each CR with the sighting compass: a primary observer (PO; observer #5 in Figure 4.9) who was
present during all nine visits, and a secondary observer who was one of three different people over the
course of the nine visits (SO; observers #2 #3 and #4 in Figure 4.9). The PO was responsible for
setting the azimuth alignment on all CR during all re-alignments.
Figure 4.10: a) Mean absolute differences of azimuth and elevation measurements made before 18 CR realignments on nine different visits. b) Zoom of plot a) to highlight the low end of the azimuth range. Circle symbols
are measurements made by the primary observer (see text) and diamond symbols are made by the secondary
observers. Error bars are 2-sigma standard errors (95% confidence interval) calculated from the sample size of 18
CRs.
For each azimuth and elevation measurement we calculate the absolute difference between the set
alignment and the measurement made at the time of re-alignment and determine the mean and
standard error (Figure 4.10). The results of this analysis show that the PO had a measurement
consistency generally less than 0.5 degrees (including 95% confidence interval). There is considerable
variation in the observations of the three SO’s. Six are greater than 2 degrees (including 95%
confidence intervals) and these correspond to an SO (observer #4, Figure 4.9) who observed on six
occasions. The second SO (observer #2), who observed on two occasions agrees very closely with
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
49
the measurements of the PO. The third SO (observer #3), who observed on one occasion did not
agree as closely with the PO, but was consistent within 1 degree (95% confidence interval).
Measurement consistency of the digital level was analysed in the same way as azimuth
measurements, with results plotted on the y-axis of Figure 4.10. The mean differences on all 9 visits
fall within 0.05 degrees (the precision of the level) and 0.15 degrees. It should be considered that
some of the measurement ‘inconsistency’ in both azimuth and elevation could be due to actual
movement of the CRs during the period between initial alignment and measurement. The magnitude
of such movement, and knowledge of whether it occurred, remains unknown. We conclude that the
relative accuracy and repeatability of our field alignment methods is much less than the absolute
accuracy determined in §4.3.3.
4.4 Processing methodology
We used the GAMMA software [Wegmüller and Werner, 1997] to process the received Single Look
Complex (SLC) imagery for each SAR sensor and the integral method of Gray et al. [1990] to extract
the RCS of each CR in each image. The integral method is commonly used to determine the
calibration factor for SAR imagery by measuring the radar response of targets of known RCS. Since all
the received SAR imagery is already externally calibrated, we simply reverse this procedure in order to
determine the RCS of the CR. The procedure used is as follows:
1.
Read the SLC imagery as provided by the SAR data provider and convert to Sigma
Nought. For TerraSAR-X, COSMO-SkyMed and RISAT-1 this involved applying the
annotated product calibration factor and then scaling the image by sin(𝜃) to get Sigma
Nought. For RADARSAT-2 this involved applying the provided Sigma Nought look-up table.
2.
For each SAR sensor, coregister (spatially align) all SLC images to a single master image
(chosen as the earliest) and coregister a DEM in order to get the geocoding look-up table.
3.
Verify the coregistration of each image and determine the range (column) and azimuth
(row) coordinates of each CR in the co-registered images.
4.
Define a square target window dependent on reflector size and the extent of side lobe
ringing in images, and a clutter window and cross width independent of target size
(Figure 4.11; Table 4.5). By computing the clutter level as the mean of all pixel values
falling within a standard-sized window but outside the cross region, a representative view
of the actual reflector RCS and SCR is obtained that removes any bias associated with
choosing the location of clutter windows manually.
5.
Determine the mean signal clutter from the four quadrants of the clutter window after
exclusion of the cross region. Excluding the cross region ensures there is no signal
contribution from the main lobe or side lobe response of the CR.
6.
Calculate the integrated point target energy:
𝐸𝐶𝑅 = 𝐸𝑛 − (
𝑁𝐶𝑅
) ∗ 𝐸𝑐𝑙𝑡
𝑁𝑐𝑙𝑡
where 𝐸𝑛 is the integrated (summed) energy in the target window, 𝐸𝑐𝑙𝑡 is the total
integrated energy in the four clutter quadrants, 𝑁𝑐𝑙𝑡 is the number of samples contained
within the clutter quadrants and 𝑁𝐶𝑅 is the number of samples in the target window.
50
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
7.
Compute the signal to clutter ratio (SCR) between the point target energy corrected for
clutter and the average clutter level per pixel:
𝑆𝐶𝑅 =
𝐸𝐶𝑅
(𝐸𝑐𝑙𝑡 ⁄𝑁𝑐𝑙𝑡 )
8.
Compute the phase error and Line-of-sight height error from the SCR as discussed in
§2.1.3.
9.
Compute the RCS of the point target by multiplying the integrated point target energy by
the area of the resolution cell (§2.1):
𝜎𝑇 = 𝐸𝐶𝑅 ∙ 𝐴
Figure 4.11: Definition of the square target (green) and clutter (red) windows and cross used in point target
analysis of a CR. The cross encompasses the main lobe and side lobe response of the CR in range and azimuth
directions. The remaining area of the full calibration window is defined as the clutter region, separated into four
quadrants. IRF is of the 2.5m CR at site 2 in TerraSAR-X image acquired on 20140120 (see Table 4.5 for window
dimensions).
Table 4.5: Definition of target window, clutter window and cross widths for CR analysis in X- and C-band imagery.
Units are pixel numbers.
X-band
CR size
(m)
C-band
Cross
width
Target window
width
Clutter window
width
Cross
width
Target window
width
Clutter window
width
1.0
7
32
48
5
24
24
1.5
7
48
48
5
24
24
2.0
7
64
48
5
36
24
2.5
7
64
48
5
48
24
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
51
4.5 Results
4.5.1 CR impulse responses in SAR imagery
Figure 4.12: Impulse response functions of CRs in TerraSAR-X SLC image of Gunning test site on 20131229.
Each CR signal is labelled by site number. Field of view in each window is approximately 880 range samples by
940 azimuth samples. The 1.0m CR response at Site 7 is highlighted with a red circle for clarity. TerraSAR-X SLC
data is © DLR.
52
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 4.13: Impulse response functions of CRs in COSMO-SkyMed-1 SLC image of Gunning test site on
20131227. Each CR signal is labelled by site number. Field of view in each window is approximately 830 range
samples by 830 azimuth samples. The 1.0m CR response at Site 7 is highlighted with a red circle for clarity.
COSMO-SkyMed SLC data is © e-geos.
Examples of the impulse response function (IRF) of each CR in SLC images from each SAR sensor
are given in Figure 4.12, Figure 4.13, Figure 4.14 and Figure 4.15. The magnitude and spatial extent
of the side lobes increases with the CR size as expected for all SAR sensors. The pixel resolution of
RADARSAT-2 imagery in Fine mode is 3.4 times coarser than COSMO-SkyMed Himage mode, 2.6
times coarser than TerraSAR-X Stripmap mode and 4.6 times coarser that RISAT-1 FRS-1 mode
(Table 1.3). As a result the spatial extent of the IRF in RADARSAT-2 is correspondingly smaller. The
CRs are generally easy to visually identify in the images, which implies that the SCR for all CR sizes is
large enough. The exception to this is the 1.0m CR installed at site 7 (circled in the imagery), which is
difficult to identify amongst high clutter targets with similar signal magnitude in the vicinity of the CR.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
53
Figure 4.14: Impulse response functions of CRs in RADARSAT-2 SLC image of Gunning test site on 20131214.
Each CR signal is labelled by site number. Field of view in each window is approximately 460 range samples by
460 azimuth samples. The 1.0m CR response at Site 7 is highlighted with a red circle. RADARSAT-2 SLC data is
© MDA.
We oversample 16 times a small image patch around each CR before extracting the azimuth and
range IRF for each CR in each SAR image (Figure 4.16). Examples of these IRF are plotted in
Figure 4.17 for a CR of each size in imagery from each SAR sensor. The width of the main peak in the
IRF indicates the relative spatial resolution of each SAR system. As can be seen, the resolution of the
image modes for RISAT-1, TerraSAR-X and COSMO-SkyMed are very similar. As expected, the
RADARSAT-2 image mode has a coarser resolution. The actual spatial resolution of a SAR system is
defined as the distance between the points registering a 3 dB drop compared to the target peak
intensity (i.e. the 3 dB width) [Zenere, 2012]. Another quality parameter is the peak to side lobe ratio
(PSLR) which is defined as the ratio of the intensities in the main lobe and the highest side lobe. It is
an indicator of how well a SAR system can resolve a weak target in the presence of a strong target
[Zenere, 2012]. From Figure 4.17 we can see that the PSLR is generally -20 dB or better for all CR
sizes in images from each SAR sensor.
54
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 4.15: Impulse response functions of CRs in RISAT-1 SLC image of Gunning test site on 20140203. Each
CR signal is labelled by site number. Field of view in each window is approximately 880 range samples by 970
azimuth samples. The 1.0m CR response at Site 7 is highlighted with a red circle. RISAT-1 SLC data is © ISRO.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
55
Figure 4.16: Example of an oversampled image of a 1.5 m CR impulse response function for site 4 in the
COSMO-SkyMed image acquired on 20140128. The IRF in range is in red and the IRF in azimuth is in blue.
Figure 4.17: Example impulse response functions in range and azimuth directions for each size of CR in
TerraSAR-X (blue; image 20140120), COSMO-SkyMed (green; image 20140128), RADARSAT-2 (red; image
20140131), and RISAT-1 (magenta; image 20140203). The full width of the extracted IRF depends on the size of
CR and SAR sensor frequency (Table 4.5). The extracted image is then oversampled 16 times.
56
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
4.5.2 Clutter
Figure 4.18: Time series of average clutter intensity for each CR site in imagery from each SAR sensor. Since the
clutter intensity is independent of target size, the CR size for each measurement is not indicated here. Also
plotted in the lower bar chart is the daily rainfall record for Gunning (data obtained from Bureau of Meteorology).
In §2.1.1 the expected clutter magnitude at C-band and X-band was summarised. We are able to
verify these expectations using observations from SAR imagery captured at Gunning. In Figure 4.18
the average clutter intensity within the four clutter quadrants at each of the 18 CR sites is plotted as a
time series. In general, clutter levels at Gunning are between -10 dB and -18 dB for both X- and Cband. The clutter level is about the same for X- and C-band which is consistent with the expectation
discussed in §2.1.1. There is a large variation in clutter values between CR sites in the RADARSAT-2
imagery, which may be a result of the coarser pixel resolution.
Also plotted in Figure 4.18 is the rainfall record at Gunning town centre (within 3 km of the nearest CR)
for the duration of the CR deployment. There is a strong correlation between rainfall and trends in
clutter level for all SAR sensors. Significant rainfall occurred in early November 2013, prior to the
installation of the CR at Gunning. Following this time a period (until February 2014) of mainly dry
conditions ensued, interspersed by sporadic rainfall events of 1 day duration of around 10 mm or less.
During this time period ground conditions at Gunning became drier, vegetation dried out and the
volume of biomass reduced (e.g. Figure 4.19). Between 14 to 17 February 2014, ~60 mm of rain fell
over the course of 4 days. Corresponding increases in soil moisture resulted in an increased level of
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
57
clutter in imagery from all 4 SAR sensors. The total increase in clutter following the February rainfall
event was about 2-3 dB for TerraSAR-X with a similar increase inferred for COSMO-SkyMed,
RADARSAT-2 and RISAT-1.
Figure 4.19: Photos of site 15 taken on 27 November 2013 (left) and 20 February 2014 (right) taken from broadly
similar viewing angles. Although significant rainfall occurred shortly before 20 February 2014, note the change in
vegetation height and density between the two photos.
Figure 4.20: Box and Whisker plot showing the statistical variation in differences of average-clutter level
calculated for all 18 CR sites. X-band differences are calculated as COSMO-SkyMed-1 minus TerraSAR-X,
whereas C-band differences are calculated as RADARSAT-2 minus RISAT-1. Pairs of numbers above each box
and whisker are the two acquisition numbers used (refer to Table 4.2). The pairs differenced were acquired up to
4 days apart and are plotted mid-way between the two acquisition dates. The box and whisker symbol represents
the minimum, maximum, median, 25th percentile and 75th percentile of the data sample.
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The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Since all SAR imagery has been calibrated to the backscatter coefficient (sigma nought) it might be
expected that the difference between the signal levels of specific SAR sensors remains constant over
time regardless of changes in soil moisture content. Using a sample population of the 18 CR sites we
compute statistics on the difference of pairs of average clutter values derived from TerraSAR-X and
COSMO-SkyMed images (i.e. X-band SAR sensors), and RADARSAT-2 and RISAT-1 images (i.e. Cband SAR sensors) (Figure 4.20). The SAR images used are selected as the two adjacent
acquisitions from the two SAR sensors with the minimum temporal separation. Although we are using
clutter estimates from the region surrounding the CR sites, this clutter analysis is independent of the
size of CR at each location because the impulse response of each CR is not sampled.
The results show that the difference between signal levels of different SAR sensors does not remain
constant through time. Although the range of differences is consistently about 2 dB through time for
TerraSAR-X and COSMO-SkyMed, the median varies by about 2.5 dB. The range of differences is
much larger (5-8 dB) and varies through time for RADARSAT-2 and RISAT-1. Furthermore the median
varies by about 1 dB. This greater inconsistency at C-band could be attributed to the fact that the
imaging modes used for both sensors vary in spatial resolution. The RADARSAT-2 Fine mode is a
medium resolution mode whereas the RISAT-1 FRS-1 mode is high resolution. As a consequence,
speckle noise may be having more of an effect on the RADARSAT-2 imagery, causing the
inconsistency between the two C-band SAR sensors.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
59
4.5.3 RCS
Figure 4.21: Derived RCS for each CR plotted against measured average clutter in TerraSAR-X images. RCS
estimates are derived from the following acquisition numbers: TerraSAR-X 2, 6, 8, 10, 12, and 15; COSMOSkyMed-1 3, 5, 9, 11 and 16; RADARSAT-2 4, 7 and 13; RISAT-1 14 and 19 (refer to Table 4.2). Theoretical Xband RCS values for each CR size are plotted as dashed lines.
For each CR in each SAR image we estimate the RCS as described in §4.4. The derived RCS is
found to be independent of clutter levels in TerraSAR-X, COSMO-SkyMed, RADARSAT-2 and RISAT1 imagery (Figure 4.21).
We normalise the derived RCS for different CR sizes by determining the difference from theoretical
RCS values (Figure 4.22). The estimated RCS values for many CR in RADARSAT-2 images turn out
to be greater than theory. Since it is not possible for the RCS to be greater than theory this highlights
that the calibration of the RADARSAT-2 imagery is not perfect. This may also be true for the other
SAR sensors, but it is harder to say for sure since the estimated RCS values for these sensors are
generally less than theory.
The RCS characterisation of the 1.0 m and 1.5 m CRs (see §3.2.2) found that at X-band there was an
average 2 dB difference between the different sizes. At C-band the difference was only about 0.4 dB
on average. Differences of these magnitudes do not occur in the RCS values derived from the SAR
imagery.
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The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
There are no obvious differences in estimated RCS that can be attributed to the differences in plate
finish of the 1.5 m CRs. Since variation in RCS exists within CR type-groups that is correlated across
different SAR sensors (particularly TerraSAR-X and COSMO-SkyMed) it appears that site-specific
conditions have a larger effect on RCS.
Generally there is a reduction in RCS difference with size. This is more apparent at X-band; For
TerraSAR-X the difference between 1.0 m and 2.5 m is on the order of 1.6 dB and for COSMOSkyMed is on the order of 0.9 dB. The trend is not as obvious in C-band RCS estimates, which agrees
with the RCS difference observations from the radar signature characterisation of the CR; departures
from inter-plate orthogonality and plate flatness are tolerated less at shorter radar wavelengths
(Figure 3.7).
Figure 4.22: Differences of measured RCS from SAR images to theoretical RCS values (theoretical minus
observed) plotted against CR ID number for easier comparison of different designs (size and plate finish; see
Table 4.4). TerraSAR-X (6 images) and COSMO-SkyMed (5 images) are plotted as box and whisker plots.
RADARSAT-2 (3 images) are plotted as box plots without whiskers due to lack of data samples. RISAT-1 (2
images) are plotted as discrete data samples.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
61
4.5.4 LOS height error
In Figure 4.23 we plot the a-priori LOS height errors derived directly from measurements of the SCR
from SAR imagery at Gunning, as described in §2.1.3. The time series of the LOS height error after
mid-February 2014 is complicated by an increase in average clutter levels as a response to rainfall
(see §4.5.2) and also because deliberate misalignment of some CR was being undertaken (see
§4.5.5). Since SCR is a ratio calculated using the background level of clutter, the time series of LOS
height error correlates well with the time series of clutter (Figure 4.18).
For acquisitions of all SAR sensors prior to mid-February 2014 (i.e. all except RISAT-1; Table 4.2), the
LOS height error remains stable for all SAR sensors. Generally we see that the LOS height error
decreases with CR size, since SCR is proportional to CR size. The LOS height error is also frequency
dependent, with C-band having greater height errors than X-band for the same CR size. At X-band, all
CR larger than 1.0 m meet the stated LOS height error criteria of 0.1 mm. The 1.0 m CRs also meet
this criterion in TerraSAR-X imagery, but not in COSMO-SkyMed. At C-band only the 2.5 m CRs come
close to the threshold of 0.1 mm, exceeding that level of error in RISAT-1 imagery. All CR larger than
1.0 m have a LOS height error less than 0.5 mm.
Using the SCR as a proxy for phase error, and therefore LOS height error, should be treated with
caution. Ketelaar et al. [2004] conducted a validation experiment with five CRs, comparing heights
derived from ERS and ENVISAT InSAR analyses with repeated levelling surveys. From this
experiment they found that the a-priori phase error derived from SCR is under-estimated by 3-4 times
compared to the a-posteriori estimates. Further work to perform an analysis of a-posteriori height
errors should be conducted on the Gunning SAR data, assuming zero differential movement between
CR sites and during the short time period of the CR deployment (as confirmed within the accuracy limit
of the RTK survey pre- and post-deployment; Table 4.1). In the meantime, a-priori LOS height error is
considered as a suitable quantity with which to assess the different prototype CR designs.
4.5.5 Impact of alignment errors
As discussed in §4.3.1, we oriented certain CR with known misalignments from the calculated
boresight for at least one acquisition of TerraSAR-X, COSMO-SkyMed and RADARSAT-2 as per the
values indicated in Table 4.4 for each CR site. To measure the drop in RCS as a result of these
misalignments, pairs of images from each SAR sensor were differenced. To ensure consistent interconstellation signal level, images from the CSK-1 satellite of the COSMO-SKYMED satellite
constellation were used. Four CR, one of each size, were used as ‘control’ and did not have a
misalignment incorporated in to their orientation and so theoretically should exhibit a zero RCS
reduction. In practice the difference is not zero due to temporal changes occurring in the scene
between the two acquisitions. To partially account for this, the standard error of the difference in RCS
measurements for these four control CRs (sites 2, 7, 13 and 16) were used to compute error bars for
the other RCS reduction measurements. The results are shown in Figure 4.24 and Table 4.6.
It is clear from these results that RCS reduction is mainly dependent on alignment error. There may be
a weak correlation with CR size for azimuth misalignments (Figure 4.25), but this is not conclusive
from this dataset. There is no discernible difference in the magnitude of RCS reduction for X-band or
C-band measurements (i.e. the level of RCS reduction is comparable in TerraSAR-X, COSMOSkyMed and RADARSAT-2 images for all CR sites). The other notable observation from this data is
that RCS reduction is more severe for elevation misalignments than equivalent azimuth misalignments
(Figure 4.26). In general, the observations at X- and C-band imply that if azimuth and elevation
alignment accuracies of 10 degrees are adhered to, the resulting RCS will be within 1 dB of the peak
62
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
value. The inferred level of RCS loss from these observations based on the derived accuracy of our
alignment methodology (§4.3.3) is less than 0.2 dB.
Figure 4.23: Time series of LOS height error estimates for each CR site at Gunning in imagery from each SAR
sensor. Each CR size is plotted as a different symbol and colour is used to differentiate between SAR sensors.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
63
Figure 4.24: RCS reduction from Gunning measurements due to misalignment of the CR from the ideal boresight
orientation for each SAR sensor. Misalignments for each CR are given in Table 4.6. Each RCS reduction is
calculated by subtracting the image with misalignment from a prior image without misalignment. The RCS of four
‘control’ CR at sites 2, 7, 13 and 16 (with no CR misalignment) are used to derive the standard error for each SAR
sensor, which are plotted here as 2-sigma error bars. The COSMO-SkyMed measurement for site 16 is discarded
from this analysis due to flooding of the CR at the time of the second acquisition (see §4.5.6).
Figure 4.25: RCS reduction measurements plotted against CR size for the four measurements at 20 degree
elevation offset (labelled “El”) and 20 degrees azimuth offset (labelled “Az”). Error bars are omitted for clarity.
64
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Table 4.6: RCS reduction measurements in dBm2 due to misalignment of the CR from the ideal boresight
orientation for each SAR sensor.
Misalignment
(degrees)
Gunning measurements
DSTO
characterisation
CR Site
CR ID
CR size
(m)
Azimuth
Elevation
TSX
CSK1
RSAT2
C-HH
X-HH
1
12
1.5
10
0
0.44
0.58
0.19
2.78
5.41
2
16
2.5
0
0
-0.21
0.37
-0.04
NC
NC
3
15
2.0
20
0
0.66
1.11
0.96
NC
NC
4
8
1.5
20
0
0.68
0.78
0.88
5.11
7.99
5
4
1.5
0
-20
2.75
3.00
2.98
5.23
8.12
6
11
1.5
0
10
0.63
0.62
0.86
2.60
4.97
7
3
1.0
0
0
-0.06
-0.74
-0.33
1.51
2.32
8
6
1.5
0
20
3.05
3.71
3.13
4.98
9.21
9
7
1.5
0
-10
0.30
0.91
0.48
2.13
6.17
10
10
1.5
10
10
0.87
0.92
0.95
NS
NS
11
1
1.0
20
0
0.33
0.60
0.58
6.94
10.03
12
9
1.5
20
20
4.02
3.85
3.77
NS
NS
13
13
2.0
0
0
-0.16
0.21
-0.13
NC
NC
14
17
2.5
20
0
0.81
0.97
0.64
NC
NC
15
2
1.0
0
20
3.44
2.83
3.41
4.55
9.61
16
5
1.5
0
0
-0.05
13.23
0.21
1.89
4.54
17
14
2.0
0
20
2.78
2.46
3.36
NC
NC
18
18
2.5
0
20
2.67
3.40
3.29
NC
NC
NC – CR not characterised
NS – Elevation and azimuth misalignment combination not sampled during radar characterisation
The RCS reductions measured from SAR imagery at the Gunning CR array can be compared against
RCS reductions measured during the radar signal characterisation exercise of the same CR
prototypes (see §3). The RCS measurement for 1.0 m and 1.5 m CR at rotation angles of ±10 and ±20
degrees is extracted from the azimuth- and elevation-cut profiles and averaged. The RCS values are
then differenced from the theoretical values and given in Table 4.6 and Figure 4.27.
Interestingly, there is an offset between measurements at X- and C-band of ~3 dBm 2 ±0.7 (2σ), which
is not seen in the measurements derived from Gunning SAR imagery. Also there is no appreciable
difference between the RCS reduction for equivalent values of azimuth and elevation misalignment
like there is for the RCS reduction measurements derived from Gunning SAR imagery. The CR is
illuminated by the synthetic aperture radar for as long as it remains within the conical beam as the
satellite platform travels past. During SAR processing the series of Doppler echoes received from the
CR are focussed to a single point in the image. The effect of a CR boresight misalignment azimuth is
therefore reduced because echoes from the actual boresight are still received. Conversely, the radar
used for radar signal characterisation does not have a synthetic aperture and only receives echoes for
particular alignment for a brief time.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
65
Figure 4.26: Contour map of RCS reduction measurements from Gunning SAR imagery as a function of azimuth
and elevation misalignments. A minimum curvature surface is fitted to the 7 RCS reduction measurements for the
1.5 m CR within the positive misalignment quadrant (i.e. the two CR with negative elevation misalignments are
excluded here). The positions in parameter space of the observed data are plotted as red stars. Observed data is
taken as the mean of the TerraSAR-X, COSMO-SkyMed and RADARSAT-2 values given in Table 4.6. Contour
interval is 0.1 dBm2 with every 0.5 dBm2 bold and annotated.
Direct comparisons of the two datasets should be treated with caution. The RCS reductions derived
from the radar signal characterisation exercise can be treated as absolute calibration for each CR.
However, the RCS reductions derived at Gunning may not be optimally calibrated for Gunning, despite
SAR imagery being absolutely calibrated because of the application of annotated calibration factors
given in the product metadata.
4.5.6 Impact of flooding
We found that when deployed at Gunning the CRs were popular roosting spots for birds. The result of
this was a build-up of dirt within the CR. In one case the single drainage hole of the CR at site 16 was
blocked as a result. This subsequently caused the CR to fill with water due to poor drainage following
a heavy rainfall event (Figure 4.28). A dramatic reduction of RCS was measured in SAR imagery due
to the inhibition of the triple bounce reflection in the CR. The drop in RCS measured in two COSMOSkyMed-1 images (acquisition numbers 16 and 24 in Table 4.2) spanning the rainfall event was 13.2
dBm2 (± 0.69 2σ). The impact of this RCS reduction on LOS height error can be seen in Figure 4.23
where a rapid change of one time series from the general trend of the others can be seen in the
COSMO-SkyMed estimates in the month of April 2014.
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The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Figure 4.27: Measurements of RCS reduction from peak RCS made during radar signal characterisation of the 1.0
m and 1.5 m CR. The RCS measured at 10 and 20 degree misalignments in azimuth and elevation are
differenced from the theoretical peak RCS and plotted against its Gunning CR site number for direct comparison
with the RCS reduction measurements from Gunning shown in Figure 4.24. Sites occupied by 2.0 m and 2.5 m
CRs are left blank here. CR sites 7 and 16 are the ‘control’ CR with no misalignment.
Figure 4.28: (left) State of flooding on 2 May 2014 due to a blocked drainage hole in the 1.5 m CR at site 16. The
level of water in the CR at the time of COSMO-SkyMed acquisitions in April 2014 is unknown, though a sudden
RCS drop was detected in the next SAR image after the rainfall. (right) Drainage hole pattern retro-fitted to all
prototype CRs and used in the design of newly manufactured CRs before deployment in Queensland.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
67
5 Conclusions and Recommendations
We have found that a triangular trihedral CR of 1.5 m inner leg dimension (or larger) will give an apriori LOS height error of 0.1 mm in X-band SAR data and 0.5 mm at C-band when deployed in
Australian conditions (Figure 4.23). Furthermore, these LOS height errors are largely maintained even
as clutter levels increase due to wetter weather and moister soils. A sub-millimetric error in InSAR
analyses due to phase instability is desirable when in the presence of other additive error sources
such as atmospheric propagation errors, orbital errors, topographic errors, processing noise,
instrument noise and unwrapping errors. The expected a-priori LOS height error for a 1.5 m CR at Lband with an SCR of 22 dB is around 10 mm (Figure 2.4).
As a result of the CR prototyping exercise described in this GA Record, we chose a 1.5 m triangular
trihedral CR as a suitable ‘compromise’ design for deployment in the regional-scale AGOS geodetic
array. This array of 40 CR co-located with survey marks, was installed by the end of 21 November
2014 in the northern Surat Basin, Queensland [Garthwaite et al., 2015]. This array includes the nine
1.5 m, three 2.0 m and three 2.5 m CR constructed as prototypes for this exercise in addition to 25
new 1.5 m CR built to a slightly improved design based on lessons learned from this prototyping
exercise.
The 1.5 m sized CR has a number of relative advantages over larger CR, including that it is cheaper to
manufacture, is easier to handle, is more discreet when deployed, is less prone to RCS reductions
due to manufacturing imperfections (see §2.4) and is less likely to saturate the SAR sensor. The 1.5 m
sized CR also has the relative advantage over smaller CRs that it is brighter in SAR imagery at any
frequency and as a result will have a more stable phase response for accurate deformation
monitoring.
The theoretical peak RCS of a triangular trihedral 1.5 m CR is 43.4 dBm 2 at X-band, 38.4 dBm2 at Cband and 25.8 dBm2 at L-band. The RCS profile of the three 1.0 m and nine 1.5 m prototype CRs were
characterised at X- and C-band. The peak RCS at C-band of the 1.5 m CRs is 2.0 ± 0.3 dBm 2 less
than theory whereas the 1.0 m CRs are around 1.6 +0.6/-0.3 dBm2 less than theory. At X-band the RCS
of individual CR is more variable, ranging between 5.0 +1.5/-1.0 dBm2 less than theory for 1.5 m CRs and
3.2 ± 1.0 dBm2 less than theory for 1.0 m CRs. These departures from theoretical RCS should not
have any negative impact on the application of the CRs. For use as SAR calibration targets it is only
important that the actual RCS is accurately known. For use as a deformation target it is only important
that the phase response remains stable.
In the prototyping exercise we found no appreciable difference in the measured response between the
1.5 m CR manufactured with a plain metal finish, a powder-coated finish, and mesh perforation. The
latter would be preferable for performance longevity in the field, but at over twice the cost it is hard to
justify choosing this design. The effect on performance of different mesh perforation patterns also
remains largely unknown. We therefore chose to use a powder-coated finish on all further CRs
manufactured.
By comparing our CR azimuth alignment methodology (that uses a sighting compass) with azimuth
measurements made using an RTK instrument we find that our technique yields an accuracy of ±2.6
degrees, based on the measurements of 5 different observers. From observations made in SAR data
at Gunning, this level of accuracy results in an RCS loss due to alignment error of less than 0.2 dB at
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The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
both X- and C-bands. Azimuth measurement repeatability is highly dependent on the individual
observer, and this should be taken in to consideration during campaigns involving frequent CR realignments. The measurement repeatability of the calibrated digital level was found to fall within 0.05
degrees (the precision of the level) and 0.15 degrees.
Some general recommendations based on the prototyping exercise are as follows:
1.
Powder coating is advisable to inhibit the build-up of heat within the CR panels. Powder
coated panels installed at Gunning were noticeably cooler to touch compared to plain
Aluminium sheet panels. This in turn will help reduce the effects of expansion and
contraction due to extreme temperature changes that could be experienced by un-coated
panels. A light grey colour is preferable to white so that the CR blend in to the scenery but
still reflect solar energy. The white powder-coating used in the prototypes exhibited high
glare in sunny conditions compared to those without a powder coated finish.
2.
A larger number of drainage holes are required to reduce the risk of flooding and
catastrophic degradation in the CR radar response. All CR prototypes were subsequently
retro-fitted with additional drainage holes on all three plates to reduce (but not eliminate)
this risk (Figure 4.28). As a result this may introduce a marginal reduction in peak RCS.
3.
Stringent quality control of materials used in manufacture of CR plates is recommended to
avoid distortions to the plates that compromise the flatness and inter-plate orthogonality.
4.
CR plates should be constructed from thick Aluminium sheeting that can better maintain its
flatness. We recommend 6 mm-thick sheeting or greater.
5.
Mesh-perforating of the CR panels has the advantage of improving drainage and ‘selfcleaning’. This is particularly important for long-term CR deployment. However, physical
punching of aluminium sheet to create the mesh has been seen to introduce significant
stresses to the sheet that cause departures from flatness that are not visible in nonpunched sheet.
6.
A central support angle parallel to the hypotenuse angle was added to the two vertical CR
panels in the revised design drawings in an attempt to reduce distortions. These changes
resulted in a better product being installed in the AGOS geodetic array in the Surat Basin.
7.
If larger CR sizes (2.0 m inner leg dimension and above) are to be considered in the future,
more effort would need to be made in eliminating distortion that occurs to panels due to
their own weight. For example, frame material that will hold its longitudinal integrity better
along longer spans such as 6–8 mm extruded aluminium “C” section should be considered
as a replacement to the current 6 mm x 50 mm extruded aluminium angle. Selecting a
sheet thickness less than 6 mm in an attempt to reduce the amount of weight being
influenced by gravity should be considered. Since larger CR are typically used with larger
wavelength SARs (e.g. L-band and lower frequencies), more aggressive mesh perforation
could also be used to reduce the weight of each panel. Furthermore, modifications will
need to be considered on the current ground mount designs for these larger panel sizes.
Thicker material, additional bracing and a larger turntable assembly should be considered
in order to decrease the susceptibility of the longer lengths of the ground mount
components to flexure under the weight of the panels.
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
69
Acknowledgements
Without the input of the following people and organisations this project would not have been a
success.
Leigh Powis and his team at DSTO were very accommodating in taking on the work of characterising
the RCS of the prototype CRs amongst an already busy work schedule.
Gunning Grazing Company are thanked for allowing us to use their property to deploy the corner
reflectors for 5 months between November 2013 and May 2014.
J&H Williams of Port Adelaide manufactured the 18 prototype corner reflectors and the reflector
mounting stand used during the DSTO RCS measurements. In particular, Michael Riese is thanked for
the great enthusiasm he showed towards our project.
Mark Sharah (GA) manufactured the 1.0 m and 1.5 m prototype reflector ground stands with great
skill.
Bart Thomas, Ryan Ruddick and Steven Curnow (all GA) did the surveying at Gunning.
Mark Williams of Horizon Geoscience Consulting is acknowledged for his work early in this project that
culminated in two technical reports on corner reflector design and orientation strategy. These reports
helped us to understand the important issues that needed addressing during our prototyping exercise.
Useful technical interactions and advice on corner reflector design options from Manfred Zink (DLR),
Marco Schwerdt (DLR), Wade Albright (Alaska Satellite Facility), and Scott Hensley (NASA-JPL) is
gratefully acknowledged.
TerraSAR-X data of the Gunning test site were acquired through DLR science project LAN1499.
RADARSAT-2 images and five COSMO-SkyMed images were funded through the AuScope AGOS
project. Four more COSMO-SkyMed images were provided by e-GEOS for product evaluation. RISAT1 data was provided by the Indian Space Research Organisation.
John Trinder (UNSW) is gratefully acknowledged for taking the time to thoroughly review the draft
manuscript.
This GA Record is published with the permission of the CEO, Geoscience Australia.
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The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
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The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
73
Table 5.1: Summary of all peak RCS measurements in dBm2 of the twelve prototype CR at X-band. The RCS at
zero degrees rotation azimuth is assumed to be the peak RCS, and is presented here for each CR.
Azimuth cut
CR ID
Elevation cut
HH-pol
VV-pol
HH-pol
VV-pol
Mean
HH
Mean
VV
Diff.
HH/VV
Mean
Std.
Error
1
Measured
29.85
32.74
33.68
32.60
31.77
32.67
0.90
32.22
0.82
2
Measured
34.26
34.08
29.82
34.34
32.04
34.21
2.17
33.12
1.10
3
Measured
34.34
34.17
34.04
34.33
34.19
34.25
0.06
34.22
0.07
1
Diff. w/ Theory
6.54
3.66
2.71
3.80
4.63
3.73
0.90
4.18
0.82
2
Diff. w/ Theory
2.14
2.32
6.57
2.06
4.35
2.19
2.17
3.27
1.10
3
Diff. w/ Theory
2.05
2.22
2.35
2.06
2.20
2.14
0.06
2.17
0.07
All 1.0m
Mean diff.
3.58
2.73
3.88
2.64
3.73
2.68
1.04
All 1.0m
Std. Error
1.48
0.46
1.35
0.58
0.77
0.52
0.61
4
Measured
38.53
38.54
38.48
38.59
38.51
38.56
0.06
38.54
0.02
5
Measured
39.25
38.90
38.59
39.15
38.92
39.03
0.10
38.97
0.15
6
Measured
36.54
37.06
37.55
36.94
37.05
37.00
0.04
37.02
0.21
4
Diff. w/ Theory
4.91
4.90
4.95
4.85
4.93
4.87
0.06
4.90
0.02
5
Diff. w/ Theory
4.18
4.54
4.84
4.29
4.51
4.41
0.10
4.46
0.15
6
Diff. w/ Theory
6.90
6.37
5.89
6.50
6.39
6.44
0.04
6.41
0.21
All SM
Mean diff.
5.33
5.27
5.23
5.21
5.28
5.24
0.07
All SM
Std. Error
0.81
0.56
0.33
0.66
0.57
0.61
0.02
7
Measured
37.39
37.85
38.07
37.95
37.73
37.90
0.17
37.81
0.15
8
Measured
38.64
38.83
38.72
38.54
38.68
38.69
0.01
38.68
0.06
9
Measured
38.03
37.85
37.67
37.96
37.85
37.90
0.05
37.88
0.08
7
Diff. w/ Theory
6.05
5.59
5.37
5.49
5.71
5.54
0.17
5.62
0.15
8
Diff. w/ Theory
4.79
4.60
4.72
4.90
4.76
4.75
0.01
4.75
0.06
9
Diff. w/ Theory
5.41
5.59
5.77
5.48
5.59
5.53
0.05
5.56
0.08
All SP
Mean diff.
5.42
5.26
5.29
5.29
5.35
5.27
0.08
All SP
Std. Error
0.36
0.33
0.31
0.19
0.30
0.26
0.05
10
Measured
38.86
39.23
39.32
39.15
39.09
39.19
0.10
39.14
0.10
11
Measured
39.55
39.55
39.28
39.50
39.41
39.52
0.11
39.47
0.06
12
Measured
38.89
38.89
38.84
38.87
38.86
38.88
0.01
38.87
0.01
10
Diff. w/ Theory
4.57
4.20
4.12
4.29
4.34
4.25
0.10
4.30
0.10
74
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Azimuth cut
CR ID
Elevation cut
HH-pol
VV-pol
HH-pol
VV-pol
Mean
HH
Mean
VV
Diff.
HH/VV
Mean
Std.
Error
11
Diff. w/ Theory
3.89
3.89
4.15
3.94
4.02
3.91
0.11
3.97
0.06
12
Diff. w/ Theory
4.55
4.55
4.59
4.57
4.57
4.56
0.01
4.57
0.01
All MM
Mean diff.
4.34
4.22
4.29
4.26
4.31
4.24
0.07
All MM
Std. Error
0.22
0.19
0.15
0.18
0.16
0.19
0.03
All 1.5m
Mean diff.
5.03
4.92
4.93
4.92
4.98
4.92
0.07
All 1.5m
Std. Error
0.32
0.26
0.21
0.26
0.25
0.26
0.02
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
75
Table 5.2: Summary of all peak RCS measurements in dBm2 of the twelve prototype CR at C-band. The RCS at
zero degrees rotation azimuth is assumed to be the peak RCS, and is presented here for each CR.
Azimuth cut
CR ID
Elevation cut
HH-pol
VV pol
HH-pol
VV-pol
Mean
HH
Mean
VV
Diff.
HH/VV
Mean
Std.
Error
1
Measured
27.73
30.32
30.27
28.07
29.00
29.19
0.19
29.10
0.70
2
Measured
30.13
30.01
29.82
29.89
29.98
29.95
0.03
29.96
0.07
3
Measured
29.73
30.32
30.05
30.00
29.89
30.16
0.27
30.03
0.12
1
Diff. w/
Theory
3.61
1.01
1.06
3.27
2.34
2.14
0.19
2.24
0.70
2
Diff. w/
Theory
1.20
1.33
1.51
1.44
1.36
1.39
0.03
1.37
0.07
3
Diff. w/
Theory
1.60
1.02
1.28
1.33
1.44
1.18
0.27
1.31
0.12
All
1.0m
Mean diff.
2.14
1.12
1.29
2.02
1.71
1.57
0.16
All
1.0m
Std. Error
0.74
0.10
0.13
0.63
0.31
0.29
0.07
4
Measured
35.16
36.46
36.31
36.22
35.74
36.34
0.61
36.04
0.30
5
Measured
36.45
36.42
36.57
36.69
36.51
36.56
0.05
36.53
0.06
6
Measured
35.43
36.90
36.66
35.98
36.04
36.44
0.40
36.24
0.33
4
Diff. w/
Theory
3.22
1.92
2.07
2.16
2.64
2.04
0.61
2.34
0.30
5
Diff. w/
Theory
1.93
1.96
1.81
1.68
1.87
1.82
0.05
1.85
0.06
6
Diff. w/
Theory
2.95
1.48
1.72
2.40
2.34
1.94
0.40
2.14
0.33
All SM
Mean diff.
2.70
1.79
1.87
2.08
2.28
1.93
0.35
All SM
Std. Error
0.39
0.15
0.10
0.21
0.22
0.06
0.16
7
Measured
35.29
36.98
36.86
35.82
36.08
36.40
0.32
36.24
0.41
8
Measured
36.35
36.95
36.74
36.61
36.55
36.78
0.23
36.66
0.12
9
Measured
36.14
36.71
36.33
36.20
36.23
36.46
0.22
36.34
0.13
7
Diff. w/
Theory
3.09
1.40
1.52
2.56
2.30
1.98
0.32
2.14
0.41
8
Diff. w/
Theory
2.02
1.43
1.64
1.77
1.83
1.60
0.23
1.72
0.12
9
Diff. w/
Theory
2.24
1.67
2.05
2.18
2.15
1.92
0.22
2.03
0.13
All SP
Mean diff.
2.45
1.50
1.74
2.17
2.09
1.83
0.26
All SP
Std. Error
0.32
0.09
0.16
0.23
0.14
0.12
0.03
10
Measured
36.29
36.59
36.61
36.49
36.45
36.54
0.09
36.50
0.07
76
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
Azimuth cut
CR ID
Elevation cut
HH-pol
VV pol
HH-pol
VV-pol
Mean
HH
Mean
VV
Diff.
HH/VV
Mean
Std.
Error
11
Measured
36.51
36.56
36.52
36.64
36.51
36.60
0.09
36.56
0.03
12
Measured
36.22
36.46
36.52
36.45
36.37
36.45
0.08
36.41
0.07
10
Diff. w/
Theory
2.09
1.79
1.77
1.89
1.93
1.84
0.09
1.88
0.07
11
Diff. w/
Theory
1.87
1.81
1.86
1.74
1.86
1.78
0.09
1.82
0.03
12
Diff. w/
Theory
2.16
1.92
1.86
1.93
2.01
1.93
0.08
1.97
0.07
All MM
Mean diff.
2.04
1.84
1.83
1.85
1.93
1.85
0.09
All MM
Std. Error
0.09
0.04
0.03
0.06
0.04
0.04
0.00
All
1.5m
Mean diff.
2.40
1.71
1.81
2.03
2.10
1.87
0.23
All
1.5m
Std. Error
0.18
0.07
0.06
0.10
0.09
0.04
0.06
The Design of Radar Corner Reflectors for the Australian Geophysical Observing System
77