AIRSAR_Chapt4

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4. REMOTE SENSING AND RADAR
4.1 Radar background
Synthetic Aperture Radar (SAR) sensors operate in the microwave region of the electromagnetic
spectrum and use an antenna to alternately transmit short pulses of energy and receive return signals
from each pulse as a result of the backscatter from the ground surface. The radar energy reaching
the ground surface can be scattered, reflected or absorbed depending on the characteristics of the
surface or object being imaged.
The sensor parameters determining the backscatter coefficient (called sigma0 or σ°) are incidence
angle, wavelength and polarisation. The incidence angle describes the angle between the radar
illumination and the ground surface and is calculated based on near range (closest to nadir) and far
range (furthest from nadir) assuming a flat topography. The incidence angle across a radar image
will vary depending on the height of the aircraft or satellite sensor.
Wavelength describes the distance between crests of a sinusoidal wave. Radar wavelengths
traditionally vary from centimetres to metres with shorter centimetre waves being more common in
imaging earth surface features (Table 4.1).
Table 4.1: Microwave band designations most commonly used in satellite and airborne SAR systems.
Band
Wavelength
Range
Frequency Range
Common
Frequencies
Sensor Platforms
X
C
2.40-3.75 cm
3.75-7.5 cm
5.75 –10-9 GHz
4.2 –5.75GHZ
9.6GHz
5.3GHz
L
15-30 cm
390MHz-1.55GHz
1.25GHz
P
77-136 cm
255-390MHz
225Mhz
SIR-C
RADARSAT
ERS-1,ERS-2,
ENVISAT,AIRSAR
SIR-C, JERS-1,
PALSAR,AIRSAR
AIRSAR
Polarization refers to the direction of the electric field of the wave being transmitted or received by
a radar antenna. Waves can be propagated and returned either vertically or horizontally.
Interaction with features on the ground may cause the propagated wave to be depolarised, that is
return some portion of the wave in a different polarisation to what was transmitted. Polarimetric
radar systems record both the amplitude and the phase angle of the co-polarized (HH or VV) and
cross-polarized (VH or HV) returns coming from the surface. This allows the synthesization of any
transmit and receive polarisation configurations and the capability of creating polarisation
signatures for any particular surface or object. Non-polarimetric radars transmit and record only
single polarisation returns.
Open water bodies (without turbulence or waves) act as specular reflectors with little or no return
directed towards the antenna and appear black on radar imagery. Wind turbulence results in some
backscatter at the wavelength befitting the roughness of the surface. Trees and vegetation usually
act as diffuse and volume scatterers relative to the wavelength being used and appear medium to
bright on radar imagery. Shorter wavelengths (X and C band) are diffusively scattered by leaves,
twigs and small branches in the canopy. Longer wavelengths (L and P band) penetrate the canopy
and interact with the larger branches and tree trunks.
The presence of a water beneath a tree or forest canopy and surrounding the trunks and understorey
can cause “double bounce” reflections where instead of the signal undergoing attenuation and loss
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at the ground interface, reflections between the various trunk and branch components and the
underlying water surface cause strong multi-path returns in the direction of the antenna (Figure 4.1).
These enhanced returns permit the detection of water beneath the canopy and provide the basis for
mapping flood extent (Hess et al., 1990, 1994; Pope, 1996; Kasischke et al., 1997; and Milne et al
2001).
Due to the three-dimensional nature of most earth surface features as well as differences in the size,
orientation and distribution of the individual components making up these features, no single band
radar system is capable of capturing the information necessary to adequately characterise that
feature and unambiguously separate it from adjacent or like features. This is a limitation with
single frequency-polarization satellite radars. However, the increased dimensionality that exists in
polarimetric and multi-band radar data contains the information required to identify and separate
objects and surface features from one another; facilitate classification and permits the estimation of
biophysical parameters such as biomass and soil moisture.
4.2 Operational satellite systems
Satellite SAR’s including ERS-1; ERS-2; RADARSAT and JERS-1 have, until recently, been
limited to single wavelength, single polarisation systems at either C-band or L-band. Airborne
SARs in addition to C and L band capabilities also operate at X- band and P- band, and most current
systems are fully polarimetric. A major limitation with airborne SAR, however, is that multi-date
imagery is not easily obtainable due to the logistics and costs involved in acquiring repetitive
coverage over the same area. For this reason airborne systems apart from AIRSAR are not
discussed in this report. For comparison between past, present and future radar systems see Table
4. 2
Table 4.2: Sensor specifications: past and current spaceborne SAR sensors (Source: adapted from Baltzer,
2001).
Instrument
Launch
date
Freq
(GHz)
Band
Polariz.
Resol.
(m)
Look
angle
(deg.)
20
Incid.
angle (deg.)
30
Swath
width
(km)
100
23-35
Repeat
cycle
(days)
3, 35, 168
ERS-1
1991-99
5.25
C
VV
JERS-1
1992-98
1.28
L
HH
18
75
35
38
44
SIR-C
1994
ERS-2
1995
1.25,
5.3
5.25
L, C
Quad
10-40
15-90
15-55
17-60
1
C
VV
30
100
20
23-35
1, 35
RADARSAT
1995
5.3
C
HH
45-500
20-59
17-50
2-3
2002
5.25
C
HH, VV
10100
30
ENVISAT
50-400
14-45
20-45
35
SRTM
2000
5.3,9.25
C, X
VV
30
48
52
57
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*ALOS
2004
1.28
L
Expt.
Quad
mode
10100
70-350
35
46
*RADARSAT - 2
2004
5.3
Expt.
Quad
mode
10100
50500
8-60
(standard)
18-43
(scansar)
10-60
C
24
(Note: Radarsat repeat cycle is 24 days, but in wide swath mode is 1 day (Arctic) and 2-3 days (Equator).
For comparison between RADARSAT-2, ENVISAT and ALOS, see Table 4.3
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Several satellite SAR’s due for launch in the next two years will have an improved but limited
polarimetric capability in that they will be able to record both co- and cross-polarised returns but
not the full phase of the return signal. This in addition to their multi-temporal capability will
provide an enhanced capacity for better feature extraction and analysis of the changing conditions
of surface phenomena. They include ENVISAT, RADARSAT and ALOS-PALSAR, details for
which are summarized in Table 4.3.
Table 4.3: Comparison between RADARSAT-2, ENVISAT and ALOS Radar Sensors
Instrument
Mode
Spatial
coverage
Spatial
resolution
Incidence
Angle range
Polarization
RADARSAT – 2
C-Band
1 mode
50-500 km
10-100 m
10-60
Multi-look fine
50
10
30-50
Selective Single Pol:
HH or VV or HV or VH
Selective Dual Pol.:
HH+HV or VV+VH
HH or VV or HV or VH
Ultra - fine
20
3
30-40
HH or VV or HV or VH
Standard Quad
Pol.
25
25
20-41
HH+VV+HV+VH
(fully polarimetric)
Fine Quad Pol
25
10
30-41
HH+VV+HV+VH
(fully polarimetric)
Image
56-100
30
15-45
HH or VV
ALTPOL
56-100
30
15-45
HH/VV or HH/HV or
VV/VH
Wide
400
150
15-45
HH or VV
Global
400
1000
15-45
HH or VV
Fine
70
10
10-51
HH or VV or HH+HV or
VV+VH
SCANSAR
250-350
100
10-51
HH or VV
Fine Quad Pol
25
10
30-41
HH+VV+HV+VH
(fully polarimetric)
ENVISAT
C-band
ALOS L-Band
ENVISAT launched by the European Space Agency in 2002 carries an Advanced SAR (ASAR) Cband radiometer that provides continuity with the ERS-1 and ERS-2 instruments. ENVISAT
acquires high resolution radar imagery with a repeat cycle of 35 days. Data acquired in the Image
Mode (IM) will provide HH or VV polarisation images from any of the seven selectable swathwidths that range of 56-100 km at a spatial resolution of 30 m. ENVISAT can also operate in an
Alternating Polarisation Mode (APM) producing two co-registered polarization pairs, either
HH/VV, HH/HV or VV/VH at any of the selectable swath widths.
Also operating in C-band, RADARSAT-2, due for launch late 2005, will retain the same beam
modes and ensure continuity of data with RADARSAT-1. RADARSAT-2 will also offer multipolarimetric option similar to ENVISAT. These include: (1) selective polarization (HH and VV)
providing one co-polarization channel (HH or VV) and the corresponding cross-polarization
channel (HV or VH); (2) high resolution (3 m) single polarization channel (HH or VV); and (3) an
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experimental fully polarimetric mode (quad polarization) providing both amplitude and phase
information. RADARSAT-2 is the first commercial satellite to offer this mode.
The ALOS PALSAR sensor to be launched by the Japanese Space Agency in 2004 will be the only
L-band radiometer operating from space and will have the capacity to operate with a large number
of polarizations at off-nadir angles and at varying resolutions. The three main modes are: HH
polarization (34.3 degree off-nadir, 10 m resolution); HH+HV polarization ( 34.3 degree off-nadir,
20 m resolution); and HH polarization (ScanSAR,100 m resolution).
All three systems have the capabilities needed for meeting the data acquisition requirements
necessary for setting up a long term mapping and monitoring program of the Mekong Basin. These
capabilities include:





spatial consistency over sub-continental scales (continuous acquisitions without gaps);
temporal consistency (regional acquisitions within short time windows);
multiple spatial resolutions (20 m, 100 m, 250 m);
adequate repetition frequency (monthly - annual); and
long-term continuation (operational life of the satellite).
4.3 AIRSAR - AIRborne Synthetic Aperture Radar
The airborne synthetic aperture radar, AIRSAR, sponsored by NASA and developed at the Jet
Propulsion Laboratory (JPL), is a three-frequency, fully polarimetric radar system that operates in
two principal modes; polarimetric SAR (POLSAR) provides high quality polarimetric data in three
frequencies; and across-track interferometric SAR (TOPSAR) which allows precision digital
elevation models (DEM) of the earth’s surface to be generated.
When operating in POLSAR mode, AIRSAR simultaneously records data in three frequencies,
including P band (441 MHz and 68 cm wavelength), L band (1.25 GHz and 24 cm wavelength) and
C band (5.35 GHz and 5.6 cm wavelength) measuring the complete scattering matrix for each 3.3 m
by 9.3 m ground cell, thereby allowing the calculation of all polarizations for each band. In
TOPSAR mode, the phase component of the C-band signal is processed to generate an elevation
model that can, in turn, be used to ortho-rectify the L and P-band polarimetry data. The combined
instrument produces simultaneous L-and P-band fully polarimetric data plus a C-band VV
polarization backscatter image in addition to the digital elevation model.
AIRSAR data provides information about the terrain and its processes that is otherwise
unobtainable from optical remotely sensed and other geophysical datasets. Whereas optical
datasets, such as Landsat Thematic Mapper (Landsat TM), provide information about the
composition of earth surface materials and the biochemistry of vegetation, AIRSAR signals are
sensitive to their spatial characteristics (texture) and dielectric properties (water content).
The advantages of AIRSAR fully polarimetric data reside in the following:




L and P-band wavelengths are most sensitive to soil moisture, green biomass and salinity;
P-band and, to a lesser extent, L band signals are able to penetrate foliage and dry sands and
map terrain features below canopy and sub-surface geometries respectively;
P- and to a lesser extent L-band, signals are highly sensitive to saturated soils and flooding of
terrain below canopy cover;
multiple wavelengths are sensitive to the micromorphology of surface materials at different
levels of roughness providing a new insight into weathering and erosional processes and
geological framework of the terrain;
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


algorithms based on the backscatter in three frequencies have been developed to optimize the
classification of vegetation communities and estimate forest biomass;
illumination geometry can be selected to highlight structure and other characteristic properties
of the terrain; and,
elevation data for each 5x5 m pixel are collected using single-pass radar interferometry, thereby
avoiding variable atmospheric attenuation of the phase information often seen in multiple-pass
interferometry that commonly leads to significant errors in the terrain height values.
4.4 Current state of optical remotely sensed data
Optically based, remote sensing systems capture reflected or emitted radiation from the earth’s
surface and are therefore dependent on the interaction of the sun’s energy with the land-ocean
interface. Not having their own source of energy, optical sensors are constrained by cloud cover,
time of day and illumination conditions, and the two directional impact of the atmosphere on the
signal received at the sensor. Despite the long and successful record of optical systems such as
Landsat, SPOT, AVHRR and more recently, ASTER and MODIS, the high frequency of cloud
cover in tropical regions such as south-east Asia largely preclude the possibility of regular data
acquisitions. ASTER and MODIS data acquired on an opportunistic basis has been used to support
this study.
4.4.1 ASTER
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) collects data in
14 spectral bands extending from the visible to the thermal infrared wavelengths. The spatial
resolution varies with wavelength being 15 m in the visible and near-infrared (VNIR); 30 m in the
short wave infrared (SWIR), and 90 m in the thermal infrared (TIR). Each ASTER scene covers an
area 60x60 km. In addition to normal nadir viewing, the instrument can change viewing angles in
order to generate stereoscopic datasets and DEM’s. As ASTER only operates for 8 minutes in each
satellite overpass, continuous coverage is not possible. Requests for acquisitions have to be
scheduled and prioritised. A sample of an ASTER scene for the TSGL is shown in Figure 5.3.
4.4.2 MODIS
The Moderate Resolution Imaging Spectrometer (MODIS) images the earth’s surface in 36 spectral
wavelengths ranging from 0.4 –14.35 micrometers. With a swath width of 2,330 km the instrument
can image the entire surface of the globe every 1-2 days collecting both daylight reflected radiation
and day/night thermal emissions. Imagery is generated at three spectral resolutions - 250, 500 and
1,000 m. Currently a MODIS instrument is operational on both the Terra and Aqua satellites
launched by the USA. Sample MODIS scenes for the TSGL and Mekong region are shown in
Figures 7.11 and 9.10.
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Figure 4.1: Main backscattering mechanisms and types of backscatter prevalent in wetland environments.
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