SECURED LINK TT & C ANTENNAS
C.Wyllie(1), D.Gould(1), G.Richards(1), R.Guy(1),
J.Fernandez(2), P.Closas(2), P.Rinous(3), F.Coromina(3)
(1)
BAE SYSTEMS Advanced Technology Centre, Chelmsford, UK
(2)
Universitat Politècnica de Catalunya, Barcelona, Spain
(3)
ESTEC, Noordwijk, The Netherlands
This paper describes a study into the use of
adaptive antenna technology to provide satellite
TT&C uplinks with an anti-jamming capability
A LEO orbit with a TT&C system was selected for
this study since this is more challenging due to the
higher angular velocity of the satellite.
An
operating frequency of 2 GHz was chosen which is
currently used by Earth Observation satellites.
INTRODUCTION
ANTENNA ARCHITECTURE
Tracking, Telemetry and Command (TT&C)
systems are a vital component for almost all space
missions. They enable the spacecraft systems to be
monitored and controlled from the ground.
Generally antennas for TT&C require wide angular
coverage and are therefore potentially susceptible
to interference from a large geographical area from
either intentional or accidental sources. A study
has been carried out into the use of adaptive
antenna technology to provide satellite TT&C
uplinks with an anti-jamming capability. An Earth
Observation mission has been chosen as the
baseline for the study. The threat considered is a
single continuous jammer of similar power level to
that of the user.
The TT&C antenna is required to provide two
coverages as follows:
Abstract
MISSION SELECTION
A study of threat scenarios was carried out.
Military systems including GPS were not
considered here since these were assumed to have
an existing anti-jam capability.
The most
vulnerable missions were identified as follows:
•
LEO/MEO earth
observation/remote
sensing (e.g. ENVISAT).
•
LEO/MEO navigation (e.g. Galileo)
•
GEO
fixed
Inmarsat)
communications
a)
Coverage of ±60° for operation in LEO orbit in
the presence of a jammer.
b) Coverage of ±95° for Launch and Early Orbit
Phase (LEOP) and emergency operation. This
is assumed to be in the absence of a jammer.
A study of alternative antenna architectures was
carried out. These included star-shaped arrays, ring
arrays and arrays of randomly-distributed elements.
The six element star-shaped array geometry
depicted in Figure 1 below was selected. This
represents a compromise between array size and
user directivity across the field of view. Figure 2
below shows how the directivity of the signal
received by the user varies when a single jammer
scans across the coverage and a null in the radiation
pattern is synthesised in the direction of the
jammer.
(e.g.
Galileo’s TT&C system already has an anti-jam
capability using a spread-spectrum, while GEO
fixed communications satellites are equipped with
high gain array-fed reflector or array antennas
which could in future be used to provide TT&C
spot beam with nulls synthesised in the direction of
jammer(s)/interferer(s). It was therefore concluded
that earth observation/remote sensing missions are
currently at most risk to threats from jamming
sources both from unintentional jamming and from
hostile sources. These operate in LEO/MEO orbits.
Element spacing=1.7λ
Figure 1
6 Element Star Array
TT&C User Directivity with 1 Jammer
2
8
1.5
6
1
4
0.5
2
0
0
-0.5
-2
-1
-4
-1.5
-6
-2
-2
-1.5
-1
-0.5
0
0.5
input file: V:\ttc\compat\1\udir_1.txt
1
1.5
2
copolar
crosspolar
-8
04-Jul-2007
LEO FOV
Outer ring of deep grating lobes
Figure 2. User Directivity with Nulled Jammer
scanned across Field of View
Note that the plot is in double “uv” space with a
range of ±2 rather than ±1 since the user may be on
one side of the coverage and the jammer on the
other side. This analysis assumed a simplified
array antenna model with isotropic radiating
elements and no mutual coupling. The user
directivity within the outer LEO Field of View
(FOV) circle is generally greater than 6dBi except
at the centre of the plot where the jammer and user
are coincident. The hot spots within the LEO circle
represent the locations of “minor” grating lobes in
the antenna pattern. If a jammer is nulled at these
locations the user directivity is decreased to 0 dBi .
The element spacing was set to 1.7λ so that the ring
of deep grating lobes lie just outside the LEO orbit
Field of View.
Two alternative array radiating elements have been
considered: a quadrifilar helix antenna or a
microstrip patch antenna. Most existing singleelement TT&C antennas are quadrifilar helices
since this design has the advantage of providing the
±95° beamwidth required for LEOP and these are
circularly-polarised with a minimum gain of 3dBic. A 2 turn quadrifilar helix was designed: the
geometry and radiation patterns of alternate tapered
and untapered geometries are depicted in Figure 3
below:
Figure 3. Radiation Patterns of Two Turn
Quadrifilar Helix Antenna Designs
Tapering the helix has resulted in a reduction in
crosspolar radiation in the rear hemisphere at the
cost of an increased variation in copolar gain in the
forward hemisphere.
A disadvantage of a
quadrifilar helix antenna is that this antenna
interacts strongly with conducting surfaces on the
spacecraft resulting in multipath nulls in the
element radiation patterns, principally due to the
following:
a) Scattering off the edge of conducting
surfaces
b) The high crosspolar in the rear hemisphere
reflects off conducting surfaces as copolar.
Figure 3 indicates that tapering the helix reduces
b).
Microstrip patch antennas are an alternative class
of radiating element which are less sensitive to
spacecraft multipath, but the beamwidth is
restricted to ±60° which would only permit in-orbit
operation. 2 alternate geometries are currently
being considered:
1) 6 element array. All elements are helices.
Centre element raised to extend
beamwidth of centre element for LEOPS.
2) 6 element array. Centre element is a helix,
edge elements are patches.
The multipath may be reduced synthesising beams
with nulls in the direction of the multipath as well
as the jammer. This is discussed later below.
ANTENNA CONTROL ALGORITHM
In principle the array main beam could be scanned
across the field of view until the optimum signal to
noise ratio is achieved. This approach can be
employed for communication satellites in
geostationary orbit where the satellite, user and
jammer positions are only slowly changing.
However, this approach does not work in the case
of LEO/MEO orbits where the satellite position is
rapidly changing. A study has been carried out into
alternative adaptive control algorithms. These fall
into 2 classes:
a)
Minimum Variance Beamforming with Sample
Matrix Inversion (SMI). This technique is
widely used in military systems where the
jammer location can be determined by range
gating and the jammer signal is higher than the
user signal. This uses a direct matrix inversion
algorithm. This method has the disadvantage
of requiring an accurate knowledge of the user
direction and the antenna radiation patterns.
b) Temporal Reference Beamforming (TRB) Reference-Based Techniques. This technique
minimises the mismatch between the output
signal and a reference signal. It is used in
mobile communications antennas to provide
multipath suppression and jamming mitigation
capabilities. In TT&C systems, the transmitted
signal has a known preamble that can be used
for reference purposes. However, this is
sensitive to jamming interferences. Thus, this
work is based on a superimposed CDMA
reference signal which fits in the TT&C
bandwidth, minimizing the impact in current
systems. One of the major advantages of the
TRB technique is that the antenna does not
need to be perfectly calibrated, as no
Direction-Of-Arrival (DOA) information is
required, in contrast to Minimum Variance
Beamformers which are sensitive to
miscalibration effects.
[A] A test case in which the user is located at θ=0°
(antenna boresight) and the jammer is moved
from θ=-60° to +60°. (this corresponds to a
vertical cut through Figure 2).
[B] The user is located at the Kiruna ground
station (Sweden) and the jammer location was
chosen to be close to the ground station so as
to provide a more difficult jamming scenario.
The satellite is SPOT 5 which moves in a sunsynchronous LEO orbit with an altitude of
822km and an inclination of 98.7°. The
analysis has been carried out with the satellite
passing over the ground station and remaining
within the field of view of the ground station
for a period of 720 seconds. This orbital path
and the variation in user to jammer angle are
shown below.
25
20
degrees
The alternative algorithms were compared by
analysing several test scenarios which included the
following:
User to Jammer Angle
30
15
10
5
-700
-600
-500
-400
-300
time(s)
-200
-100
0
Figure 4. Test [B] SPOT 5 Orbital Track and
User to Jammer Angle
In all cases, the jammer and user signal levels are
assumed to be equal and the jammer location is
unknown. It was found that the TRB algorithm
provided significantly better signal to noise levels.
This was attributed to the difficulty of separating
the user and jammer signals without an external
reference. The signal to noise characteristics,
SJNR = C/(N+J), obtained with the TRB algorithm
are illustrated in Figures 5-6 below, where C=
(wanted) carrier signal, N is the noise level and J is
the jammer level after nulling. Figure 5 displays
local minima at 40° which are consistent with the
ring of “minor” grating lobes.
Scenario 5 SJNR
14
12
10
dB
8
6
4
SQRLS
2
0
-2
-60
-40
-20
0
degrees
20
40
60
Figure 5. Test [A] SJNR Levels
The SJNR is a minimum when the user and jammer
coincide. There are minima at ±40 ° due to the ring
of minor grating lobes seen in Figure 2 above.
Helix Array on Astrium THEOS satellite
(Helix support structure not shown)
Scenario 3 SJNR
15
14
13
12
dB
11
SQRLS
10
Soyuz
launcher
boundary
9
8
TT&C Array on Sentinel-1
7
6
-700
-600
-500
-400
-300
time(s)
-200
-100
0
Figure 6
Test [B] SJNR Levels
Figure 6 shows the SJNR characteristics for Test
[B]. This shows positive margin throughout the
time period. SJNR is a minimum at the start and
finish of the orbital track when the angle between
the user and jammer is a minimum. There is also a
local minimum at time=-350s corresponding to a
user-jammer angle of 28 degrees(a maximum).
This is due to the close proximity of the “minor
ring of grating lobes discussed earlier.
INSTALLED ANTENNA
PERFORMANCE
A study of the feasibility of accommodating the 6
star array on a range of spacecraft platforms has
been performed. These included the Sentinel-1,
Sentinel-3 and Astrium THEOS platforms.
Examples are shown in Figure 7 below:
Figure 7. Installation
Spacecraft Platforms
of
Helix
Array
on
In the case of the THEOS platform, the geometry
shown is provisional and final implementation
would include a support structure so as to provide a
single unit for simpler integration. In the case of
all platforms a 95° FOV is possible. The centre
element in the array has been raised by 100mm to
improve the array gain at 95°.
As discussed earlier, the installed radiation patterns
of a quadrifilar helix antenna array are sensitive to
reflections off conducting surfaces.
This is
illustrated in Figures 8-9 below which show the
radiation patterns of each element in the array with
the array installed above a 0.8m. conducting
ground plane in isolation and also above a 0.6m
ground plane installed on a simplified model of the
THEOS satellite.
ground multipath as well as the jammer. The SJNR
analysis for Scenario [A] was analysed for these
geometries and is plotted in Figure 10 below.
Helix Array on 0.8m Diam Ground Plane
10
8
6
LHCP Directivity(dBic)
4
Scenario 5 SJNR
14
2
0
-100
-80
-60
-40
-20
-2
12
0
20
40
60
80
100
10
-4
8
SJNR(dB)
-6
-8
-10
elem 2
elem 3
no ground
0.8m ground plane
0.6m ground plane on THEOS
4
Angle(degrees)
elem 1(centre)
6
elem 4
elem 5
elem 6
2
Figure 8. Radiation Patterns of Helix Array
Elements on 0.8m. Conducting Ground Plane
0
-2
-60
-40
-20
0
Angle(degrees)
20
40
60
Helix Array on 0.6m diam Ground Plane on THEOS
10
8
6
LHCP Directivity(dBic)
4
2
0
-100
-80
-60
-40
-20
-2
0
20
40
60
80
-4
100
Figure 10.
SJNR Levels of Helix Array on
0.8m Ground Plane for Scenario [A]
This analysis demonstrates that the TRB algorithm
has successfully nulled the ground multipath so that
the maximum reduction in SJNR levels due to
multipath is only 2 dB.
-6
-8
CONCLUSIONS
-10
Angle(degrees)
elem 1(centre)
elem 2
elem 3
elem 4
elem 5
elem 5
The ground plane shown is 0.6m in diameter.
Figure 9. Radiation Patterns of Helix Array
Elements on THEOS
These analyses were carried out using the hybrid
electromagnetic solver, FEKO. The helices and
ground planes were modelled using Method of
Moments, while the THEOS platform was
modelled using UTD. The radiation pattern levels
of the individual helices show considerable
variation with angle with local nulls below the
-3dBi target. However, the nulls of the different
elements do not coincide. It is therefore possible to
use the antenna adaptive algorithm to null the
This paper has presented the results of a study of a
TT&C array antenna system with an anti-jamming
capability. The antenna comprises a 0.6metre
diameter 6 element helix array. This provides antijamming in-orbit and also has a LEOP and
emergency operations capability. It has been
demonstrated that the TRB antenna control
algorithm may be used to null a jammer and
mitigate the impact of reflections off conducting
surfaces. The signal to noise, C/(N+J), margin is
typically greater than 8 dB. On-going work is
assessing the system requirements required to
implement such a system on future satellite
payloads.
ACKNOWLEDGMENTS
The work reported here has been carried out under
ESTEC Contract No. 19674/06/NL/JA.
The authors wish to acknowledge the support
provided by Danny Daryanani and Scott McGeever
of EADS Astrium (UK).