3.2.2 Inflight Performance Verification
In-orbit beam calibration of an active
phased-array antenna is a major task and it will
include measurements in special modes, like
module stepping and external characterisation,
as well as acquisitions over rain forest. A
sophisticated antenna model will combine the
various data together with the pre-flight
characterisation and provide the elevation and
azimuth patterns for the ground processor.
Any drift in the gain and phase characteristics
of the TRMs can distort the antenna beams.
Deviations in antenna pattern and antenna
gain will potentially contribute to radiometric
errors in the SAR image. figure3.25 shows an overview
of the various processes defined to maintain the
calibration of the ASAR antenna beams.
|
| Figure 3.25 ASAR instrument calibration tasks |
Monitoring any instrument gain drifts requires a
separate calibration network, to couple out part
of the transmit signal or to inject chirp
signals into the receiver chain. This data will
be included into the high-rate data stream and
will be analysed by the ground processor in
order to estimate the necessary gain drift corrections.
The absolute overall system gain can be most
accurately determined from the image response of point
targets with high and well-known RCS. ASAR high
precision transponders will be deployed in the
Netherlands and will serve as the main external calibration
targets. A special transponder operation
mode and well-characterised distributed targets
will be used for the low-resolution Global
Monitoring mode.
In the following sections the pre-flight
measurements and the above methods for internal
calibration, module stepping, external characterisation
and rainforest measurements and absolute gain
calibration will be explained. Furthermore the
strategy for maintaining the ASAR antenna beams
will be presented.
Preflight Characterisation Measurements
In order to provide the required
image quality, the two-way antenna beam pattern
should be known to a high degree of accuracy
(0.1dB). The transmit and receive antenna
patterns have been accurately measured on ground
Ref. [3.5 ]
for all eight beams (IS1-IS7
and SS1) and both horizontal and vertical
polarisations. The radiating performances of the
antenna were measured in the Large Planar
Near Field Range at Astrium Ltd (Portsmouth).
Full characterisation was performed: 8 beams,
two modes (transmit and receive), two
polarisations (V and H), in copolar and
crosspolar components. Example patterns are
shown in figure3.26 .
|
| Figure 3.26 FM Elevation Beam Pattern Measurement |
Internal Calibration
The ASAR instrument incorporates a
very comprehensive system for internal
calibration. There is an individual calibration
path for each of the 320 transmit/receive
modules. Internal calibration will be carried
out on a row by row basis for each of the 32
rows. The calibration pulses are included in
the instruments timeline during imaging and
consist of the following (see also figure3.27 ):
- Transmit Calibration Pulses P1 (representative
of T/R module load)
The
T/R modules of the four adjacent rows in a tile
share the same power supply. In order for the
calibration sequence to be representative of
the nominal operation, the ten modules of the
selected row are set to their nominal phase and
amplitude settings whilst the phase of the
modules of the three rows sharing the same power
supplies, are set so that their combined
contribution out of the calibration network
is nominally zero. Thereby minimising their
interference to the measurement of the selected
row.
- Transmit Calibration Pulse
P1a
A second type of transmit pulse is
added in order to characterise the residual
parasitic contribution of the three unwanted
rows during P1. During P1a, the three
unwanted rows are set as for P1 and the
previously wanted row is now switched off. Even
though the load conditions on the power
supplies are not exactly representative, the
small error introduced into the estimation of
P1a is negligible.
- Receive
Calibration Pulse P2
The receive path
of the instrument is also characterised but
since no variation is expected from power supply
load variations it is possible to
characterise on a row by row basis.
-
Central Electronics Calibration Pulse P3
The central electronics transmit and receive
paths are included in the P1/P1a and P2
characterisations. The central electronics are
therefore characterised independently by means
of P3.
|
| Figure 3.27 ASAR Internal Calibration Diagram |
Using the amplitude and phase of the calibration
pulses (P1/P1a, P2 and P3) for each row it is
first necessary to calculate the amplitude
and phase of P2 relative to P3 and to subtract
P1a vectorially from P1. From these values for
each of the 32 rows and together with the
external characterisation factor, it is possible
to calculate the elevation beam pattern. This is
then used to detect any deviation to the
reference instrument gain pattern as
characterised on ground. The typical update rate
for this calculation is 5 to 35 seconds
(mode dependent).
A replica of the chirped pulse is calculated from
a complete calibration row cycle using the
P1/P1a, P2 and P3 measurements, the ground
characterised row patterns and the external
characterisation data. This is also typically
updated every 5 to 35 seconds.
Despite the comprehensive nature of the internal
calibration system, it is not possible to use it
to calibrate the passive part of the
antenna, which falls outside of the calibration
loop. This is achieved through external
characterisation by using the ground transponders.
Module Stepping
ASAR has a dedicated Module Stepping
Mode, which is used to gather data from all 320
transmit/receive modules automatically. The
entire procedure takes less than one second.
The data are downloaded to the ground for
processing. After processing, the results are
compared with the reference data from
on-ground tests in order to determine any TRM
module gain or phase drifts, temperature
behaviour and any eventual module failures.
Using this information it is possible to
implement any necessary correction to the TRM
coefficients and eventually re-synthesise
the antenna beam patterns if required.
External Characterisation
ASAR can be put into External
Characterisation Mode while flying over a
calibration transponder. This involves sending a
series of pulses from each of the 32 rows in
turn followed by each of the 10 columns in turn.
These pulses are detected both by the internal
calibration loop and the receiver embedded
in the transponder (see figure3.28 ). Comparison of
these data allows characterising the passive
part of the antenna and the calibration network.
The baseline is to repeat measurement every
six months.
|
| Figure 3.28 External Characterisation |
Rain Forest
The reasons images of the Amazonian
rain forest are used for the characterisation of
the antenna beam pattern are that it is a
stable, large-scale, isotropic distributed
target with a relatively high backscatter and a
well-understood relationship between backscatter
and incidence angle.
In order to determine the two-way beam pattern,
an uncorrected rain forest image is averaged in
the azimuth direction. In the final
processed image, the inverted beam pattern is
applied and hence the effect of the pattern on
the backscatter is removed.
Alternative distributed targets at different
latitudes are being investigated. Promising
results have been found from ERS data over Lake
Vostok in Antarctica. Antenna pattern estimates
at different latitudes could be used to verify
the round-orbit performance of the ASAR.
Gain Calibration
The purpose of the ASAR gain
calibration is to provide the users of ASAR data
with the possibility to determine the absolute
level of backscatter from any target, point
(s) or distributed (s0). For ASAR this is
achieved in fundamentally the same way as for
ERS, namely by providing an Absolute gain
Calibration Factor (ACF) in the header of the
(processed) product. Since ASAR, however, has a
total of eight beams and five different
modes and up to four polarisations more ACFs
will need to be determined for ASAR than for the
ERS single beam, single polarisation with
two modes.
The method to be used to determine the ACFs is to
image a target of known radar cross-section,
integrate the power in its Impulse Response
Function (IRF) corrected for the associated
background (clutter) power and hence calculate
the correction (the ACF) which must be
applied to the image values in order to arrive
at the same cross-section for that target. For
this purpose, precision calibration
transponders are deployed in the Netherlands.
The radar cross-section of these transponders is
65dBm
2
and is known to within ±0.13dB and
they are stable to 0.08dB
Ref. [3.6 ]
. It is necessary to use active
radar calibrators (transponders) as opposed to
passive ones (e.g. corner reflectors) since
the ratio of signal to clutter determines the
accuracy to which the calibration can be made.
Once the ACF for a particular configuration has
been calculated it will be possible to make a
direct comparison with the on-ground
measurements of the end-to-end system gain
carried out during FM testing.
Global Monitoring Mode
This is a special case since the
spatial resolution of 10001000m makes the normal
use of the transponders unfeasible. For a
reasonable calibration to be made (3s value
of ± 0.5dB), a signal to clutter ratio of
better than 30dB is required. If the clutter at
the calibration sites typically has a sigma
nought of -6dB then this would require a
transponder RCS greater than 84dBm2. This would
inevitably saturate the receiver
invalidating the calibration. As a result, it is
necessary to come up with an alternative
scenario for calibrating this mode.
The first option (baseline) is using the other
modes (namely Wide Swath and Image) to calibrate
GM mode by means of the Amazonian rain
forest. Since the sigma nought of the rain
forest is stable to within 0.3dB it will be
possible to use the sigma-nought value obtained
from a previous (or subsequent) pass in WS
or IM to calibrate GM mode. In addition, other
relatively stable distributed targets may be
used such as the ice caps and specific
desert regions (Gibson, Gobi etc).
The second option will allow direct calibration
using a special global monitoring mode setting
and a modified calibration transponder. The
intention is to use the ASAR's digital
chirp generator to offset the centre frequency
by 5MHz. This is possible since the chirp
bandwidth in GM mode is only around 1MHz. In the
calibration transponder, the received signal is
shifted back by 5MHz allowing it to be
received by the ASAR. As the clutter return will
all be outside the range of the reduced
bandwidth filter in GM mode, only the
transponder response will be seen in the
processed image against a background of noise.
The result of this operation is to provide a
transponder signal to clutter ratio of
between 25 and 30dB allowing for reliable
calibration of Global Monitoring Mode.
Calibration Transponder
Since the commissioning phase is
planned to last only six months following
switch-on of the instrument it is necessary to
make use of every possibility of imaging the
transponders during that period. Furthermore the
placement of the transponders must be optimised
to ensure that each of the seven image mode
swaths can be acquired over the transponders
sufficiently to allow for reliable calibration
of each beam. Based on a detailed coverage
analysis four locations in the Netherlands have
been selected. Three transponders will be fixed.
One mobile unit will be used for calibrating
the Wave Mode and to support interferometric
investigations during later phases of the
ENVISAT mission.
Three precision calibration transponders have
been developed by MPB (Canada) based on an ESA
prototype and were delivered to ESTEC in
April 2000 (figure3.29 ). Validation of the
calibration performance will be supported by
the four RADARSAT transponders deployed over a
latitude range from 45-74o in Canada
[3]. As RADARSAT operates at 5.3GHz, the
actual RCS of these transponders at 5.331GHz
will decrease by about 0.4dB.
|
| Figure 3.29 Prototype ASAR Calibration Transponder Deployed at ESTEC |
Beam Maintenance Strategy
The strategy for beam maintenance
throughout the instrument lifetime is
schematically shown in figure3.30 . Initially, the
on-ground characterisation data will be used.
TRM drifts occurring during the first years of
operation will be detected during module
stepping and compensated for, individually, by
applying corresponding offsets, in order to
bring the TRMs back to the initial
conditions. Should compensation not be
sufficient after TRM eventual failures, the
antenna beam characterisation data will be
updated in the ground processor with a new
calculated antenna pattern. At the end of the
instrument lifetime, beam optimisation by
re-calculation of new sets of beam coefficients
will also be possible.
The use of rain forest images, antenna synthesis
software based on pre-flight embedded row tests,
and eventually the actual near-field raw
data acquired during the pre-flight beam
characterisation will be used, in parallel, to
verify any change.
|
| Figure 3.30 ASAR Beam Maintenance Strategy |
Conclusions
ASAR antenna beam calibration is
based on combining several inputs: pre-flight
characterisation measurements, internal
calibration, module stepping and external
characterisation, rain forest and transponder
measurements. Absolute calibration will rely on
ASAR precision transponders. RADARSAT
transponders will support the verification of
round-orbit calibration performance.
It is the intention to have the Image, Wave and
Wide Swath modes calibrated within six months of
launch and the all modes within nine months
of launch.
REFERENCES
J-L.
Suchail, C. Buck, A. Schnenberg, R. Torres, M.
Zink, ESA/ESTEC. "The ASAR Instrument
Verification: Results from the Pre-Flight Test
Campaign". Proc. CEOS SAR Workshop April
2001, Tokyo, Japan.
H Jackson
ESA/ESTEC, I. Sinclair, S. Tam. MPB Technologies
Inc. "ENVISAT ASAR Precision
Transponders". CEOS SAR Workshop 26-29
October 1999, ESA-SP450
R.K.
Hawkins, et al. "RADARSAT Precision
Transponders". Adv. Space Res. Vol. 19, No.
9, pp. 1455-1465, 1997.
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