3.2.1 Preflight Characteristics and Expected Performance
Compared to the ERS AMI-SAR, ASAR offers
five different modes of operation, dual
polarization and a total of seven beams with a
combined ground-range of 485km covering
incidence angles from 15° to 45°. In
spite of this substantial increase in
functionality, the intention from the outset
was that there would be no significant
degradation in the various performance
parameters such as spatial and radiometric
resolution, sensitivity and ambiguity
suppression compared to ERS.
result and in order to be convinced that this
high goal would be met, performance prediction
analyses have been repeatedly made and
updated since the earliest days of the project
using principally the specially developed PEAS software (Performance
Evaluation and Assessment Software).
Confidence in the results obtained using
PEAS is high, thanks to independent
corroborative analyses and comparison, where
possible, of the predictions and
measurements made on the FM instrument.
Performance Objectives are shown in Level 1B Accuracy 18.104.22.168. .
Performance Prediction Methodology
The ASAR antenna with its 320 T/R modules requires a
different philosophy when determining
performance than say, ERS, since instead of a
single point failure it has 320. That being
said, it is reasonable to assume that not
all of the modules will survive launch and four
years of continuous operation. This forms the
basis of the performance analysis philosophy
the aim of which is to predict the worst case
expected performance at end of life following a
period of so-called graceful degradation.
To achieve this, two main issues with
respect to the T/R modules are considered,
namely: module failure and phase and amplitude setting errors.
From assessments of component reliability a
figure of 6% failure rate has been estimated by
industry for the T/R modules at end-of-life.
This corresponds to about 20 failed modules.
Module Setting Errors
Each T/R module can be set to provide a
certain phase and amplitude in order
to generate the required elevation beam patterns. Six bits are
available the required phase from 0° to
360° and a further six to specify the
amplitude in the range 0 to -20dB. From
measurements made at component and
instrument level it has been determined that the
phase setting accuracy is approximately
±5° and the amplitude setting accuracy
Generating "End-of-Life" Antenna Patterns
In order to model accurately the impact of
T/R module failures and setting errors on the
ASAR performance it is possible to create a
failure/error matrix representing the antenna
and use it in producing representative simulated
elevation beam patterns. Each pattern thus
generated would have a slightly different shape
to the one desired with some sidelobes becoming
larger and nulls appearing at different look
angles. The problem with this approach is
clearly the extremely large number of possible
permutations of failures and setting errors.
Instead, the method chosen was to calculate
a disturbance level which could be applied
linearly to the calculated elevation patterns
with the effect of increasing sidelobe
levels and "filling-in" nulls.
Ideally T/R modules would fail in an evenly
distributed random pattern across the antenna.
However, the possibility that failures might be
clustered could not be ignored. This was
accounted for by calculating the variance in the
sidelobe level as a result of failures and
basing the disturbance level to be applied
on a 90% probability (two standard deviations)
of the antenna being able to generate the
required pattern within the sidelobe levels
set (figure3.21 ).
|Figure 3.21 Effect of Disturbance Level on Elevation Beam Pattern. Black: FM measured pattern, Red: sidelobe disturbed pattern at 90% probability
Sigma Nought Model
A similar "worst case" approach
was taken in order to determine the distributed
target ambiguity suppression ratio with respect
to the normalized radar cross section of the
in-swath target region and the ambiguous
regions. From measurements provided in the
Ref. [3.1 ]
) on C-band backscatter over land and sea
or ice two graphs were produced. In swath, the
sigma nought would be assumed
to be the lower of the two curves while outside
the swath, the ambiguous regions, would be
assumed to have a sigma nought
corresponding to the higher curve (see figure3.22 ).
|Figure 3.22 ASAR sigma nought model showing the position of the IS3 swath
At low incidence angles this can lead to more
than 15dB difference between target and
ambiguous region sigma nought which puts extra
constraints on the elevation pattern sidelobe levels.
The predicted performance of ASAR has been
determined using the ASAR PEAS software which
makes use of algorithms based on those used
successfully to predict the performance of the
ERS SARs. All FM instrument measured
characteristics have been used as input
parameters to PEAS together with the
sidelobe disturbed FM beam patterns to produce
the most accurate possible performance
predictions for end-of-life operation.
Figure3.23 shows the sensitivity
expressed as noise equivalent sigma
nought for all image beams (IS1-IS7) compared
to the ASAR sigma nought model. There is several
dB margin everywhere for all beams compared
to the sigma nought curves with the exception of
the far range of IS5 over
land which in any case overlaps with the near range of IS6.
|Figure 3.23 Noise Equivalent Sigma Nought for Image Mode beams IS1-IS7
In figure3.24 the range distributed
target ambiguity suppression for beams IS1-IS7
is shown. For each beam the measured FM pattern
has been used in the calculation to show the
start-of-life situation (lower set of curves)
as well as the sidelobe disturbed version of the
FM patterns to demonstrate the worst case (90%
probability) expected suppression at
end-of-life (upper set of curves). This shows
that at start-of-life the DTRAR is better everywhere
than about -25dB and even at end-of-life it
should never be worse than about -20dB.
|Figure 3.24 Distributed Target Range Ambiguity Ratio
A summary of all the end-of-life
performance parameters and how they compare to
those calculated or estimated for ERS is given
in table 2.
Throughout the pre-flight assessment of the
performance of the ASAR instrument a worst case
scenario has been assumed including all
component related issues. The result of this
approach is that there is now high confidence
that the ASAR will meet all the requirement
specifications throughout its planned life. This
means that the high standards which were set by
the ERS AMI SARs will be maintained allowing
ASAR to become a valuable instrument in terms of
its potential for applications.
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Breaking Synthetic Aperture Radar: The
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J-L. Suchail, R. Torres, J. Guijarro and C. Buck
(ESA-ESTEC, NL). "The ENVISAT-1
Advanced Synthetic Aperture Radar. The
Development Status". CEOS'98.
Guijarro, C. Buck, P. Mancini, J-L. Suchail and
R. Torres (ESA-ESTEC, NL). "The
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