2.11.4 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. 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.
2.11.4.1 Characterisation of the Antenna Beam Pattern
For the characterisation of the antenna beam
pattern, images of the Amazon Rain Forest
are used. This is because the rai forest 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.
2.11.4.2 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 65dBm2 and is known to
within ±0.13dB and they are stable
to 0.08dB
Ref. [2.14 ]
. 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.
2.11.4.3 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 s0 of the rain
forest is stable to within 0.3dB it will be
possible to use the s0 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.
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