369
RX-calibration
TX-calibration
transmit mode
O T- T- CO
Figure 2: RX- and TX-calibration sequence at the beginning and end of each data take.
SAR data and stored in the raw data stream. The evaluation of these calibration loop data during
precision processing allows to derive parameters to correct variations of the transmit power and
receiver gain.
3 - MEASUREMENT OF THE INFLIGHT ANTENNA PATTERN
The cross-track antenna pattern is a key element for radiometric corrections [1], thus the precise
inflight antenna pattern should be known. The antennas of the present and future spacebome SAR
missions (ERS-1, X-SAR/SIR-C, JERS-1, PRIRODA, RADARSAT) have dimensions of up to 15
m, which makes pre-launch pattern measurements on a test range difficult (fair field restrictions).
Furthermore, distortions in the antenna geometry due to launch vibrations, incomplete antenna
deployment, interference with the carrier structure, or thermal effects cannot be taken into account
by means of pre-launch measurements. These restrictions require a precise method to measure the
actual inflight antenna pattern.
We use high precision ground calibration receivers deployed across the imaged swath and
beyond to register azimuth cuts of the three-dimensional antenna pattern. These cuts can then
be time correlated to obtain the main cut in the cross-track direction for later use in radiometric
corrections of the image data. This approach does not include the SAR receiver and the processor
and therefore seems to be the most direct approach to measure the full array pattern on transmit.
Our colleagues from the Institute of Navigation at the University of Stuttgart have developed
and produced some 20 precision calibration receivers and 5 polarimetric active radar calibrators
(PARCs) having also full receiver channels in L-, C- and X-band. A detailed description of these
devices and the calibration procedure can be found in [2]. All these receiver units are capable of
registering single pulse envelopes and the azimuth pattern shapes within a 65 dB dynamic range.
The calibration receivers digitize each received radar pulse with sixteen samples. This feature gives
the opportunity to analyze the transmitted radar pulses in detail. Fig. 3 shows a typical example
of the almost ideal rect-shaped ERS-1 pulses measured by one of our ground receivers.
The sixteen samples are comprised to one pulse power value by applying a special integration
process. Precision internal clocks, which have to be synchronized using GPS, provide the time
of receipt for each pulse. This makes it possible to relate the measurement of a single pulse to
the corresponding orbit position and the platform attitude of the SAR sensor. Together with the
position of the ground receivers (from differential GPS survey) we know the measurement geometry.
