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The Calibration Concept of TerraSAR-X

Figure 1: Challenges to calibrate a multiple mode high resolution SAR such as TerraSAR-X . Figure 2: Design of the Calibration System as part of the TerraSAR-X Ground Segment . Figure 3: Test site configuration for TerraSAR-X Stripmap calibration with 7 target positions (black and red crosses). The yellow region on the Earth surface is the crossing area covered by an ascending and a descending pass, whereby the red crosses are covered by both passes . Figure 4: Three test sites near Oberpfaffenhofen (OP) for TerraSAR-X Stripmap calibration: blue for beams S007 and orange for beam S002 and S013 (Me: Memmingen)

TerraSAR-X (Web page 'TerraSAR-X') is designed to be a multiple mode high resolution SAR and features the following operational modes, as shown in (Figure 1):

• Stripmap Mode at up to 26 different look angles (swath positions),
• ScanSAR Mode in 9 different wide swath con­figurations each consisting of 4 sub-swaths,
• Spotlight Mode at up to 125 different look angles implemented by switching between more than 100 different azimuth beams.

On top of the nominal right-looking mode, by performing a roll manoeuvre of the satellite, data can also be acquired in left-looking geometry. Beyond the 3 basic modes, operation in the following experimental modes is planned:

• Wide-band Operation with up to 300 MHz bandwidth,
• Dual-Receive-Antenna (DRA) mode achieved by electronically splitting the antenna in flight direction, enabling along-track interferometry and quad-pol operations.

Due to this multitude of operation modes based on an active phased antenna array with hundreds of transmit/receiver modules (TRM), we obtain a large number of different antenna beams (~ 10000 for TerraSAR-X). Furthermore, the demand on the absolute radiometric accuracy with only a couple of tenths of dB means that the complete TerraSAR-X system has to be adjusted with the accuracy of laboratory equipment.

Thus, a conventional calibration approach of SAR systems like ERS or X-SAR, involving internal calibration of a single transmit/receive chain (via special calibration loops), the determination of a single antenna beam pattern and the absolute calibration in one or two operational modes, is not feasible. Hence, new, more efficient and affordable methods have been developed. The two most important innovations are:

• the Antenna Model Approach (Web page ‘Antenna Model Approach’), deriving the settings of the antenna array for best performance even for drifted and/or failed TRMs as well as providing the reference patterns required for the radiometric correction of SAR data products,
• the PN-Gating Method (Web page ‘TRM Characterisation PN-Gating’), characterising the actual settings of the TRMs during operation in amplitude and in phase.

Taking these aspects into account, the main efforts have been concentrated on developing the calibration concept for TerraSAR-X since 2003. As part of the TerraSAR-X Ground Segment the concept has been realized according to the design depicted in Figure 2.

The TerraSAR-X Calibration System provides all necessary sub-systems to perform the different calibration tasks. The major functions are:

• internal calibration, monitoring the instrument stability and linearity of the TerraSAR-X Front-End (XFE) and Central Electronics (CE), characterisation of T/R Modules, providing specific reference chirps for range compression,
• external calibration, measuring the TerraSAR-X system against standard targets and featuring in-flight measurements, absolute radiometric calibration, geometric calibration and polarimetric calibration,
• antenna characterization, sub-divided into the excitation generator for optimum instrument operation and the antenna pattern generator providing an accurate estimation of the actual antenna pattern,
• noise characterization, providing profiles for the noise correction of processed image data,
• maintenance and deployment of calibration ground equipment, supporting the external calibration activities.
• characterizing the instrument on ground. This task is performed by EADS Astrium GmbH.

With regard to these functions, new software tools have been implemented and existing algorithms were updated (Web page ‘Algorithms and Tools’). Further calibration targets are being developed and purchased.

Currently the detailed calibration activities required during the commissioning phase of TerraSAR-X are being planned. Starting from the calibration strategy, different constraints have to be considered here, e.g. the coverage on the Earth’s surface or the required number of point target measurements assumed in the radiometric accuracy budget. One example is shown in Figure 3. In order to obtain as many as possible passes over deployed calibration targets, the test sites will be set up in the cross-over points of ascending and descending orbits. Mainly for logistic reasons we aim to select these test sites near Oberpfaffenhofen, as shown in Figure 4.

The calibration strategy of TerraSAR-X has been adapted to TerraSAR-L (Web page ‘TerraSAR-L’) and to Sentinel-1 (Web page ‘Sentinel-1’).