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TerraSAR-L

Figure 1: Artist’s view of TerraSAR-L (© Astrium Ltd.) . Figure 2: Block diagram of the calibration section including interfaces to the long-term data base

TerraSAR-L was originally planned to complement TerraSAR-X under the Infoterra/TerraSAR initiative of EADS Astrium Ltd., BNSC, and DLR, see Figure 1. While TerraSAR-X is being implemented as a German national mission (Link to ‘TerraSAR-X Cal’), TerraSAR-L was proposed as an element of ESA’s Earth Watch Programme, where the system definition was developed up to Phase B level concluding in a Preliminary Design Review (PDR). The Institute contributed to the Phase B study with the definition of the System Calibration and Verification Plan and the specification of the Instrument Calibration Segment. Furthermore, jointly with CNES the option of a micro-satellite constellation flying in formation with TerraSAR-L was studied.

The snapdragon platform is optimised for and built around the active phased array antenna of the L-band Synthetic Aperture Radar (L-SAR). The L-SAR instrument is based on an 11 m x 2.9 m active phased array antenna build up of 160 transmit/receive modules (TRMs). This instrument features on top of standard Stripmap and ScanSAR operations, full-polarimetric capabilities, repeat-pass ScanSAR interferometry and a Wave Mode.

As for TerraSAR-X the calibration and verification of a multi-mode, multi-beam instrument in a short commissioning phase (in the case of TerraSAR-L only 3 months) is a major challenge requiring new concepts and strategies. Dynamic re-calibration of the antenna due to graceful degradation of the TRMs throughout the mission lifetime, polarimetric calibration, and ionospheric propagation effects (most importantly Faraday rotation) are further issues that had to be covered.

Therefore, the methodology developed for TerraSAR-X (Link to ‘ TerraSAR-X Cal’) has also been applied for the Terra-SAR-L calibration concept. The active antenna has the advantage of the ability to be mathematically modelled, enabling the antenna patterns to be accurately computed from the commanded excitation values, pre-launch antenna and in-orbit TRM characterisation data. This antenna-model approach puts special emphasis on precise pre-launch characterisation and allows replacing traditional beam-to-beam calibration reducing the in-flight effort to the verification of the antenna model and absolute calibration measurements.

For in-flight TRM characterisation, a coding technique like PN-gating for TerraSAR-X (Link to ‘TRM Characterisation PN-Gating’) has been designed, that allows taking measurements in a realistic operational scenario and power supply load conditions, i.e. all modules are on. Algorithms for polarimetric calibration of different product levels have been developed including a combined correction of Faraday rotation in the case of quad-pol data. For non quad-pol data, a strategy to detect potential propagation effects via externally provided TEC maps has been conceived. In the PDR, the robust design of the calibration concept has been confirmed.

Again, following the TerraSAR-X model for TerraSAR-L, a dedicated Instrument Calibration Segment (see Figure 2) was specified including all the tools required for instrument and product calibration and verification, the generation of auxiliary products required in the ground processor, as well as for the update of instrument parameter tables on-board and in the ground segment.

After a successful PDR the TerraSAR-L programme has been halted for the time being. However, enough interest in a European L-band mission has been created and options for implementing this mission have been actively sought, our HABITAT proposal in response to ESA’s last call for new Earth Explorer missions being the most promising one.