Efficient Calibration of Active Phased Array SARs

Calibration of the first space-borne SAR instruments, with only one swath and based on passive antenna tech-nologies, (e.g. ERS-1) required the in-flight characterisation of that single-swath instrument over well-known scatterers (reflectors, transponders and rainforest sensing data). Modern systems evolved to more access flexibil-ity therefore, multiple beams , and multi-polarisation that led to the use of complex active phased-array anten-nas. In-orbit calibration of such complex SAR system can be a very time consuming and expensive exercise that takes time off the spacecraft lifetime. The full characterisation on ground presents the additional risk of a limited validity if during the early phases of the in-flight operations a number of sub-arrays suffer degradation (i.e. failure of T/R module functions or irrecoverable phase and gain drifts). If this type of degradation occurs, an in-flight re-characterisation of the instrument throughout the different beams (i.e. using repeating passes over the rain forest, etc.) is the only way to keep the required accuracy. After the lessons learnt during the ENVISAT ASAR commissioning, this paper present a new concept for SAR calibration built around a mathematical antenna model based on accurate on-ground measurement of the instru-ment, a set of post-launch external measurements to be performed during the initial commissioning period, peri-odic in-flight internal characterisation, and the internal calibration data to be taken during and together with the sensing data. Such an accurate antenna model is a very powerful tool both for pre-flight characterisation of all antenna beams, and for in-flight estimation of the actual patterns. Eventually at the SAR processor, the actual calibration data together with the antenna model can replace the use of fixed antenna patterns and calibration constants. The swath characteristics (pattern and gain) would then be given by the calibration data taken during the SAR sensing and the use of a validated antenna model. This concept is conceived to achieve a very high ra-diometric quality but reducing as much as possible the in-flight characterisation that requires deployment, main-tenance and data collection of transponders, and the need of long-lasting antenna beam characterisation using re-petitive passes over the rainforest, for each antenna beam