IQ Bias Channel Deviation on TerraSAR-X and TanDEM-X

Abstract:
The SAR satellites TerraSAR-X and TanDEM-X are experiencing periodically varying solar conditions due to their orbit geometry. As a consequence multiple system parameters are affected by these variations mainly induced by corresponding temperature fluctuation. A long-term analysis has been conducted showing that in particular one parameter, the in-phase and quadrature bias (IQ bias), deviates considerably from expected results. This deviation occurs between the respective channels and also between both spacecraft. In order to identify the driving mechanisms behind this behaviour, the signal path between the antenna front-end and the analogue-to-digital conversion (ADC) block is analysed, putting a special focus on the IQ demodulation block. Within this block, mixer imbalances are the main contributors to the observed behaviour and provide an explanation for the differing channel outputs. The mixer is also the reason for IQ bias deviations between the satellites since the channel imbalance shows a reversed tendency when comparing both systems. Although the effects are corrected in calibration, the results provide valuable information to interpret monitoring results and to gain a deeper understanding of the system.