In-Orbit Relative Amplitude and Phase Antenna Pattern Calibration for Tandem-L

Abstract

Precise knowledge of the far-field antenna patterns associated with individual receive channels of a multichannel SAR system is of fundamental importance to operate the radar instrument in advanced imaging modes. A prominent example is Tandem-L which uses a large deployable parabolic reflector that is illuminated by a digital feed array with multiple receive channels. This architecture enables, in combination with an appropriate onboard signal processing, the acquisition of a wide image swath by simultaneously recording multiple scattered radar echoes with a set of narrow elevation beams that are steered in real time towards the angles of the arriving wavefronts. As such a multiple elevation beam technique is prone to range ambiguities, optimized real-time beamformers are considered that maximize their gain in the direction of the desired radar echo and suppress, at the same time, the range ambiguous radar echoes arriving from different angles. The implementation of these advanced real-time beamforming techniques requires, however, precise knowledge of the amplitude and phase of the secondary far-field antenna patterns associated with the excitation of single feed elements. As it is impossible to measure these complex antenna patterns on ground with sufficient accuracy, we propose here a novel technique that enables a highly accurate in-orbit measurement of the relative amplitude and phase of the far-field secondary antenna patterns associated with individual feed elements. The key idea is to collect SAR data in space by a set of dedicated calibration flights, where the signals from all feed elements are simultaneously recorded in a transparency mode, i.e., without any real-time beamforming on board the satellite. The multichannel data are then transferred to the ground, where the relative antenna pattern information is extracted and calibrated beamforming coefficients are derived, as required for the implementation of advanced SAR modes. This paper provides an overview of the proposed multichannel antenna calibration technique and demonstrates the superior SAR imaging performance that can be achieved by employing this technique in conjunction with a series of especially designed calibration flights over natural terrain with known topography.