Pointing Error Calibration for NISAR S-SAR

NASA ISRO Synthetic Aperture Radar (NISAR) is a joint SAR mission between NASA and ISRO having the capability to image a wide swath at high resolution using the SweepSAR technique. Nominally, NISAR uses onboard Digital Beam Forming (DBF) to generate wide swath image at uniform radiometry. Precise Spacecraft pointing is crucial to achieve good radiometric accuracy. Error in pointing leads to improper antenna pattern correction during DBF affecting the radiometric quality of the processed images. Pointing error calibration aims at estimating the mean deviation between the nominal pointing and actual pointing executed onboard NISAR spacecraft. Deviation of nominal spacecraft pointing corresponding to the Zero Doppler Steering (ZDS) results in non-zero Doppler centroid frequency (FDC). Raw data from different antenna beams is combined to generate Null patterns. The Null and Doppler information is exploited to estimate the errors in pointing in the radar coordinate system (RCS) which is further converted to delta errors at Spacecraft coordinate system (SCCS). These errors are used to correct the spacecraft pointing to achieve Zero Doppler steering. This paper discusses the methodology to estimate and validate these errors using simulated SAR raw data for upcoming NISAR mission.