Overview: NISAR L-band SAR instrument pre-launch verification, on-orbit calibration and validation

The NISAR Science Payload consists of an L-band SAR instrument (L-SAR) and an S-band SAR instrument (S-SAR) each with their own feed sharing a single 12m reflector. The instrument design, based on “SweepSAR” architecture for wide-swath coverage, is a multi-beam polarimetric system employing on-board time-to-angle digital beam forming to each of the receive channels. The instrument exhibits a few “features” that are not present in the conventional SAR system that utilizes a rectangular phased-array antenna and electronically steering the antenna beam in “ScanSAR” operation mode. The most prominent feature is the large reflector-feed antenna, whose geometry is more susceptible to the on-orbit thermal environment. When the geometry gets changed thermally, the mechanical boresight will be perturbed, and the antenna patterns will be distorted. For NISAR science objectives, the instrument can be used as a repeat-pass interferometer with polarimetric capability; Science imposed a set of “Performance” requirements and a set of “Pointing” requirements on L-SAR and these two sets are inter-related. The large reflector-feed antenna cannot be tested with the instrument in a deployed configuration and is the driver of several test-as-you-fly exceptions. To verify that the system is meeting project requirements before launch and to understand what’s expected once on-orbit, a set of inter-related system models were developed and used to evaluate L-SAR’s sensitivity when the radar antenna is subject to on-orbit thermal environment influences affecting pointing and performance. The results from these models run as a whole, portends what to expect during the mission and the results also are used to certify L2 requirements.