NASA-ISRO Synthetic Aperture Radar (NISAR) Mission: System Integration & Test

Abstract

The NASA-ISRO Synthetic Aperture Radar (NISAR) mission is a multi-disciplinary Earth-observing radar that makes global measurements of land surface changes to greatly improve Earth system models and elucidate spatially and temporally complex phenomena, including ecosystem disturbances and natural hazards such as earthquakes, tsunamis, volcanoes, and landslides. The mission, a partnership between the U.S. and Indian space agencies, uses dual frequency L- and S-band fully polarimetric Synthetic Aperture Radars (L-SAR and S-SAR) in exact repeating orbits every 12 days that allow interferometric combination of data on repeated passes. NASA provides the L-SAR instrument, a shared deployable Antenna, and an Engineering Payload (EP) that supports mission-specific data handling and communication. ISRO provides the S-SAR instrument, spacecraft bus, and launch vehicle. The project is currently in Payload-level System Integration and Test (SIT), which integrates the L-SAR, S-SAR, Antenna, and EP subsystems after having completed their individual test campaigns. • The L-SAR SIT campaign faced unique challenges verifying performance during ambient and thermal vacuum conditions. Due to the SweepSAR digital beamforming architecture and onboard calibration, a complex radar data analysis system was developed to compare and correlate the test data with the measured data. • ISRO's S-SAR test campaign was performed in India where it demonstrated and characterized its stable performance in both ambient and thermal vacuum conditions and after completing standard process of Test and Evaluation (T&E) at ISRO. • The Antenna system test campaign focused on the deployable mechanical elements (9 m Boom and 12 m Reflector) using a flight spare structure to verify launch loading, on-orbit thermo-elastic loading, shock, and functional performance. Functional testing required conducting gravity offloaded deployments of the Boom and Reflector, and a thermal vacuum characterization of the thermo-elastic strains and First Motion. • The EP test campaign demonstrated its capability to bridge the communication between the L-SAR instrument and spacecraft Bus by interfacing the EP electronics with a simulator of the satellite On-Board Computer (OBC) provided by ISRO. Once the L-SAR and S-SAR individual test campaigns were complete, the S-SAR was delivered to JPL where it was integrated with the L-SAR into the Radar Payload structure and subjected to joint testing. After EP integration, a variety of functional tests are performed to verify commandability and data handling. In addition, transmit tests, electromagnetic interference/compatibility (EMI/EMC) tests, and Mission Scenario Tests (MSTs) are performed to ensure compatibility and represent on orbit scenarios. The final activity for Radar Payload integration is mounting the Boom and Reflector on to the Radar Instrument Structure (RIS). Boom full deployment and reflector First Motion tests are performed to confirm alignments and functionality of the mechanical elements with the EP electronics. The completed Radar Payload assembly undergoes dynamic testing followed by two thermal vacuum tests allowing for both the stowed and science (antenna deployed) configurations to be verified over temperature. This paper discusses the test results and design challenges of the completed SIT campaigns, the challenges that are still ahead, and the verification and validation activities that were conducted throughout.