Gapless Imaging with the NASA-ISRO SAR (NISAR) Mission: Challenges and Opportunities of Staggered SAR

The NASA-ISRO SAR (NISAR) is a science and applications mission that will map the Earth’s surface every 12 days by using a reflector-feed system with scan-on-receive (“SweepSAR”) capability. Gapless imaging of a swath of over 240 km at fine resolution, however, requires staggered SAR operation of the radar instrument, i.e., continuous variation of the pulse repetition interval (PRI). While NISAR presents a challenge in that the system’s size is constrained by available resources, and it is planned to operate with lower pulse rate than would be optimum, interesting opportunities arise from subaperture processing and adaptive spectral estimation techniques that greatly improve the image quality at the cost of modestly more computationally intensive image processing

NISAR Mission Overview and Performance Summary

The National Aeronautics and Space Administration (NASA) in the United States and the Indian Space Research Organisation (ISRO) are developing a synthetic aperture radar (SAR) mission to map Earth's surface every 12 days, known as the NASA-ISRO SAR (NISAR) Mission. NISAR is being designed to measure changes in Earth's land surface using repeat pass synthetic aperture radar interferometry and polarimetry methods from space, in three science areas: 1) solid earth, including earthquakes, volcanoes, mountain building, and erosion, including landslides; 2) terrestrial ecosystems on which all of life depends, including global carbon distribution and change; 3) cryosphere, including changes in ice sheets, sea ice, and glaciers as key indicators of climate effects. The L-band radar provided by NASA, with its performance characteristics of wide-swath and complete coverage of land and ice twice in 12 days, is suited to measuring surface displacements of the solid earth, ice sheet velocities in Greenland and Antarctica, sea-ice motion, and ecosystem variables such as biomass, disturbance, and wetlands and agriculture area. In addition, the fast and reliable sampling in time will be used to develop reliable applications such as subsidence mapping, hazard assessment and disaster response. The radar is a scan-on-receive system that can be operated with fixed or variable pulse repetition frequency (PRF). With fixed PRF, because of the wide swath and required Doppler sampling, there will be gaps in the received swath during transmit events. With variable PRF, these gaps can be filled in at some cost of added multiplicative noise. In this paper, we will review the mission characteristics and observation plan, describe the radar performance with regard to traditional imaging metrics, and also describe the performance relative to the science requirements as predicted by the mission's performance tool