Characterization of NISAR S-SAR 1-D Feed Array Antenna: A Time Efficient Approach

ISRO (Indian Space Research Organization) and JPL (Jet Propulsion Laboratory)/NASA are jointly developing a state-of-the-art L&S band SAR, planned to be launched through GSLV during 2024-25. The conventional SAR systems has a limitation of providing higher spatial resolution as well as wider swath coverage simultaneously [6]. The NASA-ISRO Synthetic Aperture Radar (NISAR) is based on a novel SAR concept, which will be utilized to image wide swath as high resolution of stripmap SAR, also called SweepSAR [1]. It uses a large deployable reflector antenna with a phased array antenna feed. To illuminate wider swath during transmit, all feed elements are excited to sub-illuminate reflector. While on receive, a much smaller portion of phased array antenna is used to collect the return echo. NISAR S-SAR payload comprises of 48 Transmit Receive integrated modules (TRiMs) for signal transmission and reception ($24 \mathrm{H}+24 \mathrm{~V}$). For proper collimation and beamforming in transmit, it is essential to characterize the primary antenna feed system i.e. all the feed elements to be in equi-phase within phase shifter error limits. Digital beamforming on receive requires that all the receive path gain and phase are balanced. Characterization of 1-D feed array antenna is feasible using either the Near Field Antenna Test Facility (NFTF) or Compact Antenna Test Range Facility (CATF). In this paper, a time efficient characterization of NISAR S-SAR primary antenna using CATF facility is discussed. This paper gives insights and comparison of 1-D array antenna characterization time using NFTF and CATF. The methodology adopted for full characterization of feed array antenna with $48 \mathrm{Tx} / \mathrm{Rx}$ elements in CATF is also described, which has reduced the characterization time by more than 70% as compared to conventional NFTF method.