Chandrayaan-2 GaN-based DFSAR in Detail (and More)

DRAFT-1/DSD14092019/Parth/AMCA240
The Chandrayaan-2 is a composite spacecraft comprising of an orbiter, a lander containing a rover. Each module is equipped with a wide array of scientific payloads. The orbiter features 8 payloads out of a total of 13 planned for the lunar mission. It's one of the most advanced spacecraft ever developed by ISRO and very much likely to accomplish many firsts.

Primary Sensor Package of Chandrayaan-2 Orbiter (RF and Optical only)

  1. Dual-band Synthetic Aperture Radar (DFSAR)
    DFSAR is a lightweight radar operating in S and L band (used for the first time on the moon). It's a follow-up of MiniSAR experiment of Chandrayaan-1. MiniSAR was an S-band synthetic aperture radar supplied by NASA. Weight and power are always constraining factors when it comes to planetary missions. The entire SAR instrument package of C2 orbiter weighs only 15 kg. To ensure high efficiency, the transmitter package of the radar uses Gallium Nitride (GaN) based solid-state power amplifier (SSPA) chain. The transmit chain feeds to a planner-microstrip array antenna of dimensions 1.35x1.1 m. The antenna is capable of transmitting in both L and S bands simultaneously. DFSAR has a maximum resolution of 2 meters from varying incidence angles. The SAR data provided by DFSAR instrument will facilitate the study of surface and subsurface properties of the lunar regolith. Furthermore, high ground-penetration capability of L-band frequencies will be useful for subsurface water-ice detection and estimation.
    ISRO's Space Application Centre (SAC) has been quite successful in development of GaN-Based synthetic-aperture radars, for example- NISAR dual-band SAR satellite which is being jointly developed by ISRO and NASA is going to feature SAC-developed S-band SAR module. NISAR's S-band SAR module uses GaN-based TRMs developed in India. NASA will provide L-band SAR. Both L and S band modules of NISAR will share a unfurlable reflector of 12 meter diameter. Another SAC project that features GaN SSPA is Airborne MiniSAR (not to be confused with Chandrayaan-1 MiniSAR) - an X-band lightweight SAR being developed for airborne high-resolution imaging applications.
    radar, antenna
    MiniSAR, featuring GaN-based X-band TRM
  2. Optical High-Resolution Camera (OHRC)
    OHRC is the primary optical imaging system onboard Chandrayaan-2 orbiter. With 0.32 meter spatial resolution, it's the most powerful imaging system ever deployed on an Indian spacecraft. It's also the highest resolution camera ever used on a lunar mission. For comparison, NASA's Lunar Reconnaissance Orbitor captures images at 0.5 m resolution.
    One of the tasks of OHRC was to scout safe landing sites for Vikram Lander which included hazard detection i.e. boulders larger than 32 centimeters in size and ground slope greater than 15 degrees. To put that in perspective, OHRC can detect objects slightly larger than a football from an altitude of 100 km. It's speculated that OHRC is based on an enhanced Cartosat design.
    The telescope of a Cartosat-2 satellite
Extra tidbits -

  • RISAT-2A, unlike its predecessor (RISAT-2/TecSAR), will have a 3x3 meter planar active-antenna configuration with 256 X-band TR modules. 
  • SAC developed an airborne dual-band (L/S) SAR as a precursor to NISAR. It has been flight-tested.
  • S-band SAR module of NISAR uses 200 Watt GaN SSPA.
  • X-band miniSAR developed by SAC uses 80 Watt GaN SSPA.

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