The design and development of airborne and space borne Synthetic Aperture Radar (SAR) sensors was initiated in 1989, with the initiation of ISRO’s Microwave Remote Sensing Programme (MRSP). SAR is an active imaging radar, with varied applications in the fields of agriculture, Disaster Management, Oceanography, land-use, geology, cartography etc. During 1990s, a C-band Airborne Synthetic Aperture Radar (ASAR) was developed at SAC, ISRO. ISRO's C- Band ASAR is a 5.3 GHz monostatic radar aboard Beechcraft Super king B-200 aircraft, that flies at 8 km altitudes with 120 m/s velocity. Since 1997, ASAR was regularly utilized for various user and experimental / calibration flight campaigns to cater to numerous land and ocean applications.
It was needed to develop X- band SAR for airborne platform ahead of the space borne version. Thus, the development of Miniaturized Synthetic Aperture Radar at X-Band was initiated in 2013 to have a lightweight, compact SAR to fly in small aircrafts and provide high-resolution imagery for various applications. A miniaturized X-Band airborne SAR payload (MinisSAR) was developed for the purpose. MiniSAR operates in Single/Dual/Circular Polarization, with high spatial resolution of less than 30 cm. The system was designed to accommodate altitude variation of 3 km to 10 km with a nominal velocity of 120 m/s. In-house SSR was designed for on-board data storage, MMIC based FG-Receiver, GaN based Transmitter and miniaturized high speed digital system for this mission. The MiniSAR was mounted on Beechcraft-200 aircraft System and had many successful flights overAhmedabad region. Generated data products in operational scenario with minimum turnaround time, provided high resolution mode (less than 30 cm) imagery for various applications.
This MiniSAR with cente frequency at 9.6 GHz, targets a resolution is 0.25 m (Az) x 0.31 m (Gr. Range) with 5 km swath, in Stripmap mode operation, with three imaging modes single, dual and circular polarization and a reference calibration mode. The system has been configured with a dual polarized planar array antenna with the dimensions 0.5 m x 0.2 m. It accommodated SAR electronics viz. 50 W SSPA, Receiver, Frequency Generator, Digital Chirp generator (DCG), Data acquisition and compression unit (DACS) and payload controller (PLC) in a single bboard along with electronic power conditioners (EPC) at the backside of the planar antenna. A NAND Flash based Indigenous Solid State recorder was designed and developed with storage capacity of 1 Tb and capable of handling 1.6 Gbps sustained input data rate. Overall weight is 6 kg and DC power requirement is 130 W.
Airborne SAR is prone to irregular motion due to atmospheric turbulences at the low altitude flight path and intentional aircraft manoeuvres. Motion compensation of SAR data before azimuth matched filtering rectifies these motion errors. A GPS assisted ‘inertial navigation system’ (INS) incorporated provides accurate flight information for this purpose.
This MINISAR has provided excellent results during number of airborne test flights in all modes. Results are shared with the users and to industries for further application in UAV and drone platforms.
MiniSAR Payload on Airborne Platform
MiniSAR High Resolution Processed image