Tapan Misra, and A.S. Kirankuma

From a nascent beginning with development of X-band SLAR, ISRO has come a long way in mastering the SAR based imaging technology. Imaging Radar development started with a development of X-band SLAR in late 1980’s and early 1990’s. Though the technology was comparatively simple, this experiences led to development of India’s first Airborne SAR (ASAR) in C-band in 90’s. The system was capable of imaging a swath of 25 km with 6 m resolution. SAR technology requires not only mastering sophisticated radar hardware but more importantly, its complex processing algorithm and implementation of immensely large computing speed required for processing. ASAR also ushered in sophisticated SAR processing systems, capable of processing SAR data under severe motion errors.

A more sophisticated version of C-band airborne SAR was developed for operational use, specifically for disaster management, called DMSAR (SAR for Disaster management) in 2005 time frame. The system was capable of imaging at various resolutions and various swath configurations. The best resolution possible by this system is 1 m. Total two versions of DMSARs were built and it is still under operation.

ISRO graduated from airborne SAR systems to sophisticated spaceborne SAR systems leading to the development of Radar Imaging Satellite (RISAT-1). RISAT-1 was launched on 26th April, 2012 and is available to international SAR community and researchers as a source of multi resolution/multi-swath/multi-polarisation SAR data. The first satellite RISAT-1 carries a multi-mode Synthetic Aperture Radar (SAR) in C-band as the sole payload. RISAT-SAR payload operates from a sun synchronous orbit at a nominal altitude of 536 km. RISAT-1 supports sophisticated imaging modes from strip map to scanSAR to spotlight. All the above mentioned imaging modes can be operated in Hybrid polarimetry mode, available in spaceborne SAR systems for the first time.

RISAT-1 incorporates a crucial antenna technology called active antenna. The active antenna has nominal aperture of 6mx2m, consisting of 12 tiles which are grouped into 3 panels. There are 288 pairs of TR modules feeding H and V polarizations separately..

The RISAT-1 spacecraft has been built around the SAR payload in order to optimise the spacecraft weight and structure. In-orbit mass of the satellite will be around 1858 kg, of which the SAR payload contributes around 950 kg. The spacecraft resources permit operation of maximum of 10 minutes duration in each orbit. The satellite is placed in a sun-synchronous orbit with 6 am-6 pm equatorial crossing, to maximize solar power availability.

From RISAT-1 experience, ISRO has embarked upon, in a joint development with NASA, a sophisticated, Digital Beam Former based SAR for both interferometric as well as polarimetric operation. This SAR is called NISAR (NASA-ISRO SAR) and is capable of providing high resolution over a very wide swath, with dual frequency operation in L and S band. By 2020 time frame, NISAR system will be the sole operational dual frequency SAR, one of its kind.

The SAR technology is going to be mainstay of imaging sensors in ISRO’s remote sensing programme. Already plans are on to provide imaging capability in other microwave bands.