Paradigm shifting contributions of ISRO in Radar technology
Vikram Sarabhai once said, “He who can listen to the music in the midst of noise can achieve great things.” And ISRO as an institution not only listened to the music but also created its own.
Our founder Tapan Misra had been a part of ISRO, the country’s national space agency, for over 3 decades. Here is what he has to say about the institution:
ISRO suffers from typical Indian mindset - not ready to claim what is its genuine achievement. Do you know that ISRO's first Radar Imaging Satellite was just not a path breaking technical marvel, but rewrote the new standards and innovations with which SAR systems are to be designed?
ISRO introduced many innovations in its RISAT programme:
1. As far as polarimetry is concerned, there are two eras: Pre RISAT and Post RISAT. In Pre RISAT era, polarimetry is based on 4 linear polarisation based distinct SAR imaging mode, an exclusive polarimetric mode over a very limited range of low incidence angle. In Post RISAT era, polarimetry was democratised. RISAT introduced for the first time the hybrid polarimetry, cutting down the data rate by half. The hybrid polarimetry can be operated seamlessly in any imaging mode and at any incidence angle. So RISAT was the first to demonstrate availability of polarimetry in spotlight and scanSAR modes.
2. To support Hybrid polarimetry, RISAT rewrote the SAR architecture. Among spaceborne SARs, it introduced operation with both linear as well as circular polarisation, a unique feature itself.
3. ISRO delivered the first spotlight SAR image in C band, a big departure from conventional wisdom that spotlight is a too complicated an operation and SAR processing for it to be available in microwave bands lower than X-band.
4. ISRO introduced for the first time the concept of sliding spotlight imaging mode. In Pre RISAT era, spotlight mode imaging was truly limited to a small spot, typically of 1 km x 1 km to 5 km x 5 km size. Users also had to bear with the annoying antenna pattern modulation along both azimuth and range direction. RISAT's sliding spotlight mode gave for the first time a spotlight imaging capability over 10 km x 100 km, with antenna pattern modulation only along range, but strip map like uniformity along azimuth, essentially improving SNR at the edges by 6 dB. Just unbelievable.