This paper presents an overview of the estimation of noise, techniques for noise removal in Single Look Complex (SLC) as well as hybrid polarimetry decomposed images and the effects of noise removal in SAR images, using RISAT-1 data. Thermal noise affects the signal to noise ratio as well as radiometry of the SAR images. Different approaches adopted for estimating the thermal noise using onboard noise measurements and from the noise equivalent area of the images like calm reservoirs, lakes, etc., for RISAT-1 SAR are discussed. Subsequent to noise removal, its effect on minimum detectable noise and SNR of the images is addressed. Traditional noise removal methods affect the phase of the data, which in turn affect advanced SAR applications. A brief overview of the hybrid polarimetry configuration of RISAT-1 SAR, one of the emerging trends in polarimetry domain, is given and the effect of using noise removed single look complex (SLC) images for polarimetry decomposition is brought out. Thereby, a new technique for thermal noise removal in polarimetry decomposed data is presented.
The Indian Space Research Organisation (ISRO) and the National Aeronautics and Space Administration (NASA) have jointly embarked on NASA-ISRO Synthetic Aperture Radar (NISAR) operating in L-band and S-band, which will map Earth’s surface every 12 days. As a pre-cursor to the NISAR mission, ISRO is planning an airborne SAR (L&S band) which will deliver NISAR analogue data products to the science community. ISRO will develop all the hardware with the aim of adhering to system design aspects of NISAR to the maximum extent possible. It is a fully polarimetric stripmap SAR and can be operated in single, dual, compact, quasi-quad and full polarimetry modes. It has wide incidence angle coverage from 24°–77° with swath coverage from 5.5km to 15 km. Apart from simultaneous imaging operations, this system can also operate in standalone L/S SAR modes. This system is planned to operate from an aircraft platform with nominal altitude of 8000meters. Antenna for this SAR will be rigidly mounted to the aircraft, whereas, motion compensation will be implemented in the software processor to generate data products. Data products for this airborne SAR will be generated in slant & ground range azimuth dimension and geocoded in HDF5/Geotiff formats. This airborne SAR will help to prepare the Indian scientific community for optimum utilization of NISAR data. In-order to collect useful science data, airborne campaigns are planned from end of 2016 onwards.