The Indian Air Force has lost contact with AN-32 (transport aircraft) on a mission. ISRO is going to deploy RISAT to search for the missing plane as the search area has widened. How will such satellites help? Are the new navy P8I Posiedon sub-hunter planes not fit to do such a task?
RISAT-1 and RISAT-2 both carry synthetic aperture radar systems which can create detailed maps of the surface below. There has been talk of using these systems to identify wreckage on the ground for quite a while now, but in the late 1990s, when a lot of these ideas were being proposed, the resolution was simply too low to make it a reliable option. NASA actually did some research in this area too, but that's been indefinitely on hold for quite some time.
Here's a sample paper from the late 90s
Abstract: (emphasis mine)
This paper summarizes some results of studies at the Canada Centre for Remote Sensing (CCRS) using Synthetic Aperture Radar (SAR) imagery from spaceborne systems for the detection of crashed aircraft. Studies have been carried out using detected products (intensity values only) and interferometric methods (using complex imagery). Due to the low resolution of single polarization single frequency spaceborne SAR imagery (approximately 8 metres ground range being the best currently available from operational remote sensing satellites), it is seen that such imagery cannot currently be used with much optimism although the techniques themselves show promise. Further study is needed to examine if the better resolutions that will be available from future systems such as RADARSAT-2 make possible the reliable detection of crashed aircraft. Other research, not described in this manuscript, is underway at CCRS through the support of the National Search and Rescue Secretariat examining the contributions to Search and Rescue that can be made using spaceborne polarimetric SAR systems including RADARSAT-2.
According to this article, RISAT-1 has a resolution that can be varied between 50m and 3m, and also has a "spotlight mode" where this resolution can be as good as 1m. This is definitely in the range where it could be useful if we have a general idea of where to look.
Realistically, I'm not sure how much good the satellites will do, especially compared with the aircraft and submarines already deployed, but it is an idea that works in theory, so you can't really fault them for going for it.