Why Do We Need More Radar Imaging Satellites (Naval Point of View)?

NISAR Radar Satellite- ISRO/NASA JV: Image for representation only

Radar Imaging Satellites are an indispensable part of remote sensing capabilities of a nation. Unlike satellites with sensors operating in the visible spectrum, radar satellites can 'see through' clouds and record data even at night. Radar imaging satellites employ Synthetic Aperture Radar (SAR) to obtain images of the terrain. These satellites can cover a wide swath of terrain with high resolution.
Radar satellites play an important role in maritime security and safety, as they can detect and locate targets as small as fishing boats. Radar imagery is very useful for this role because -

• Wide coverage area
• Operation unaffected by weather conditions

Fun fact: When operating in surveillance mode, radar satellites are actually looking for ship wakes, not the ship itself!

'The wake is the region of disturbed flow (often turbulent) downstream of a solid body moving through a fluid, caused by the flow of the fluid around the body.' (Wake-Wikipedia)
Ship wakes are divided into Kelvin and turbulent wakes, with the latter being more visible to SAR. Any object travelling on the water surface will create wakes, and if the wakes are large enough to be detected by SAR then it wouldn't matter if the ship itself is stealthy or not -it will be detected.

freighter as seen by SAR, Turbulent wake (yellow lines) and boundaries of the ship wake (red lines), Image: Centre for Remote Imaging, Sensing and Processing (CRISP), licensed under CC BY 4.0

CC BY 4.

During the cold war, Soviet Russia launched a fleet of radar satellites (39 launches, including test units) called Upravlyaemy Sputnik Aktivnyy or US-A, also known as Radar Ocean Reconnaissance Satellite or RORSAT. These satellites were launched to monitor/track NATO and merchant vessels. Another mission objective of RORSATs was to acquire targets for long-range anti-ship missiles used by Soviet Navy (Range of shipborne radars is limited to radar horizon). These satellites would enable the Soviet navy to launch a massive missile strike on NATO flotilla without putting their own ships and submarine at risk.

Up until 2016, India operated two radar satellites, namely RISAT-2 (TacSAR) and RISAT-1. Even though RISAT-1 was already under development, special priority was given to Radar Imaging Satellite program with the fast-tracked launch of RISAT-2 (equipped with an Israeli X-band antenna) after 2008 Mumbai attacks. It is likely (even though never officially admitted) both satellites are equipped with Vessel Detection System (VDS). India has lost RISAT-1 to orbital debris and RISAT-2 is well past planned mission duration. New RISAT-1A is planned to be launched this year, but, the numbers are nowhere near actual requirement.

why do we need more RISAT? 

1. Additional surveillance assets are required to monitor Chinese incursions in IOR and India doesn't have anything like Global Hawk...yet. The endurance of Searcher-2 and Heron UAVs is relatively short. 
2. The Navy needs more eyes in the sky (more like space), that's the only way navy could effectively monitor huge expanse of IOR in near real time. 
3. Maximum exploitation of the extended range of BrahMos (450-600 km) without putting lives at risk when operating outside UAV coverage area - IN would use helicopters/submarines to scout ahead for targets(Over the Horizon Targeting, OTH-T) and relay targeting data back to the ship, which is a dangerous task, especially for helicopters. After detection of an enemy flotilla by satellites, additional assets could be called in to identify and classify targets.

IN585 Kamov KA.28 Indian Navy aeroprints.com [CC BY-SA 3.0 via Wikimedia Commons]

Additional 'stuff' 

AESA Radar of RISAT : 

RISAT-1 active antenna, deployed form

 Dual Polarised Antenna

printed microstrip TRM unit