Airborne Ground Surveillance(AGS) systems

From wikipedia:

Airborne Ground Surveillance (AGS) refers to a class of military airborne radar system used for detecting and tracking ground targets, such as vehicles and slow moving helicopters.

In layman’s terms they’re like the air-to-ground version of AWACS. Surprisingly despite the obvious significance of their applications they’re not as widely used as AWACS, nor is information on them as easy to find.

This thread is to discuss the operational as well as experimental AGS systems and their capabilities.

To start off, there’s the E-8 JSTARS, operated by the USAF.
http://en.wikipedia.org/wiki/E-8_Joint_STARS
http://www.northropgrumman.com/protectjointstars/

The Raytheon Sentinel/ASTOR, operated by the RAF.
http://en.wikipedia.org/wiki/Raytheon_Sentinel
http://www.raytheon.co.uk/capabilities/products/astor/index.html

How does the Sentinel compare to the JSTARS? I imagine the smaller platform would mean a reduced capability overall.