The AN/TPQ-37 Firefinder anti-artillery system is a mobile phased-array radar
system used by the US Army to detect enemy artillery
The radar system finds the location of enemy artillery units by tracking the ballistic path of the incoming shells and back-plotting their course to determine where they came from. This information can then be sent via the AN/GSG-10 TacFire automated artillery control system to friendly artillery units to guide their counterbattery fire.
The only real drawback to the system is that the enemy has to be shooting at you before you can find out where he is.