1. Environment that surrounds and affects a system or piece of equipment that operates on the ground.

2. System or part of a system, as of a guidance system, that functions on the ground; the aggregate of equipment, conditions, facilities, and personnel that go to make up a system, or part of a system, functioning on the ground.

In air defense, the portion of the air defense system that provides for the detection, surveillance, and control of airborne objects; normally these are ground-based facilities, but also include overwater facilities, such as picket vessels, and airborne early warning and control aircraft.