Ground sensors were a type of security system installed around the perimeter of Global Chemicals to deter intruders. The were hidden in the brush fields near the electric fence that encamped the facility.
The Third Doctor accidentally set off a ground sensor when he used a South Wales Electricity board Land Rover to jump the electric fence. He had been given the floor plans of Global Chemicals, but had no knowledge of the sensors in place, and ran too close to one which he couldn't see, tripping it. This alerted BOSS and Stevens to his location. (TV: The Green Death)
