A sensor tower uses passive and active thermal and electromagnetic scanning along with input from seismic sensors and Doppler analysis, enabling it to pinpoint any target within range. Even foes hiding behind terrain features can be detected, although they won't be identified without visual verification.
The multi-spectrum output enables foes to easily detect the presence of a sensor tower and identify the location of the base that holds it.
Other players can see the radius as a colored circle on their minimap, revealing its location.
In the single-player campaign, the player is given access to the sensor tower in the mission "Outbreak". In this mission a tower is pre-constructed on a hill overlooking the main entrances to their base to allow forward knowledge of attacks from the infested colonists.
Some strategies revolve around "spoofing" the tower with decoy units or simply staying outside its range. Terran commanders have been known to counter by erecting sensor towers in fake base locations just to keep enemies guessing.