A system for the surveillance of terrain and the detection of intrusions over a plane extending into that terrain. A curtain array of light beams is projected along the plane and reflections from the terrain are detected by a sensor array essentially spatially coincident with the array of light sources. The times of flight of the beams are determined, and these characterize the form of the terrain being surveilled. The initial background reflection pattern is acquired and stored by the system. A sudden change in this detected background pattern can be defined as arising from an unexpected reflection, indicative of an intrusion. Signal processing systems are described utilizing modulated laser beams and detection at a frequency at least twice that of the modulation, such that reflected signals arising from the ON and the OFF periods of the laser modulation can be subtracted to eliminate the background signals.