Radio Tracking
Radio collars transmit radio waves of a specific frequency at a rate of approximately one signal per minute (can be varied). A directional antennae and receiver are needed in order to pick up the transmitted signal. This equipment can either be hand held as the biologist walks or drives slowly through the area, or it can be mounted on the wings of a plane. A very accurate location of the collared animal can then be determined by a field triangulation method which requires the use of GPS locates, compass directions and maps.
Excellent resources on radio telemetry:
Handbook of Animal Radio-Tracking
University of Minnesota Press
ATS (Advanced Telemetry Systems)