A Tracking transmitter broadcasts a radio

Radio

Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 signal which can be detected by a directional antenna

Directional antenna

A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....

 (typically a Radio Direction Finder

Radio direction finder

A radio direction finder is a device for finding the direction to a radio source. Due to low frequency propagation characteristic to travel very long distances and "over the horizon", it makes a particularly good navigation system for ships, small boats, and aircraft that might be some distance...

.)
By rotating the antenna one can determine the direction the signal lies in and of course whatever it may be attached to. The EPIRB is an example of a similar device.

It is commonly used in model rocketry and remote control aircraft to locate lost equipment.