A bistatic electronic sensing system (e.g., radar or sonar) has one transmitting and one receiving antenna. The antennas are usually separated by some number of wavelengths. At least some electronics will be at both locations, although the signals are usually sent to a processing system, perhaps at a third location.
Some early applications included radar altimeters, in which the transmitting antenna is on the top of the aircraft and the receiver is on the bottom. The difference between the time of sending the signal from the transmitting antenna, and the time of arrival at the receiving antenna is proportional to the combination of aircraft speed and altitude. Radar altimeters usually modify the transmitted signal in some way, such as frequency modulation, so the receiver can identify the offset from the start of a modulation time cycle.
Bistatic radars also can help detect stealth aircraft. One of the principles of stealth is that it reflects little or no signal in the direction of the transmitting antenna. If the receiver is in a different location than the transmitter, however, it may receive a stronger return than if the transmitting and receiving antennas were colocated. Multistatic radars are even more powerful in counter-stealth applications.