Transponder

Original analog video only has one channel per transponder, with subcarriers for audio and automatic transmission identification service (ATIS).

Secondary radar overcomes these limitations but it depends on a transponder in the aircraft to respond to interrogations from the ground station to make the plane more visible.

However, in the field of general aviation there have been objections to these moves, because of the cost, size, limited benefit to the users in uncontrolled airspace, and, in the case of balloons and gliders, the power requirements during long flights.

[citation needed] Transponders are used on some military aircraft to ensure ground personnel can verify the functionality of a missile’s flight termination system prior to launch.

Such radar-enhancing transponders are needed as the enclosed weapon bays on modern aircraft interfere with prelaunch, flight termination system verification performed by range safety personnel during training test launches.

[citation needed] Sonar transponders operate under water and are used to measure distance and form the basis of underwater location marking, position tracking and navigation.

Electronic toll collection systems such as E-ZPass in the eastern United States use RFID transponders to identify vehicles.

[citation needed] Passive and active RFID systems are used in motor sports, and off-road events such as Enduro and Hare and Hounds racing, the riders have a transponder on their person, normally on their arm.

[citation needed] NASCAR uses transponders and cable loops placed at numerous points around the track to determine the lineup during a caution period.