| Satellite/broadcast communications | | | | radar screen. |
| For more details on this topic, see Transponder | | | | Automotive |
| (satellite communications). | | | | Many modern automobiles (especially the more |
| A communications satellite channels are called | | | | expensive models) have keys with transponders |
| transponders, because each is a separate transceiver | | | | hidden inside the plastic head of the key. The owner of |
| or repeater. With digital video data compression and | | | | the car may not even be aware that the transponder |
| multiplexing, several video and audio channels may | | | | is there because there are no buttons to press. When |
| travel through a single transponder on a single | | | | a key is inserted into the ignition lock cylinder and |
| wideband carrier. Original analog video only has one | | | | turned, the car's computer sends a radio signal to the |
| channel per transponder, with subcarriers for audio and | | | | transponder. Unless the transponder replies with a valid |
| automatic transmission identification service ATIS. | | | | code, the computer will not allow the engine to be |
| Non-multiplexed radio stations can also travel in single | | | | started. Transponder keys have no battery; they are |
| channel per carrier (SCPC) mode, with multiple carriers | | | | energized by the radio signal itself. |
| (analog or digital) per transponder. This allows each | | | | Road |
| station to transmit directly to the satellite, rather than | | | | The E-ZPass system in the eastern United States is |
| paying for a whole transponder, or using landlines to | | | | one of many systems for paying bridge and road tolls |
| send it to an earth station for multiplexing with other | | | | by a RFID transponder in the car. The 407 ETR in |
| stations. | | | | Ontario is one of the world's first completely |
| Aviation | | | | automated toll highways. |
| For more details on this topic, see Transponder | | | | Motorsport |
| (aviation). | | | | Transponders are used in motorsport for lap timing |
| Another type of transponder occurs in identification | | | | purposes. A cable loop is dug into the race circuit near |
| friend or foe systems in military aviation and in air | | | | to the start/finish line. Each car has an active |
| traffic control secondary surveillance radar (beacon | | | | transponder with a unique id code. When the racing |
| radar) systems for general aviation and commercial | | | | car passes the start/finish line the lap time and the |
| aviation. Primary radar works best with large all-metal | | | | racing position is shown on the score board. |
| aircraft, but not so well on small, composite aircraft. Its | | | | Passive and active RFID systems are used in off road |
| range is also limited by terrain and rain or snow and | | | | events such as Enduro and Hare and Hounds racing, |
| also detects unwanted objects such as automobiles, | | | | the riders have a transponder on their person, normally |
| hills and trees. Furthermore it cannot always estimate | | | | on their arm. When they complete a lap they swipe or |
| the altitude of an aircraft. Secondary radar overcomes | | | | touch the receiver which is connected to a computer |
| these limitations but it depends on a transponder in the | | | | and log their lap time. The Casimo Group Ltd make a |
| aircraft to respond to interrogations from the ground | | | | system which does this. |
| station to make the plane more visible. | | | | NASCAR uses transponders and cable loops placed |
| Depending on the type of interrogation, the | | | | at numerous points around the track to determine the |
| transponder sends back a transponder code (or | | | | lineup during a caution period. This system replaced a |
| "squawk code") (Mode A) or altitude information (Mode | | | | dangerous race back to the start-finish line. |
| C) to help air traffic controllers to identify the aircraft | | | | Underwater |
| and to maintain separation. Another mode called Mode | | | | Sonar transponders operate under water and are |
| S (Mode Select) is designed to help avoiding | | | | used to measure distance and form the basis of |
| overinterogation of the transponder (having many | | | | underwater location marking, position tracking and |
| radars in busy areas) and to allow automatic collision | | | | navigation. |
| avoidance. Mode S transponders are 'backwards | | | | Gated communities |
| compatible' with Modes A & C. Mode S is | | | | Transponders may also be used by residents to enter |
| mandatory in controlled airspace in many countries. | | | | their gated communities. |
| Some countries have also required, or are moving | | | | References |
| towards requiring, that all aircraft be equipped with | | | | ^ XPDR |
| Mode S, even in uncontrolled airspace. However in the | | | | ^ XPNDR |
| field of general aviation, there have been objections to | | | | ^ TPDR |
| these moves, because of the cost, size, limited benefit | | | | ^ TP |
| to the users in uncontrolled airspace, and, in the case | | | | Department of Defense Dictionary of Military and |
| of balloons and gliders, the power requirements during | | | | Associated Terms |
| long flights. | | | | External links |
| Marine | | | | Transponding with DCC - Transponding in model |
| Navigational aids often have transponders called | | | | railroading. |
| RACON designed to make them stand out on a ship's | | | | |