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A transponder is a coupled radio or radar receiver-transmitter that will generate a reply signal, upon proper interrogation. Transponders aboard airplanes are the basis of modern air traffic control (ATC); it is incorrect to assume that ATC tracks aircraft by radar alone.

In maritime safety, the new automatic identification system will provide similar collision avoidance and traffic management for vessels.

Military identification-friend-or-foe systems are based on transponders, which, when sent an appropriately verifiable signal, respond with a "don't kill me" response that avoids fratricide. Such systems, in combat, have to be designed to respond only to securely authenticated interrogators, so they do not give away position information to an enemy.

Aviation Types

Originally, military and civilian air traffic control were totally separate. There has been an initiative, however, to enable military types to work in both systems. There are and will be different mechanisms needed in a military environment.

Type User Function
Mode C Civilian Obsolescent
Mode S Civilian Current, although ADS-B in future
Mode 1 Military One of 64 aircraft type and mission codes
Mode 2 Military One of 4096 "tail number" codes
Mode 3/A Military and civilian Altitude information
Mode 4 Military Current secure challenge/response
Mode 5 Military Future secure challenge/response usable with ADS-B

While radar is part of the air traffic control system, the reality is that the primary identification technique is a transponder response, which is displayed in preference to radar-only. One of the problems of the defense response to the 9-11 attacks is that the hijackers turned off the transponders, so fighters could identify the specific aircraft only by visually checking the "tail number" painted on the tail. Had transponder information been available, fighters would still have had difficulty making an intercept, but without it, only great luck would lead them to the correct aircraft.

The Traffic Collision and Avoidance System (TCAS) and Enhanced TCAS (ETCAS) are separate from, but complement, the ATC oriented transponder system. In TCAS, aircraft broadcast identification information to others nearby, and an aircraft computer determine if there is a threat of collision. If so, it will immediately alert both pilots about the threat and what action to take. The actions must be "deconflicted", so one plane, for example, knows it should climb while the other knows it should dive.

A combination of Modes C and 3/A is the standard air traffic control mode, but Mode S is the next generation, and Automatic Dependent Surveillance-Broadcast (ADS-B) goes beyond.

Mode C

Civilian interrogators challenge in Mode C, and then alternating 3 and A. Mode C is labor intensive, nd subject to error if a code is entered incorrectly. Each aircraft under an air traffic control flight plan is given a identification code, for that flight, by the airport departure controller.

That four-digit code must be manually set into the transponder control panel. It is used internationally, in conjunction with the automatic altitude reporting mode (Mode C), to provide positive control of all aircraft flying under instrument flight rules. Such aircraft are assigned unique mode3/A codes by the airport departure controller. General aviation aircraft not under a flight plan all use Mode 3/A code of 1200. There are several codes that can be used to signal emergencies, such as 7700 as the basic "MAYDAY".

Once en route, the aircraft leaves the terminal air control center and contacts the en route traffic center, who may assign a new code if the one assigned on takeoff already is in use. On long flights, the aircrew may have to work with many en route controllers, changing the code as necessary, until they reach their destination and contact the approach controller, who may assign yet another code.

The en route controller provides additional flight instructions and may assign a new Mode 3/A code in the event of conflicts in his control zone. On a transcontinental flight, the aircraft passes through dozens of such zones until it is handed over to the approach controller at its destination.

At any time, a controller may ask the pilot to "squawk ID", which retransmits the current code and gives the exact position.

Mode S

The greatest difference in Mode S is there will be no more manual code setting. Every physical aircraft will be assigned a unique beacon code, from a list of over 16 million. As the Mode S aircraft enters a controller's responsibility, the Mode S transponder will give the aircraft unique code, which will be looked up by ATC computers and automatically associated with a specific flight. Once that match is made, the controller can select "Delta flight DL 102", and the computer will map that to the unique aircraft number and interrogate that aircraft alone, getting its current position, altitude, and other information.

Mode S can coexist with mode C.


TIS-B is a service which provides Automatic Dependant Surveillance - Broadcast (ADS-B) equipped aircraft with position reports from secondary surveillance radar on non-ADS-B equipped aircraft.

Tests by general aviation pilots in Alaska and air transport carriers in the Ohio River Valley, the FAA determined in 2005 that ADS-B is ready to be made operational throughout the national airspace system.

With ADS-B, both pilots and controllers will see radar-like displays with highly accurate traffic data from satellites – displays that update in real time and don't degrade with distance or terrain. The system will also give pilots access to weather services, terrain maps and flight information services. The improved situational awareness will mean that pilots will be able to fly at safe distances from one another with less assistance from air traffic controllers.[1]


Military requirements grew out of identification-friend-or-foe, but that is no longer practical when military aircraft have to fly through civilian airspace. A representative military transponder, for civilian compliance, needed, will support Modes S and C, TCAS, and ADS-B. The most widely used military IFF transponder is the AN/APX-100.


The equivalent of transponders for ships is the automatic identification system (AIS). The "fast broadcast' is sent every 2 to 10 seconds while underway, and every 3 minutes while at anchor at a power level of 12.5 watts. "Slow broadcasts" repeat every 6 minutes.[2]

Fast broadcast datum Source Slow broadcast datum source
Maritime Mobile Service Identifier (MMSI) Administratively assigned and manually programmed Maritime Mobile Service Identifier (MMSI) Administratively assigned and manually programmed
Navigation status Standard codes such as "at anchor", "not under command", etc. International Maritime Organization (IMO) number Administratively assigned and manually programmed
Rate of turn Specific rate of turn indicator or autopilot Radio call sign Administratively assigned and manually programmed
Speed over ground Navigation computer, possibly autopilot Cargo type Defined in IMO table; manually programmed
Position accuracy GPS type (e.g., regular, DGPS) and if RAIM[3] is in use Name of ship Manually programmed
Longitude GPS Dimensions of ship Manually programmed
Course over ground Fluxgate compass or GPS compass computer Location on ship where reference point for position reports is located Manually programmed
True Heading Gyro input Type of position fixing device Manually programmed (e.g., GPS, undefined)
Time stamp GPS Draught of ship Manually provided
Destination Manually provided
Estimated time of Arrival at destination Manually provided


  1. Federal Aviation Administration, ADS-B General Information
  2. United States Coast Guard Navigation Center, What AIS broadcasts
  3. Research and Innovative Technology Administration (RITA). U.S. Department of Transportation (US DOT), Integrity Monitoring