Another type of transponder occurs in
identification friend or foe (IFF) systems in
military aviation and in
air traffic control secondary surveillance radar (beacon radar) systems for
general aviation and
commercial aviation.
Primary radar works best with large all-metal aircraft, but not so well on small, composite aircraft. Its range is also limited by terrain and rain or snow and also detects unwanted objects such as automobiles, hills and trees. Furthermore, it cannot always estimate the altitude of an aircraft.
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. Depending on the type of interrogation, the transponder sends back a
transponder code (or "squawk code", Mode A) or altitude information (Mode C) to help air traffic controllers to identify the aircraft and to maintain separation between planes. Another mode called Mode S (Mode Select) is designed to help avoiding over-interrogation of the transponder (having many radars in busy areas) and to allow automatic collision avoidance. Mode S transponders are
backward compatible with Modes A and C. Mode S is mandatory in
controlled airspace in many countries. Some countries have also required, or are moving toward requiring, that all aircraft be equipped with Mode S, even in
uncontrolled airspace. 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. 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. The transponders re-radiate the signals allowing for much longer communication distances.
Marine The
International Maritime Organization's
International Convention for the Safety of Life at Sea (SOLAS) requires the
Automatic Identification System (AIS) to be fitted aboard international voyaging ships with , and all passenger ships regardless of size. AIS transmitters/receivers are generally called
transponders, but they generally transmit autonomously, although
coast stations can interrogate
class B transponders on smaller vessels for additional information. In addition,
navigational aids often have transponders called
RACON (radar beacons) designed to make them stand out on a ship's radar screen.
Sonar transponders operate under water and are used to measure distance and form the basis of underwater location marking, position tracking and
navigation. ==Other applications==