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Lecture 2.12 Module 2. AVIATION TELECOMMUNICATION SYSTEMS Topic 2.12. EQUIPMENT FOR SATELLITE COMUNICATION.
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Lecture 2.12Module 2. AVIATION TELECOMMUNICATION SYSTEMSTopic 2.12. EQUIPMENT FOR SATELLITE COMUNICATION
TransponderIn telecommunication, the term transponder (short-for Transmitter-responder and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) has the following meanings:- An automaticdevice that receives, amplifies, and retransmits a signal on a different frequency (see also broadcast translator).- An automatic device that transmits a predetermined message in response to a predefined received signal.- A receiver-transmitter that will generate a reply signal upon proper electronic interrogation.
Satellite/Broadcast communicationsA communications satellite’s channels are called transponders, because each is a separate transceiver or repeater. With digital videodata compression and multiplexing, several video and audio channels may travel through a single transponder on a single widebandcarrier. Original analog video only has one channel per transponder, with subcarriers for audio and automatic transmission identification service ATIS. Non-multiplexed radio stations can also travel in single channel per carrier (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder, or using landlines to send it to an earth station for multiplexing with other stations.
Transponder (satellite communications)A communications satellite’s channels are called transponders, because each is a separate transceiver or repeater. Most comsats are microwave radio relay stations in orbit, and carry dozens of transponders, each with a bandwidth of tens of megahertz. Most transponders operate on a "bent pipe" principle, referring to the sending back of what goes into the conduit with only amplification and a shift from uplink to downlink frequency, as opposed to a 'regenerative' system whereby the signal is used to remake and remodulate the signal.
With data compression and multiplexing, several video (including digital video) and audio channels may travel through a single transponder on a single widebandcarrier.Original analog video only has one channel per transponder, with subcarriers for audio and automatic transmission identification service ATIS. Non-multiplexed radio stations can also travel in single channel per carrier (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder, or using landlines to send it to an earth station for multiplexing with other stations.
AviationAnother type of transponder occurs in identification friend or foe systems in military aviation and in air traffic controlsecondary 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. Another mode called Mode S (Mode Select) is designed to help avoiding overinterogation of the transponder (having many radars in busy areas) and to allow automatic collision avoidance. Mode S transponders are 'backwards compatible' with Modes A & C. Mode S is mandatory in controlled airspace in many countries. Some countries have also required, or are moving towards 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.
Transponder (aviation)A transponder (short-for Transmitter-responder and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) is an electronic device that produces a response when it receives a radio-frequency interrogation. In aviation, aircraft have transponders to assist in identifying them on radar and on other aircraft's collision avoidance systems.
Air traffic control units use the term "squawk" when they are assigning an aircraft a transponder code, e.g. "Squawk 7421". Squawk or squawking thus can be said to mean "select transponder code" or "I have selected transponder code xxxx".HistoryThe aviation transponder was originally developed during World War IIby the British and American military as an "Identification friend or foe" (IFF) system to differentiate friendly from enemy aircraft on radar. The concept became a core of NORAD technology in the defence of North America during the Cold War.This concept was adapted in the 1950s by civil air traffic control using secondary surveillance radar (beacon radar) systems to provide traffic services for general aviation and commercial aviation.
Secondary Surveillance RadarSecondary Surveillance Radar is referred to as "secondary", to distinguish it from the "primary radar" that works by passively bouncing a radio signal off the skin of the aircraft. 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 not all primary radars can 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 aircraft more visible and to report the aircraft's altitude.
Transponder modesOperationA pilot may be requested to squawk a given code by the air traffic controller via the radio, using a phrase such as "Cessna 123AB, squawk 0363". The pilot then selects the 0363 code on their transponder and the track on the radar screen of the air traffic controller will become correctly associated with their identity.
