Transponder

transponder is a component which, on receiving an ElectroMagnetic (EM) signal, often coded, will respond by sending a similar signal, usually after a known, controlled delay time. In telecommunications, the term transponder is a contraction of transmitter responder, sometimes abbreviated to XPDR, XPNDR, TPDR, has the following meanings:

  • an automatic device that receives, amplifies, and retransmits a signal on a different frequency;
  • an automatic device that transmits a predetermined message in response to a received and predetermined signal;
  • a receiver-transmitter (transceiver) that generates a signal in response to a specific query;
  • a device that accomplishes the conversion of a bidirectional (full duplex) electrical signal to a bidirectional (full duplex) optical signal and vice versa.

In particular, the telecommunications satellite channels are called transponders because they perform the transposition of the receiving channel (called uplink) with the transmitting channel (called downlink). With digital television, data compression, and multiplexing, several video and audio channels can travel through a single transponder on a single broadband carrier. The original analog video signal has only one channel per transponder, with sub-carriers 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 an entire transponder, or using a terrestrial communication line to send the satellite to a terrestrial station to multiplex with other stations.

Another example of transponders is found in military employment systems with IFF (friend-enemy identification), air traffic control systems, and secondary radars (beacon radars). Transponders are used, according to ICAO standards, by both military and commercial aviation aircraft to identify aircraft and make the aircraft visible for air traffic purposes. Most transponders are capable of transmitting information about altitude, type of flight, and even the presence of hijackers on board; they use a four-digit code known as a transponder code or “squawk code,” and it is used by air traffic controllers to enforce previously planned safety distances and airways.

Sonar transponders operate underwater and are used to measure distance and form the basis of underwater position marking, position tracking and navigation.

Transponder code

In aviation, the transponder code is a numerical code that is selected on the transponder equipment of an aircraft to facilitate flight management by an air traffic controller. In aeronautical phraseology it is called squawk code, while in military jargon it is also known as IFF code, acronym of identification friend or foe.

The transponder code selected by the pilot appears on the radar screen on the ground and helps to match the tracks observed on it to the various flights, a procedure that is called radar identification.

Depending on the model of the equipment, the code is composed of two or four digits and the selectable numbers are in Base 8 (from 0 to 7), allowing in the first case the potential selection of only 64 codes and in the second case of 4096 codes. The code, once selected on the on-board device, is sent to a secondary radar. The code, then, appears on the radar screen located at a ground station and displayed in correspondence of the aircraft’s light trail. This combination allows the operator, who has instructed the pilot to select this code, to be certain about the identity of the electronic track.

An aircraft whose equipment allows for the selection of only two digits for a maximum total of 64 potential codes, will not be assigned a four digit code, even though on the screen of a radar system capable of processing 4096 codes it will still appear as a four digit number with the last two equal to double zero (00): for example the assigned code 42 will appear on the radar screen as 4200.

On the other hand, when an aircraft whose on-board equipment allows the selection of 4 digits for a total of 4096 codes is operated by a secondary surveillance radar capable of only 2 digits, it will still have to select on the transponder four digits, the last two of which will be double zeros: for example, if the radar agency assigns the code 54, the pilot will select on the on-board transponder the code 5400.

Applications

Specifically, channels on telecommunications satellites are called transponders because they perform the transposition of the receiving channel (called the uplink) with the transmitting channel (called the downlink). With digital television, data compression and multiplexing, multiple video and audio channels can be carried on a single transponder on a single broadband carrier. The original analog video signal has only one channel per transponder, with subcarriers for audio and Automatic Transmission Identification Service (ATIS). Non-multiplexed radio stations can also operate 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 instead of paying for an entire transponder, or to use a fixed line to send the satellite to a ground station for multiplexing with other stations.

Another example of transponders is found in military IFF (Identification Friend or Foe) systems, air traffic control systems, and secondary radar (beacon radar). Transponders are used by both military and commercial aircraft in accordance with ICAO standards to identify aircraft and make them visible to air traffic. Most transponders are capable of transmitting information about altitude, type of flight, and even the presence of hijackers on board; they use a four-digit code, known as a transponder code or “squawk code,” which is used by air traffic controllers to enforce pre-planned safety distances and airways.

Sonar transponders operate underwater and are used to measure distance and provide the basis for underwater position determination, tracking and navigation.

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