Transponder (aeronautics)

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Cessna ARC RT-359A transponder (beige box), beneath a VHF radio. In this example, the transponder code selected is 1200 for VFR flight (in North American airspace). The green IDENT button is marked "ID". CessnaARC-RT-359ATransponder04.jpg
Cessna ARC RT-359A transponder (beige box), beneath a VHF radio. In this example, the transponder code selected is 1200 for VFR flight (in North American airspace). The green IDENT button is marked "ID".

A transponder (short for transmitter-responder [1] and sometimes abbreviated to XPDR, [2] XPNDR, [3] TPDR [4] or TP [5] ) is an electronic device that produces a response when it receives a radio-frequency interrogation. Aircraft have transponders to assist in identifying them on air traffic control radar. Collision avoidance systems have been developed to use transponder transmissions as a means of detecting aircraft at risk of colliding with each other. [6] [7]

Contents

Air traffic control units use the term "squawk" when they are assigning an aircraft a transponder code, e.g., "Squawk 7421". Squawk thus can be said to mean "select transponder code" or "squawking xxxx" to mean "I have selected transponder code xxxx". [6]

The transponder receives interrogation from the secondary surveillance radar on 1030 MHz and replies on 1090 MHz.

Secondary surveillance radar

Secondary surveillance radar (SSR) is referred to as "secondary", to distinguish it from the "primary radar" that works by passively reflecting a radio signal off the skin of the aircraft. Primary radar determines range and bearing to a target with reasonably high fidelity, but it cannot determine target elevation (altitude) reliably except at close range. SSR uses an active transponder (beacon) to transmit a response to an interrogation by a secondary radar. This response most often includes the aircraft's pressure altitude and a 4-digit octal identifier. [7] [8]

Operation

A pilot may be requested to squawk a given code by an 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 air traffic controller's radar screen will become correctly associated with their identity. [6] [7]

Because primary radar generally gives bearing and range position information, but lacks altitude information, mode C and mode S transponders also report pressure altitude. Mode C altitude information conventionally comes from the pilot's altimeter, and is transmitted using a modified Gray code, called a Gillham code. Where the pilot's altimeter does not contain a suitable altitude encoder, a blind encoder (which does not directly display altitude) is connected to the transponder. Around busy airspace there is often a regulatory requirement that all aircraft be equipped with altitude-reporting mode C or mode S transponders. In the United States, this is known as a Mode C veil. Mode S transponders are compatible with transmitting the mode C signal, and have the capability to report in 25-foot (7.5 m) increments; they receive information from a GPS receiver and also transmit location and speed. Without the pressure altitude reporting, the air traffic controller has no display of accurate altitude information, and must rely on the altitude reported by the pilot via radio. [6] [7] Similarly, the traffic collision avoidance system (TCAS) installed on some aircraft needs the altitude information supplied by transponder signals.

IDENT

All mode A, C, and S transponders include an "IDENT" switch which activates a special thirteenth bit on the mode A reply known as IDENT, short for "identify". When ground-based radar equipment [9] receives the IDENT bit, it results in the aircraft's blip "blossoming" on the radar scope. This is often used by the controller to locate the aircraft amongst others by requesting the ident function from the pilot, e.g., "Cessna 123AB, squawk 0363 and ident". [6] [7]

Ident can also be used in case of a reported or suspected radio failure to determine if the failure is only one way and whether the pilot can still transmit or receive, but not both, e.g., "Cessna 123AB, if you read, squawk ident". [7]

Transponder codes

Transponder codes are four-digit numbers transmitted by an aircraft transponder in response to a secondary surveillance radar interrogation signal to assist air traffic controllers with traffic separation. A discrete transponder code (often called a squawk code) is assigned by air traffic controllers to identify an aircraft uniquely in a flight information region (FIR). This allows easy identification of aircraft on radar. [6] [7]

Codes are made of four octal digits; the dials on a transponder read from zero to seven, inclusive. Four octal digits can represent up to 4096 different codes, which is why such transponders are sometimes described as "4096 code transponders". [10]

The use of the word "squawk" comes from the system's origin in the World War II identification friend or foe (IFF) system, which was code-named "Parrot". [11] [12]

Codes assigned by air traffic control

Some codes can be selected by the pilot if and when the situation requires or allows it, without permission from air traffic control (ATC). Such codes are referred to as "conspicuity codes" in the UK. [13] Other codes are generally assigned by ATC units. [6] [7] For flights on instrument flight rules (IFR), the squawk code is typically assigned as part of the departure clearance and stays the same throughout the flight. [6] [7]

Flights on visual flight rules (VFR), when in uncontrolled airspace, will "squawk VFR" (1200 in the United States and Canada, 7000 in Europe). Upon contact with an ATC unit, they will be told to squawk a certain code. When changing frequency, for instance because the VFR flight leaves controlled airspace or changes to another ATC unit, the VFR flight will be told to "squawk VFR" again. [6] [7]

In order to avoid confusion over assigned squawk codes, ATC units will typically be allocated blocks of squawk codes, not overlapping with the blocks of nearby ATC units, to assign at their discretion.

Not all ATC units will use radar to identify aircraft, but they assign squawk codes nevertheless. As an example, London Information—the flight information service station that covers the southern half of the UK—does not have access to radar images, but does assign squawk code 1177 to all aircraft that receive a flight information service (FIS) from them. This tells other radar-equipped ATC units that a specific aircraft is listening on the London Information radio frequency, in case they need to contact that aircraft. [13]

Emergency codes

The following codes are applicable worldwide.

CodeUse
7500 Aircraft hijacking (ICAO) [6] [14]
7600Radio failure (lost communications) (ICAO) [6] [14]
7700Emergency (ICAO) [6] [14]

See List of transponder codes for list of country-specific and historic allocations.

See also

Related Research Articles

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<span class="mw-page-title-main">Transponder</span> Device that emits an identifying signal in response to a received signal

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<span class="mw-page-title-main">Air traffic control</span> Service to direct pilots of aircraft

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<span class="mw-page-title-main">Identification friend or foe</span> Command or control enemy distinction through radio frequencies

Identification, friend or foe (IFF) is a combat identification system designed for command and control. It uses a transponder that listens for an interrogation signal and then sends a response that identifies the broadcaster. IFF systems usually use radar frequencies, but other electromagnetic frequencies, radio or infrared, may be used. It enables military and civilian air traffic control interrogation systems to identify aircraft, vehicles or forces as friendly, as opposed to neutral or hostile, and to determine their bearing and range from the interrogator. IFF is used by both military and civilian aircraft. IFF was first developed during World War II, with the arrival of radar, and several friendly fire incidents.

<span class="mw-page-title-main">Traffic collision avoidance system</span> Aircraft collision avoidance system

A traffic alert and collision avoidance system (TCAS, pronounced TEE-kas), also known as an Airborne Collision Avoidance System (ACAS), is an aircraft collision avoidance system designed to reduce the incidence of mid-air collision (MAC) between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of MAC. It is a type of airborne collision avoidance system mandated by the International Civil Aviation Organization to be fitted to all aircraft with a maximum take-off mass (MTOM) of over 5,700 kg (12,600 lb) or authorized to carry more than 19 passengers. CFR 14, Ch I, part 135 requires that TCAS I be installed for aircraft with 10-30 passengers and TCAS II for aircraft with more than 30 passengers. ACAS/TCAS is based on secondary surveillance radar (SSR) transponder signals, but operates independently of ground-based equipment to provide advice to the pilot on potentially conflicting aircraft.

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<span class="mw-page-title-main">Secondary surveillance radar</span> Radar system used in air traffic control

Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC), that unlike primary radar systems that measure the bearing and distance of targets using the detected reflections of radio signals, relies on targets equipped with a radar transponder, that reply to each interrogation signal by transmitting encoded data such as an identity code, the aircraft's altitude and further information depending on its chosen mode. SSR is based on the military identification friend or foe (IFF) technology originally developed during World War II; therefore, the two systems are still compatible. Monopulse secondary surveillance radar (MSSR), Mode S, TCAS and ADS-B are similar modern methods of secondary surveillance.

The air traffic control radar beacon system (ATCRBS) is a system used in air traffic control (ATC) to enhance surveillance radar monitoring and separation of air traffic. It consists of a rotating ground antenna and transponders in aircraft. The ground antenna sweeps a narrow vertical beam of microwaves around the airspace. When the beam strikes an aircraft, the transponder transmits a return signal back giving information such as altitude and the Squawk Code, a four digit code assigned to each aircraft that enters a region. Information about this aircraft is then entered into the system and subsequently added to the controller's screen to display this information when queried. This information can include flight number designation and altitude of the aircraft. ATCRBS assists air traffic control (ATC) surveillance radars by acquiring information about the aircraft being monitored, and providing this information to the radar controllers. The controllers can use the information to identify radar returns from aircraft and to distinguish those returns from ground clutter.

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The aviation transponder interrogation modes are the standard formats of pulsed sequences from an interrogating Secondary Surveillance Radar (SSR) or similar Automatic Dependent Surveillance-Broadcast (ADS-B) system. The reply format is usually referred to as a "code" from a transponder, which is used to determine detailed information from a suitably equipped aircraft.

<span class="mw-page-title-main">Automatic Dependent Surveillance–Broadcast</span> Aircraft surveillance technology

Automatic Dependent Surveillance–Broadcast (ADS-B) is an aviation surveillance technology and form of electronic conspicuity in which an aircraft determines its position via satellite navigation or other sensors and periodically broadcasts its position and other related data, enabling it to be tracked. The information can be received by air traffic control ground-based or satellite-based receivers as a replacement for secondary surveillance radar (SSR). Unlike SSR, ADS-B does not require an interrogation signal from the ground or from other aircraft to activate its transmissions. ADS-B can also receive point-to-point by other nearby equipped ADS-B equipped aircraft to provide traffic situational awareness and support self-separation.

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<span class="mw-page-title-main">Traffic information service – broadcast</span> Aviation information service

Traffic information service – broadcast (TIS–B) is an aviation information service that allows pilots to see aircraft that are not emitting ADS-B data but have a basic transponder.

Airport surveillance and broadcast systems are a set of runway-safety tools that display aircraft on and near an airport.

IFF Mark X was the NATO standard military identification friend or foe transponder system from the early 1950s until it was slowly replaced by the IFF Mark XII in the 1970s. It was also adopted by ICAO, with some modifications, as the civilian air traffic control (ATC) secondary radar (SSR) transponder. The X in the name does not mean "tenth", but "eXperimental". Later IFF models acted as if it was the tenth in the series and used subsequent numbers.

References

  1. ↑ Mangine, Robert (2007). "The Truth About Transponders". Archived from the original on August 16, 2011. Retrieved March 5, 2019.
  2. ↑ Farlex, Inc. (2008). "XPDR" . Retrieved December 24, 2008.
  3. ↑ Farlex, Inc. (2008). "XPNDR" . Retrieved December 24, 2008.
  4. ↑ Farlex, Inc. (2008). "TPDR" . Retrieved December 24, 2008.
  5. ↑ Farlex, Inc. (2008). "TP" . Retrieved December 24, 2008.
  6. 1 2 3 4 5 6 7 8 9 10 11 12 Transport Canada (May 20, 2010). "TP 14371 – Transport Canada Aeronautical Information Manual (TC AIM) RAC 1.9 Transponder Operation". Archived from the original on July 9, 2010. Retrieved August 21, 2010.
  7. 1 2 3 4 5 6 7 8 9 10 Peppler, I. L. (1996). From The Ground Up (27th revised ed.). Ottawa, Ontario: Aviation Publishers. pp. 238–239. ISBN   0-9690054-9-0.
  8. ↑ Vabre, Phil. "Air Traffic Services Surveillance Systems". The Airways Museum & Civil Aviation Historical Society. Retrieved April 15, 2010.
  9. ↑ Rogers, Tom (September 6, 1998). "Transponder Basics". AVweb. Archived from the original on March 4, 2016. Retrieved March 18, 2014.
  10. ↑ "Chapter 14: Airport Operations". Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C ed.). Federal Aviation Administration. July 17, 2023. p. 25.
  11. ↑ Getline, Meryl (April 17, 2006). "Ask the Captain: Strangle my WHAT?". USA Today . Archived from the original on January 23, 2009. Retrieved March 13, 2008.
  12. ↑ Vabre, Phil. "Air Traffic Services Surveillance Systems, Including An Explanation of Primary and Secondary Radar". The Airways Museum & Civil Aviation Historical Society. Retrieved March 13, 2008.
  13. 1 2 "ENR 1.6.2 — SSR Operating Procedures". July 29, 2021. Retrieved September 11, 2021.
  14. 1 2 3 Federal Aviation Administration. "JO 7110.66D, National Beacon Code Allocation Plan". Federal Aviation Administration. Retrieved September 23, 2023.
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