Secondary surveillance radar

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(Redirected from Identification friend or foe)

Secondary Surveillance Radar (SSR) is a radio interrogator device installed in air traffic control facilities that interacts with radio transponders installed in aircraft.

1 Overview
2 Operation
3 Modes
4 See also
5 External links

[edit] Overview

The rapid wartime development of radar had obvious applications for air traffic control (ATC) as a means of providing continuous surveillance of air traffic disposition. This increased precision would also permit a reduction in the normal procedural separation standards, which in turn promised considerable increases in the efficiency of the airways system. The advantage of ATC — or Primary Surveillance Radar (PSR) — is that it operates totally independently of the target aircraft; however, that is also one of its key disadvantages, because the correlation of a particular radar return with a specific aircraft requires an identification process. When PSR was the only type of radar available, this was typically achieved by the Controller observing a directed turn by the aircraft.

This conundrum led to the employment of another wartime radar development, the Identification Friend or Foe (IFF) system, which had been created as a means of positively identifying friendly aircraft from enemy. This system, which became known in civil use as Secondary Surveillance Radar (SSR) or in the USA as the Air Traffic Control Radar Beacon System (ATCRBS), relies on a piece of equipment aboard the aircraft known as a 'transponder'. The transponder is a radio receiver and transmitter operating on a radar frequency. The target aircraft's transponder responds to interrogation by the ground station by transmitting a coded reply signal. Furthermore, because the signals being sent in each direction are electronically coded, it is possible to transmit additional information between the ground station’s PSR and the aircraft’s SSR.

[edit] Operation

The purpose of this system is to improve the ability to detect and identify aircraft while it additionally provides automatically the Flight Level (pressure altitude) of a flight. An SSR continuously transmits interrogation pulses (selectively rather than continuously in Mode-4, Mode-5, and Mode-S) as its antenna rotates, or is electronically scanned in space. A transponder on an aircraft that is within line-of-sight range 'listens' for the SSR interrogation signal and sends back a reply that provides aircraft information. The reply sent depends on the mode that was interrogated (see below). The aircraft is then displayed as a tagged icon on the controller's radar screen at the calculated bearing and range. An aircraft without an operating transponder may still be observed by primary radar, but without an identifying tag.

A cross-band beacon is used, which simply means that the interrogation pulses are at one frequency (1030 MHz) and the reply pulses are at a different frequency (1090 MHz).

[edit] Modes

There are several transponder modes, each offering different information

Mode 1 - provide 2-digit 5-bit mission code (military only - cockpit selectable).
Mode 2 - provide 4-digit octal unit code (military only - set on ground for fighters, can be changed in flight by transport aircraft).
Mode 3/A - provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller (military and civilian).
Mode 4 - provide a 3-pulse reply to crypto coded challenge (military only).
Mode 5 - provide a crypto secure version of Mode S and, ADS-B GPS position (military only).
Mode C - provide 4-digit octal code for aircraft's pressure altitude (military and civilian).
Mode S - provide multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24-bit address. (military and civilian).

For civilian flights according to ICAO the modes of operations are the A , C and S

The A mode is based on a 4-digit code using numbers between 0 and 7 assigned by the ATC and set by the pilot enabling identification and monitoring. The mode C is working automatically reading directly the pressure altitude from the aircraft altimeter. The mode S is triggered by a mode-S interrogation and can provide the information that is included in the interrogation signal. For modes A and C, all aircraft receiving the interrogation signal will reply, whereas mode S allows aircraft to be addressed individually. In modern ATC systems the data appear with alphanumeric characters in a tag or label linked to the flight position symbol on the radar screen.