Secondary surveillance radar
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Secondary Surveillance Radar (SSR) is a radar system used in air traffic control (ATC), which not only detects and measures the position of aircraft but also requests additional information from the aircraft itself such as its identity and altitude. Unlike primary radar systems, which measure only the range and bearing of targets by detecting reflected radio signals, rather like seeing an object in a beam of light, SSR relies on its targets being equipped with a radar transponder, which replies to each interrogation signal by transmitting its own response containing encoded data. SSR is based on the military Identification Friend or Foe (IFF) technology originally developed during the second world war, and the two systems are still compatible today.
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[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. Precise knowledge of the positions of aircraft would permit a reduction in the normal procedural separation standards, which in turn promised considerable increases in the efficiency of the airways system. This type of radar (now called a 'primary' radar) can detect and report the position of anything that reflects its transmitted radio signals including, depending on its design, aircraft, birds, weather and land features. For air traffic control purposes this is both an advantage and a disadvantage. Its targets do not have to co-operate, they only have to be within its coverage and be able to reflect radio waves, but it only indicates the position of the targets, it does not identify them. When primary radar was the only type of radar available, the correlation of individual radar returns with specific aircraft typically was achieved by the Controller observing a directed turn by the aircraft.
The need to be able to identify aircraft more easily and reliably led to 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 replies to signals from an interrogator (usually, but not necessarily, a ground station co-located with a primary radar) by transmitting a coded reply signal containing the requested information.
Both the civilian SSR and the military IFF have become much more complex than their war-time ancestors, but remain compatible with each other, not least to allow military aircraft to operate in civil airspace. SSR can now provide much more detailed information and it also permits the exchange of data directly between aircraft for collision avoidance. Given its primary military role of reliably identifying friends, IFF has much more secure (encrypted) messages to prevent 'spoofing' by the enemy, and also is used on all kinds of military platforms including air, sea and land vehicles.
[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 still may be observed by primary radar, but would be displayed to the controller without the benefit of SSR derived data.
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 - provides 2-digit 5-bit mission code (military only - cockpit selectable).
- Mode 2 - provides 4-digit octal unit code (military only - either set on the ground or changed in flight depending on the particular aircraft type).
- Mode 3/A - provides a 4-digit octal identification code for the aircraft, known as a squawk code, assigned by the air traffic controller (military and civilian).
- Mode 4 - provides a 3-pulse reply to crypto coded challenge (military only).
- Mode 5 - provides crypto secure capabilty similar to Mode S including transmision of ADS-B and GPS position (military only).
- Mode C - provides a 10-bit binary Gray Code for the aircraft's pressure altitude (military and civilian).
- Mode S - originally envisioned as a data packet standard in both uplink data (1030 MHz) and downlink data (1090 MHz) formats, also used to provide a radar design wherein the transponder responds to selective interrogations (each aircraft can be assigned a fixed 24-bit address for selective interrogation purposes). The downlink data format can also be utilized independently to squitter information such as position and velocity (AIS-P/TailLight). (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. Mode C transmits pressure altitude, read automatically from the aircraft altimeter. The mode S is triggered by a mode-S interrogation and can provide the particular information that is requested by 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.
[edit] Future uses
Weapon scientists and science fiction authors have both envisaged similar systems for general use on the battlefield — identifying vehicles, installations or even individual soldiers. Apart from the difficulty of providing reliable systems of sufficiently compact size and low weight, there are also concerns that the use of these devices may reduce concealment from enemy fire — unlike in aerial combat, where the mere presence of enemy units is usually well known.
[edit] See also
- Air Traffic Control Radar Beacon System, all encompassing description.
- Automatic Dependent Surveillance-Broadcast, Free Flight enhancement.
- Automatic Independent Surveillance-Protocol, Free Flight enhancement with ATCRBS transponder TSO corrections.
