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SSR-antenna of Deutsche Flugsicherung at Neubrandenburg, in Mecklenburg/Western Pommerania

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 World War II, and the two systems are still compatible today.

Monopulse secondary surveillance radar (MSSR) is a modern improved version of SSR.




Primary radar

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. Primary radar is still used by ATC today as a backup/complementary system to secondary radar, although its coverage and information is more limited.[1][2][3][4]

Secondary radar

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 which receives on one frequency (1030 MHz) and transmits on another (1090 MHz). 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.

Monopulse secondary surveillance radar

Monopulse secondary surveillance radar[5] (MSSR) is an improved version of the classic SSR of the 50s. In the middle of the 70s engineers tried to avoid some standard problems of SSR. In particular Garbling and the False Replies Unsynchronized with the Interrogation Transmissions or simply FRUIT. Garbling was happening when flights close to each other were sending very narrowly spaced replies to the SSR and its decoder was unable to detect them separately. The FRUIT was the result of many SSRs working in the same area where the reply from a flight due to the interrogation of one was also received by another SSR that has not yet sent out an interrogation for this flight. Both problems resulted in loss of the aircraft position producing inaccuracies.

At the end of the 80s the SSR antenna was modified to the LVA (Large Vertical Aperture) type where a series of many dipoles independently read the reply from a flight and the radar was then calculating the received difference in strength and phase (delay) of each one. The mathematical result was able to calculate and resolve simultaneous replies from various flights with a directional angle difference of some 0,5 degrees whereas the classic SSR could not see the difference within an angle of some 3 degrees. Actually the mathematical model was able to calculate the flight position using a single pulse of the many (maximum 15 possible) in the reply signal. This provided the term Mono.

MSSR can reduce garbling in multi-radar environments.[6]

The MSSR replaced most of the existing SSRs by the 90s and its accuracy provided for a reduction of separation minima in en-route ATC from 10 nautical miles (19 km) to 5 nautical miles (9.3 km).


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).


There are several transponder modes, each offering different information. For civilian flights according to ICAO the modes of operations are A, C and S.

See also


External links


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