Definition

A surveillance radar system which uses transmitters/receivers (interrogators) and transponders.

Source: ICAO Doc 4444 PANS-ATM

Description

Principle of operation

The radar antenna rotates (usually at 5-12 rpm) and transmits a pulse which is received by the onboard equipment (transponder). The transponder sends back a reply containing at least a code (if operating in Mode A) but more often this is combined with level (mode C) or other information, e.g. aircraft identification, selected level, etc. (Mode S). The information received depends on the interrogation mode (A, C or S) and the transponder capability. For example, interrogation in Mode A will receive a reply in mode A even though the transponder may have Mode C or Mode S capability and an interrogation in Mode C will not trigger a response from a Mode A transponder. Typically, two Mode A interrogations are followed by a Mode C interrogation. The reason for using Mode A more frequently is that the identity of the aircraft (the SSR code) is of greater importance to the controller.

Note that the level received from the transponder is always in respect to standard pressure (1013.25 hPa, 29.92" Hg) regardless of the altimeter setting selected by the pilot. This is a mitigation against human error. As a result, when the controller observes that two aircraft are separated by 1000 feet, it means that this separation exists regardless of the altimeter settings of the two aircraft.

When the reply is received, aircraft position (range and bearing) is determined. The range is calculated by knowing the time difference between the interrogation and the reply (the speed of propagation is the speed of light). The azimuth is taken from the antenna position. The antenna usually rotates at 5-12 rpm.

SSRs may be used as interrogators for passive MLAT/WAM arrays.

Advantages

• Requires much less power to achieve the desired range, in comparison to PSR. This is because the transmitted signal only needs to reach the aircraft, while the PSR needs to emit a signal strong enough to reach the aircraft and travel back to the antenna.
• The information provided is not limited to range and bearing from the antenna but also includes additional data based on the transponder mode of operation (A, C or S).
• Targets are easier to distinguish due to the different SSR codes.
• SSR is immune to clutter as it uses different frequencies for interrogation (1030 MHz) and replies (1090 MHz). Consequently, even if an echo on 1030 MHz is received, it is not processed by the system. Therefore, terrain, buildings and weather phenomena (clouds and precipitation) do not produce clutter, as is the case with PSR. These can interfere with the signal, though, thus sometimes making detection of real targets more difficult.

Limitations and issues

• The SSR relies on the onboard equipment to discover aircraft. In case of transponder failure the SSR will receive no reply and will therefore not discover the target. This is mitigated by combining the SSR with a PSR. If proper signal processing is used, it is possible to continue to track an aircraft (and preserve Correlation) even if the transponder has failed completely provided that reliable primary data is received. Note that in this case level information will be less reliable and more frequent pilot reports will be necessary.
• Sometimes two replies are received at the same time (if the slant range and the bearings of the aircraft the same). This phenomenon is called "garbling" and may result either in the "detection" of a false (non-existing) aircraft or in a target not being detected (the radar considers that there is a false target).
• Another phenomenon that may produce false indication is FRUIT (False Replies Unsynchronised In Time or False Replies Unsynchronised to Interrogator Transmissions). This happens when the radar receives a reply from a transponder that has been interrogated by another radar. Since all SSRs operate on the same frequencies, it is not possible to detect that the reply is related to another radar's transmission. Moreover, as the time of the interrogation is not known, the range calculation will most likely be wrong. As a result, a false target may appear on the situational display. Additionally, if another (valid) transponder reply is received at the same time, garbling could occur. Garbling and FRUIT are aggravated by the need of "classic" SSRs to use several interrogations for proper azimuth determination and can be mitigated by:
  • Using an MSSR (monopulse SSR). This is an advanced radar that uses a different beam pattern that provides more accurate azimuth determination. As a result, fewer interrogations are required to determine the azimuth.
  • Mode S selective interrogation. This is a feature of the Mode S SSRs that allows them to interrogate a specific aircraft (other aircraft may receive the interrogation but would ignore it).
• SSRs are vulnerable to antenna shadowing (i.e. the onboard antenna is shadowed by the aircraft fuselage, e.g. due to the bank angle). This is mitigated by placing more than one antenna (usually two - one on top of the aircraft and one at the bottom).

The advantages of the PSR and SSR are often combined by co-locating them.