Secondary surveillance radar
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A Secondary Surveillance Radar (SSR) is a radio interrogator device installed in air traffic control facilities, and radio transponder devices that are installed in aircraft. The purpose of this system is to improve the ability to detect and identify aircraft. 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. This enhances primary radar. 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).
Modes
There are several transponder modes, each offering different information- Mode 1 - provide 2 or 4-digit octal mission code (military only).
- Mode 2 - provide 4-digit octal unit code (military only).
- 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).
See also
- Air Traffic Control Radar Beacon System, all encompassing description.
- Automatic Dependent Surveillance-Broadcast, Free Flight enhancement.
External links
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