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  • What is Secondary Surveillance Radar SSR

    Also known as transponder

Air traffic controllers need to know the “name and surname” of each aircraft, which is why the secondary surveillance radar or SSR was created. Just imagine the number of aircraft flying overhead… Can’t you imagine? Take a look at Flight Radar, come on.

In today’s post, we explain the origin of secondary surveillance radar and how it works.


Origins of Secondary Surveillance Radar SSR

In 1935, aircraft were becoming increasingly sophisticated and the first air raids began. The need arose for a system to identify aircraft in flight.

Well, the physicist Robert Watson-Watt presented what was a great advantage for the Royal Air Force of Great Britain: RDF, Radio Direction Finding, the predecessor of what we know today as radar.

RDF showed on a screen the different aircraft flying in an area, but could not distinguish between friendly and enemy aircraft.

It was then, during the Second World War, that the IFF or Identification Friend or Foe was developed, an identification system consisting of a piece of equipment that was colloquially known as ‘the parrot’ (see below).

This equipment was installed in aircraft and emitted coded signals called Squawk codes. In this way, they were able to differentiate between allied and enemy aircraft on the radar screens.

How does SSR or Secondary Surveillance Radar work?

Secondary Surveillance Radar (SSR), also known as transponder, is a system composed of two stations, one on the ground, called the interrogator, and one on the aircraft, called the responder.

The interrogator interrogates on the 1030 MHz frequency, to which the aircraft responds with electromagnetic pulses on the 1090 MHz frequency.

Did you know that…?

During World War II, so that enemies would not know of its existence, the transponder was called ‘the parrot’ (because it never shut up). When it was no longer necessary to identify the aircraft, they said “strangle de parrot”, i.e., turn off the transponder.

Elements of the transponder or Secondary Surveillance Radar

The transponder is an instrument similar in appearance to a radio and allows the selection of a 4-digit number, ranging from 0 to 7.

Pilots can choose various functions:

  • On/Off: On and off.
  • STBY: The system is on standby, receiving interrogations but no responses; this is the mode used when the aircraft is started on the ground but not yet about to take off.
  • On: When the responder has no code selected it operates in A mode.
  • Alt: The responder transmits in mode C, i.e. it transmits position and altitude.
  • Test: Self test by the responder.
  • Ident: Transmits a 25 second pulse; this mode should only be used when requested by the tower or if the aircraft is in an emergency.

Operation modes of the SSR

Depending on the time between pulses and the reporting capability, there are different types of operation of the secondary surveillance radar:

  • Mode A: This is the simplest of all; it only displays the position of the aircraft.
  • Mode C: Displays the position and pressure altitude of the aircraft. The pressure altitude is the altitude marked by the altimeter with reference 1013 Hpa.
  • Mode S: This is the most sophisticated mode, also used by commercial aircraft. It provides position, altitude, speed, weather reports, ground speed and TCAS, which is a system that prevents in-flight collisions between aircraft.
Photo by US Navy
Photo by Ahunt

Special codes on Secondary Surveillance Radar

Air traffic controllers have a system for assigning different codes to aircraft, but there are some exceptions:

  • Series 00 – Code 0000: Available to any state for general purposes.
  • Series 20 – Code 2000: For aircraft that have not been instructed by air traffic control units to use the responder.
  • Series 70 – Code 7000: Used for VFR flights in Spain.
  • Series 75 – Code 750: Used to recognise aircraft subject to unlawful interference.
  • Series 76 – Code 7600: Used in case of communications failure.
  • Series 77 – Code 7700: Available to aircraft in a state of emergency. Codes 7711-17 and 7721-27 are reserved for SAR operations, while code 7777 is for ground transponder monitoring.

Mode A and Mode C SSR issues

  • If a transponder is in range of two interrogators it will respond to both, so that one station will receive an erroneous indication, this is known as fruiting.
  • If two aircraft are on the same heading from the station, and are within 1.7 nautical miles of each other, the signals will overlap, resulting in an error known as garbling.
  • Mode A can only operate with 4096 codes at a time, while Mode S increases this number to 1677721424 codes.

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