Radar / Transponder / Secondary Surveillance Radar

Radar and Transponder

Radar (Radio Detection and Ranging) is a ground- or airborne system that, by transmitting radio pulses and receiving reflections or replies, determines position and direction of aircraft. Two main categories: primary radar (reflection) and Secondary Surveillance Radar (SSR) with a transponder (active reply from the aircraft).

Source: ICAO Annex 10 Vol IV Surveillance and Collision Avoidance Systems; ICAO Doc 4444 PANS-ATM Ch. 8; EASA AMC1 SERA.13005.

Ground radar — operating principle

Distance and direction can be instantaneously obtained through ground radar:

• Distance is determined by time measurement between transmitted pulse and received echo.
• Direction of the airplane in relation to the antenna is determined by the orientation of the antenna — when the rotating radar antenna points in a certain direction, the echo comes from that direction.

The radar antenna typically rotates at 5-15 RPM (one antenna revolution = 4-12 seconds).

Primary vs Secondary radar — difference

The difference between primary and secondary radar is that the pulses of a primary radar are reflected by the aircraft's surface whereas the pulses of a secondary radar system are answered by a transponder:

Transponder — the onboard equipment

The onboard equipment of the Secondary Surveillance Radar (SSR) is called transponder:

• Transmits on 1090 MHz in reply to interrogation on 1030 MHz.
• Code is set by the pilot as a 4-digit octal number (squawk) — e.g. 7000 (VFR standard EU), 7700 (emergency), 7600 (radio failure), 7500 (hijack).
• Several modes: Mode A (identification code), Mode C (altitude), Mode S (digital + selective).

Transponder modes

Mode C altitude — pressure altitude

An altitude inquiry by the air traffic control unit via the transponder with attached 'encoding altimeter' is always related to pressure altitude:

• Pressure altitude = altitude referenced to standard pressure 1013.25 hPa, independent of current QNH.
• An encoding altimeter is connected in the cockpit — altitude is digitally read via a coding disk and transmitted to the ground station ("the ATC transponder delivers altitude information to a Mode C interrogation. The altitude information is digitally sensed via coding disk of the on-board altimeter and decoded in the ground station").
• ATC decodes the pressure altitude on the Mode-C display into QNH altitude (below transition altitude).

IDENT button — squawk ident

When the controller instructs "Squawk ident", the pilot must briefly press the IDENT button, which transmits an additional pulse that highlights the aircraft on the radar screen ("when instructed by the controller to 'Squawk ident' the IDENT button has to be pushed briefly which transmits an additional pulse which highlights the aircraft on the screen"):

• On the ATC screen the aircraft symbol appears marked (e.g. blinking or enlarged).
• Controller confirms: "DEMRA, identified, ...".
• Used for identification when several aircraft have the same code or confusion arises.

Squawk SBY (stand-by) — pre-heating

When the controller instructs "Squawk standby", the pilot must select operating mode SBY ("when instructed to 'Squawk standby' the operating mode SBY has to be selected"):

• SBY (standby) = transponder switched on, but sends NO replies.
• From OFF to SBY: after switching on, the transponder is available for operation after about 2 minutes of pre-heating ("when switched to STBY from the OFF position, the transponder is available for operation after about 2 minutes of pre-heating time").
• Use: before take-off as preparation (warm up); after landing to reduce TCAS confusion on the ground.

Reset 4500 — function check

"Reset 4500" means the pilot switches the device to STBY and on again immediately afterwards to check it for new operability:

• Specifically: pilot sets code 4500 (test code), cycles mode select from ON to SBY and back, watches the display.
• Used in certain test or maintenance protocols.

Radar types for ATC

Different radar facilities are used depending on the ATC service task:

DFS ground radar tasks

Ground radar facilities of the DFS serve to survey and control the controlled traffic mainly — i.e. for IFR and VFR traffic in controlled airspace. VFR in uncontrolled airspace is not actively surveyed.

Transponder use in practice

• Before take-off: SBY mode, squawk code set (typically 7000 for VFR EU, or per ATC assignment).
• On take-off: mode "ON" or "ALT" (Mode C / Mode S active).
• In controlled airspace: ATC can assign any squawk code, pilot must set.
• Emergency:
  • 7700: general emergency.
  • 7600: radio communication failure.
  • 7500: unlawful interference (hijack).
• VFR standard: 7000 in Europe (unless ATC otherwise).

Squawk code reservations