Secondary Surveillance Radar and Transponders

Secondary Surveillance Radar (SSR) is the cooperative radar on which most air traffic control depends. A ground interrogator transmits a coded question on 1030 MHz and the aircraft's transponder (XPDR) answers on 1090 MHz. The ground station decodes the reply into an identity code, a pressure altitude and, with Mode S, a unique aircraft address and further data. Because the aircraft transmits its own answer instead of reflecting energy, the technique is called secondary radar, and it sees only aircraft whose transponder is working.

The same transponder serves more than the controller's screen. Its replies let collision avoidance equipment in other aircraft track it (see ACAS and TCAS), its Mode S version carries the extended squitter used for ADS-B, and its four-digit code is the fastest way for a crew to declare a hijack, a radio failure or an emergency. EASA theory examinations test the pulse structure and the codes in detail; FAA tests concentrate on codes, phraseology and where a transponder is required.

Primary versus secondary radar

A primary surveillance radar (PSR) detects the echo of its own pulse from the aircraft's skin. It needs nothing on board, so it still sees an aircraft whose transponder has failed, but it also sees weather, terrain and birds, cannot tell one aircraft from another and gives no height. The energy travels out and back, so the returned power falls with the fourth power of range and a PSR needs a powerful transmitter.

SSR works on the interrogator and transponder principle used by DME, with the roles reversed. Each leg is a one-way path, so power falls only with the square of range, a far smaller transmitter gives the same coverage, and the reply stands out clearly from clutter. The price is dependence on the aircraft: no transponder, no SSR return. Range comes from the time between interrogation and reply, and bearing from the direction of the rotating ground antenna, often mounted on the primary antenna. The aircraft's antenna is omnidirectional.

Interrogation pulses and side-lobe suppression

A Mode A or Mode C interrogation is built from the SSR interrogation pulses (P1, P2, P3, P4). P1 and P3 are radiated in the narrow main beam, and their spacing tells the transponder what is being asked: 8 µs for Mode A (identity), 21 µs for Mode C (altitude). P2 follows P1 by 2 µs and comes from a separate, broad control antenna. P4 appears only in the intermode interrogations that manage Mode S, described below.

P2 exists for side-lobe suppression (SLS). A directional antenna leaks weaker energy in side lobes, and a nearby aircraft replying to one would appear at a false bearing. An aircraft in the main beam receives P1 much stronger than P2 and replies. An aircraft in a side lobe receives P2 at least as strong as P1 and stays silent.

Mode A and Mode C replies

The reply sits between two framing pulses 20.3 µs apart. Between them are 12 information pulses in four groups of three, each group coding one octal digit from 0 to 7. Four digits of eight values give 4,096 codes, 0000 to 7777, which is why no squawk contains an 8 or a 9. A further position, 4.35 µs after the last framing pulse, carries the special position identification (SPI) pulse.

Transponder Mode A answers with the code set by the crew and identifies the aircraft. Mode C answers with pressure altitude. Altitude reporting (ALT RPTG) is selected separately on the control panel, because the altitude source can fail or be wrong while the transponder itself still works. The ground station interleaves the two interrogations, so the controller sees code and level together in one label.

Altitude encoding and Mode C verification

The altitude comes from an encoding altimeter, a blind encoder or an air data computer, and is always referenced to 1013.25 hPa (29.92 inHg) whatever is set on the pilot's subscale. It is coded in 100 ft steps using the Gillham code, a modified Gray code in which only one bit changes between adjacent values. Below the transition level the ground system converts pressure altitude to altitude using QNH (see altimeter settings), so winding the subscale changes nothing the transponder sends.

Before a controller uses the displayed level for separation it is checked. Mode C verification, also called Mode C level verification, compares the readout with the level reported by the pilot, normally on first contact. The ICAO tolerance is ±300 ft, and an authority may set a tighter value. If the difference is larger, the pilot is asked to check the pressure setting and confirm the level. If it persists, the controller may ask the crew to stop altitude transmission, provided that does not also remove position and identity, and tells the next unit. Once verified, the readout shows that an aircraft has left a level when it moves more than 300 ft away in the expected direction. In US traffic calls, an unverified readout is introduced as "altitude indicates".

Garbling and FRUIT

Garbling occurs when two aircraft on nearly the same bearing are within about 1.7 NM of each other in slant range from the station. Each reply lasts 20.3 µs, so their pulses overlap at the receiver and cannot be sorted, producing false or missing codes.

False Replies Unsynchronised in Time (FRUIT) are replies that a ground station receives but did not trigger: answers to neighbouring stations or to airborne interrogators such as TCAS. They arrive at random times relative to the station's own interrogations. Fruiting shows as scattered spurious returns; defruiters discard replies that do not repeat consistently from one interrogation to the next. The problem grows with overlapping coverage and traffic density, one of the reasons Mode S was developed.

Mode S and the 24-bit address

Mode S (S for selective) gives each aircraft a unique 24-bit aircraft address, usually called the ICAO 24-bit aircraft address. It is allocated by the State of Registry and belongs to the airframe, not to the flight. With 2²⁴ combinations there are more than 16 million addresses. An interrogator can call one aircraft and only that transponder replies, so two aircraft close together no longer garble each other, and the station can schedule its calls.

To learn the address the station uses the Mode S all-call interrogation; Mode S transponders also broadcast an acquisition squitter about once a second. Intermode interrogations keep old and new equipment compatible. P1, P3 and a long P4 form a Mode A/C/S all-call: Mode A/C transponders give their normal reply and Mode S transponders reply with their address. A short P4 makes an A/C-only all-call that Mode S transponders ignore. Once an aircraft is acquired, the station can lock it out of further all-calls, reducing FRUIT. Mode S replies are 56 or 112 bits long; the long format also carries the extended squitter used by ADS-B and the messages that coordinate RAs between TCAS II aircraft.

Mode S transponder levels describe data-link capability:

Elementary and enhanced surveillance

Mode S elementary surveillance (ELS) provides the address, Mode A code, altitude, flight status (airborne or on the ground), capability reports and the aircraft identification. That identification is the callsign as filed in item 7 of the flight plan, and it must be entered exactly. Mode S enhanced surveillance (EHS) adds downlinked aircraft parameters (DAP): selected altitude, roll angle, track angle rate, true track, ground speed, magnetic heading, indicated airspeed or Mach number and vertical rate. The selected altitude is the most valuable of these, because the controller sees what the crew has set on the MCP or FCU and can catch a level bust before it happens.

In the Single European Sky, Regulation (EU) No 1207/2011, as amended by Regulation (EU) 2020/587, requires Mode S ELS on aircraft flying IFR as general air traffic. Fixed-wing aircraft with a maximum certificated take-off mass above 5,700 kg or a maximum cruising true airspeed above 250 kt, whose individual certificate of airworthiness was first issued on or after 7 June 1995, must also have EHS and ADS-B Out, a requirement in force since 7 December 2020.

The FAA takes a different route. Under 14 CFR 91.215 a transponder with altitude reporting (Mode C is enough) is required in Class A, B and C airspace, within 30 NM of Class B primary airports (the Mode C veil), and at and above 10,000 ft MSL except within 2,500 ft of the surface. 14 CFR 91.225 has required ADS-B Out in much the same airspace since 1 January 2020. The airspace classification article covers these areas.

Mode S data link

The Mode S data link carries standard-length messages in both directions. Comm-A places 56 bits of data from the ground inside an interrogation, and Comm-B returns 56 bits from the aircraft inside a reply. Enhanced surveillance uses ground-initiated Comm-B: the station asks for a particular register in the transponder and the reply carries its contents. Extended-length messages chain several segments together, uplink at Level 3 and downlink at Level 4.

Transponder codes and squawk phraseology

A discrete code is a four-digit code whose last two digits are not 00. ATC assigns such a discrete SSR code, or discrete transponder code, to each flight it wants to identify individually, and the pilot reads it back. Codes ending in 00 are shared by many aircraft and identify only a category of flight.

In an emergency the pilot sets 7700 unless ATC has already assigned a code; in that case the crew keeps the assigned code unless told otherwise, but may select 7700 whenever that seems the better course. 7600 goes with the radio communication failure procedures; since 1 May 2025, SERA.14083 adds 7601 in the EU for an IFR flight that loses communications and continues in visual conditions to land at the nearest suitable aerodrome. 7500 is treated as genuine the moment it appears: the controller asks the crew to confirm the code, and either a confirmation or silence is taken as confirmation.

View full schematic

SERA.13001 requires a serviceable transponder to be operated at all times in flight, inside or outside radar coverage. At many aerodromes it must also be on, with altitude reporting, whenever the aircraft moves on the manoeuvring area, because surface surveillance uses its replies; the AIP gives the local rule. When changing code, pilots avoid passing through the emergency codes: the AIM example is to go from 2700 to 7200 through 2200, never through 7700. Selecting standby while changing code, once the textbook method, is now discouraged, because a transponder in standby does not reply to ground radar or to other aircraft's TCAS.

Transponder phraseology is standardised in ICAO Doc 4444:

Reset squawk is used when a wrong or garbled code is displayed and does not mean restarting the equipment. Squawk standby leaves the transponder warm but silent, removing the aircraft from the secondary picture. Stop squawk Charlie removes only the altitude, usually after a failed verification. The FAA uses "IDENT" rather than "SQUAWK IDENT", "SQUAWK ALTITUDE" and "STOP ALTITUDE SQUAWK" for Charlie, and "SQUAWK VFR" for 1200.

Transponder ident sends the SPI pulse, or in Mode S an equivalent flag, for between 15 and 30 seconds under the equipment standard, and the aircraft's label flashes or brightens on the controller's display. SERA forbids pressing IDENT unless ATS requests it. A controller can identify an aircraft by recognising its Mode S aircraft identification or a verified discrete code in the label, by watching it set a newly assigned code or squawk ident, or by transfer from another controller.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.