Field Guide · protocol

Also known as: Mode A/C, Mode A, Mode C, Mode 3/A

Mode A/C is the legacy secondary surveillance radar transponder scheme in which an aircraft replies on 1090 MHz to a ground radar’s 1030 MHz interrogation with a burst of pulses encoding either a four-digit squawk code (Mode A) or pressure altitude (Mode C).1 It predates the selective-addressing Mode S protocol that underlies ADS-B, and the two coexist on the same frequency pair because Mode S was designed to be backward-compatible with Mode A/C interrogators.

radar 1030 MHz interrogation aircraft 1090 MHz reply F1F2 12 info pulses (20.3 µs frame)
Mode A/C: a 1030 MHz ground interrogation triggers a 1090 MHz reply whose two framing pulses (F1, F2) bracket up to 12 information pulses.

Overview

Mode A/C is the pulse-only heart of the international Secondary Surveillance Radar (SSR) system. A rotating ground antenna transmits interrogation pulse pairs on 1030 MHz; the mode is selected by the spacing between the pulses (8 µs for Mode A, 21 µs for Mode C). Every transponder in the beam that hears the interrogation replies on 1090 MHz, so unlike Mode S there is no addressing — the radar sorts replies by the azimuth of its antenna and the round-trip timing.

Technical characteristics

Property Value
Interrogation frequency 1030 MHz
Reply frequency 1090 MHz
Mode A trigger P1–P3 pulses spaced 8 µs
Mode C trigger P1–P3 pulses spaced 21 µs
Reply frame F1 … F2 framing pulses 20.3 µs apart
Information pulses 12 (A, B, C, D groups)
Mode A capacity 4096 codes (octal 0000–7777)
Mode C encoding Gillham/Gray-coded altitude in 100 ft steps

The reply is a train of up to 12 information pulses bracketed by two framing pulses (F1 and F2) exactly 20.3 µs apart. In Mode A those 12 bits carry the pilot-set four-digit octal squawk (e.g. 7500 hijack, 7600 radio failure, 7700 emergency). In Mode C the same 12 positions carry pressure altitude in 100-foot increments using a Gray-code-derived Gillham encoding. A special position identification (SPI) pulse can follow F2 when the pilot presses “ident”.

History

Mode A/C descends directly from wartime IFF (“Identification Friend or Foe”) and was standardised by ICAO in Annex 10. It served as the sole cooperative surveillance mode for decades. Its weaknesses — no unique address, garbled overlapping replies (“FRUIT” and synchronous garble) when many aircraft share a beam, and only 4096 possible codes — drove the development of Mode S selective interrogation and, later, ADS-B.

Deployment

Mode A/C transponders remain in wide use, especially in general aviation, and every Mode S transponder still answers legacy Mode A/C interrogations for backward compatibility. Ground SSR and airborne TCAS units both interrogate Mode A/C. The squawk code is the tag air-traffic control assigns to correlate a radar return with a flight plan.

Decoding it with GopherTrunk

Not decoded. Mode A/C carries no aircraft identity or absolute position on its own (a bare squawk plus altitude), and its unaddressed, radar-triggered replies are of limited value to a passive scanner. GopherTrunk’s aviation support targets the information-rich Mode S extended squitter and ADS-B instead. Mode A/C replies are, however, the raw material for ground-based multilateration networks, which time-difference the 1090 MHz bursts across several receivers to locate non-ADS-B aircraft.

Sources

  1. Secondary surveillance radar — Wikipedia, for the 1030/1090 MHz interrogation-reply scheme, Mode A squawk codes, Mode C altitude encoding, framing pulses, and the relationship to Mode S. ↩

See also