Field Guide · technology

Also known as: TCAS, ACAS, TCAS II

TCAS (Traffic Collision Avoidance System), known internationally as ACAS, is an airborne system that independently interrogates the transponders of nearby aircraft on 1030/1090 MHz and, when a collision threatens, issues an aural and visual resolution advisory (“CLIMB, CLIMB” / “DESCEND, DESCEND”) to the pilots.1 It works without any ground infrastructure, reusing the same Mode S and Mode A/C transponder signalling that secondary radar uses.

own intruder 1030 MHz interrogation 1090 MHz reply + RA coordination DESCEND CLIMB
TCAS interrogates an intruder's transponder, and the two units coordinate over Mode S so one climbs and the other descends.

How it works

TCAS carries its own interrogator. Roughly once per second it transmits Mode S and Mode A/C interrogations on 1030 MHz and listens for replies on 1090 MHz. From each reply’s round-trip delay it measures range, from the change in range it estimates closure rate, and from the reported (Mode C / Mode S) altitude it tracks vertical separation. It does not rely on bearing for the core logic — it reasons primarily in the range/altitude “tau” domain, projecting time-to-closest-approach.

The alerting has two tiers:

  • Traffic Advisory (TA) — “TRAFFIC, TRAFFIC” draws the crew’s attention to a converging target but commands no maneuver.
  • Resolution Advisory (RA) — a vertical command (climb, descend, or maintain rate) that provides guaranteed vertical miss distance.

Crucially, when two TCAS II aircraft threaten each other, they coordinate their RAs over the Mode S data link so the maneuvers are complementary — if one is told to climb, the other is told to descend. This handshake is what makes TCAS safe against the failure mode where both aircraft dodge the same way. TCAS II is mandated for large transport aircraft worldwide; the simpler TCAS I gives only traffic advisories.

Relevance to SDR

TCAS is not a signal an SDR user “decodes” as a product in itself, but its interrogations and coordination messages are a prominent, high-power feature of the 1030/1090 MHz environment that anyone monitoring aviation bands will encounter. TCAS interrogations add to the interrogation load that a passive 1090 MHz receiver sees, and TCAS-equipped aircraft’s own Mode S replies (including any ADS-B extended squitters) are decoded like any other. TCAS also illustrates the same range-from-timing principle that ground-based multilateration networks exploit to locate non-ADS-B aircraft.

GopherTrunk does not implement or decode TCAS: it is a receiver focused on land-mobile trunking plus 1090 MHz ADS-B, and it neither interrogates transponders nor reconstructs collision-avoidance logic. TCAS is included here as context for the 1030/1090 MHz aviation ecosystem GopherTrunk’s ADS-B decoder lives alongside.

Sources

  1. Traffic collision avoidance system — Wikipedia, for TCAS/ACAS operation, 1030/1090 MHz Mode S/Mode A/C interrogation, range-and-altitude threat logic, and coordinated resolution advisories. ↩

See also