Field Guide · technology

Also known as: TACAN

TACAN (Tactical Air Navigation) is a military radio navaid that provides an aircraft with both bearing and distance from a single beacon in the L-band (960–1215 MHz). Its distance function is identical to civil DME — the same coded pulse-pair round trip — while its bearing function is built into the modulation of those same pulses, so one compact, often transportable or shipborne, ground unit delivers a full fix.1 TACAN is the military counterpart to the civil VOR/DME pairing, and the two are deliberately made to interoperate.

rotating pattern → 15 Hz + 135 Hz bearing modulation distance: DME pulse-pair timing bearing: AM of the pulse envelope one beacon = full position fix
TACAN reuses the DME pulse-pair ranging and adds bearing by amplitude-modulating the pulse stream with a rotating antenna pattern.

How it works

TACAN’s ranging half is DME: the aircraft interrogates with pulse pairs and the beacon replies after the standard fixed delay, and round-trip time gives slant range. The bearing half comes from the beacon’s antenna radiating a rotating pattern that amplitude-modulates the pulse stream at 15 Hz (a coarse indication) and, from a finer nine-lobe pattern, at 135 Hz. Reference bursts mark the pattern’s orientation, and the airborne set compares the phase of the received 15 Hz and 135 Hz envelope modulation against those references to resolve bearing to about a degree — more precisely than a conventional VOR, because the 135 Hz component sharpens the reading.

Where a civil VOR and a TACAN are co-located, the facility is called a VORTAC: civil aircraft take VOR bearing plus TACAN’s DME distance, and military aircraft take full TACAN. TACAN is compact enough to mount on ships and vehicles, which is central to its tactical role.

Relevance to SDR

TACAN shares the pulsed L-band environment of DME and Mode S, so it is not a beginner SDR target, but it illustrates how one signal can multiplex distance (pulse timing) and bearing (envelope amplitude modulation) at once. GopherTrunk does not decode TACAN; it is a land-mobile trunking scanner, and TACAN is included as reference context alongside DME, VOR, and ILS.

Sources

  1. Tactical air navigation system — Wikipedia, for TACAN’s combined bearing/distance operation, its shared DME ranging, the 15 Hz/135 Hz bearing modulation, and the VORTAC arrangement. ↩

See also