Field Guide · protocol

Also known as: UAT, UAT-978, Universal Access Transceiver

UAT (Universal Access Transceiver) is the 978 MHz datalink adopted by the U.S. FAA as the second, general-aviation link for ADS-B, running in parallel with the worldwide 1090 MHz Mode S Extended Squitter link.1 Beyond position reports it uniquely carries two free ground uplinks: FIS-B weather and aeronautical information, and TIS-B traffic — services that 1090 MHz ADS-B does not provide.

ADS-B message segment ground-uplink segment 0 s1 s (UTC) aircraft TX FIS-B / TIS-B down 978 MHz · CPFSK 1.04 Mbps · TDMA
The one-second UAT frame: aircraft broadcast ADS-B in an early time segment; ground stations fill the rest with FIS-B and TIS-B uplinks.

Overview

UAT was designed from scratch as a broadband, general-aviation-friendly alternative to the congested 1090 MHz channel. It uses a single 978 MHz channel shared in time: each one-second UTC-aligned frame is split into a message segment, in which airborne transponders transmit short ADS-B reports in pseudo-random time slots, and a ground-uplink segment, in which fixed ground stations blast large FIS-B and TIS-B payloads to every aircraft in range.

Technical characteristics

Property Value
Frequency 978 MHz
Bit rate 1.041667 Mbps
Modulation Continuous-phase FSK (deviation ≈ ±312.5 kHz)
Occupied bandwidth ≈ 1.3 MHz
Access TDMA, 1-second UTC frame
ADS-B payload 18-byte (basic) or 34-byte (long) message
Ground uplink up to ~3.5 kbytes per slot
FEC Reed–Solomon
Standard RTCA DO-282

The physical layer is continuous-phase binary FSK at about 1.04 Mbps, giving a constant envelope that suits inexpensive class-A/B avionics. Every message is protected by a Reed–Solomon code so partial reception can still be corrected.

History

UAT was developed in the late 1990s and standardised as RTCA DO-282. The FAA selected it as the low-cost link for aircraft operating below 18,000 ft, mandating either UAT or 1090ES for ADS-B Out by 2020. The dual-link approach let the FAA offload light aircraft from 1090 MHz while adding the free FIS-B/TIS-B uplinks as an incentive to equip.

Deployment

UAT is a U.S.-only system; the rest of the world uses 1090ES exclusively. Because the two links do not interoperate directly, U.S. ground infrastructure rebroadcasts 1090ES traffic to UAT-only aircraft (and vice versa) via TIS-B, and pushes weather via FIS-B. Popular portable receivers (e.g. dual-band “ADS-B In” units) tune 978 MHz to give pilots free in-cockpit weather and traffic.

Decoding it with GopherTrunk

Not decoded. GopherTrunk’s ADS-B support targets the 1090 MHz Mode S Extended Squitter link, not the 978 MHz UAT link, which uses a different modulation and framing. UAT is well within reach of a wideband SDR, and open tools exist for it, but it is outside GopherTrunk’s current aviation scope. The companion pages FIS-B and TIS-B describe the two ground-uplink services carried here.

Sources

  1. Universal Access Transceiver — Wikipedia, for the 978 MHz UAT link, its CPFSK physical layer, one-second TDMA frame, ADS-B message segment, and FIS-B/TIS-B ground uplinks. ↩

See also