FINAL REPORT - Stakeholders - Ofcom

3.3.8.2 UAT
The Universal Access Transceiver (UAT) is the result of an internally funded project at
MITRE/CAASD in the US, started in December 1994 to develop and demonstrate a
transceiver system designed specifically to support the function of ADS-B. UAT has been
developed to a prototype status and is being trialled in the US.
In addition to ADS-B, UAT is intended to support uplink broadcast data from ground
stations. This could include TIS-B and/or FIS-B data. Note that UAT only supports
broadcast applications and not point to point applications.
The UAT prototype operates in a single channel with a bandwidth of approximately 2-3
MHz, using the same frequency for transmit and receive. All aircraft access the channel
autonomously at random, and there is no centralised ground control or on-board logic for
this function.
The first UAT prototype operated with experimental authorisation at 966MHz, which is in
the range used for DME (960-1215 MHz). The large scale trials presently underway in
Alaska (“Capstone”) operate at 981 MHz. The channel that will be used in a final
operational system is not yet fixed. Within the US, the FAA has initiated procedures to
secure a permanent frequency assignment.
UAT has been specifically designed to provide air-air, air-ground and ground-air
broadcast services.
UAT does not provide point-to-point functionality.
System description
UAT messages are transmitted using continuous phase frequency shift keying. Data is
transmitted at a rate just over 1 Mbit/sec (precisely 1.041667 megabit/second), thus each
bit period is 0.96 microseconds.
In the UAT system, the ‘frame’ is the most fundamental time unit. Frames are one second
long and begin at the start of each Universal Time Coordinated (UTC) (or GNSS)
second 4 . Each frame is divided up into two segments – one for ground station
transmissions and another for mobile station transmissions (aircraft or surface vehicle).
Each segment is further subdivided into message start opportunities (MSOs) spaced 250
ms apart for a total of 4,000 MSOs per frame. The MSO is the smallest time increment
used for scheduling Ground Uplink messages or ADS-B message transmissions.
4 UTC is a global standard for time which was defined by the International Consultative
Committee (CCIR), a predecessor of the ITU-T. CCIR Recommendation 460-4, or ITU-T
Recommendation X.680 (7/94) contains the full definition. The UTC second can be
obtained from GPS or, more generally, GNSS signals.
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