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Figure 3-12 shows the format and components of the ADS-B message burst transmission
from aircraft (or ground vehicles).
number of bits:
Tx power stabilisation
Synchronisation
Length identifier
4 36
8 128/256 24 48 4
ADS-B data
(‘payload’)
Tx ramp down
Forwarded Error Correction
Cyclic Redundancy Check
Figure 3-12 – UAT ADS-B message transmitted from aircraft/ground vehicle
UAT is the only one of the ADS-B data link systems to incorporate a Forward Error
Correction (FEC). This gives it increased robustness to bit errors that may occur in the
message since a small number of these can be corrected. The Cyclic Redundancy Check
(CRC) is still applied following any corrections by the FEC. This maintains the integrity of
the message.
The UAT message set is defined with a “Basic” Message that includes only State Vector
information, and a set of 3 “Extended Length” Messages (types 0, 1, 2) that each include
the State Vector plus other variable information required by the RTCA MASPS. In trials
(e.g. the Capstone trial), aircraft were configured to only transmit the basic and extended
type 0 messages.
The ADS-B message transmission rate for UAT is fixed at once per second. This rate has
been designed to support all applications identified in RTCA DO-242.
Airborne terminal design
Different airborne terminal configurations are possible depending on the class of aircraft.
The configuration illustrated in. Figure 3-13 may be sufficient for low-end general aviation
users.
T/R
UAT transceiver
GPS time/position sensor
& altitude source
ADS-B Reports
TIS-B, FIS-B
Application
processor(s)
CDTI and
surveillance
functions
Figure 3-13 – UAT Low-end Airborne Architecture
A processing unit linked to the transceiver processes data from ADS-B reports and
external data sources such as TIS-B and FIS-B, and provides surveillance reports and
track data to the CDTI and other surveillance functions.
Under normal conditions it is expected that the UAT transceiver would interface with a
GNSS sensor and barometric altitude source for a minimal installation. The GNSS sensor
would provide the position and velocity information as well as timing for ADS-B
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