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Ground broadcasts
(188 ms = 752 MSOs)
Timeslot consists
of 22 MSOs
(5.5ms)
Ground message
(3712 bits payload)
Ground station messages are
synchronised to timeslots.
Timeslot usage is planned so
ground station transmissions
do not overlap.
UAT frame of 1 second
ADS-B reports from aircraft
(812 ms = 3248 MSOs)
ADS-B message
(128/256 bits payload)
Message Start
Opportunities
(MSOs, 250 µ s)
Aircraft ADS-B reports are synchronised to an
MSO if the aircraft has an accurate time
reference available.
MSOs are selected randomly by aircraft.
Figure 3-11 – UAT timing structure
= Guard time of 48 MSOs (12ms)
Ground transmissions
The first segment of the frame is allocated to transmissions from UAT ground stations and
consists of 752 MSOs (Message Start Opportunities). This portion of the frame contains
32 time-slots, each containing 22 MSOs and therefore lasting 5.5 ms, with a guard time of
48 MSOs at the end of the frame.
Each ground station is assigned one of the 32 time-slots in such a way that transmissions
from nearby ground stations can be received without interference. Each ground station
transmits a ground broadcast message once each second, starting at the beginning of its
assigned slot.
ADS-B message transmissions
The second segment of the frame is devoted to ADS-B message transmissions. Within
this portion of the frame, aircraft and surface vehicles are required to transmit at randomly
selected times from among the 3200 MSOs in the segment. The selection algorithm is
designed to prevent any two stations from repeatedly selecting the same MSO. Aircraft
messages may contain 128 or 256 bits of ADS-B data (payload).
A substantial guard time, specifically for timing drift, is accommodated at both the
beginning and the end of an ADS-B segment. This allows for clock drift in airborne units
for a period of time before there would be any possibility of ADS-B transmission overlap
with a ground message.
UAT, like VDL4, needs a source of precise time to ensure that it is aware of the timeslot
structure to which it should adhere when transmitting messages. The source could most
easily be a GNSS receiver. However, in the event of loss of the timing source, a mobile
UAT receiver may make timing measurements on ground station signals to determine
timing information.
Exactly one ADS-B message is transmitted per aircraft every second. An ADS-B message
is either 252 or 380 bit intervals in length.
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