User's Manual

TT21 Transponder Installation Manual 19 January 2009
00560-00 Issue AA
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Trig Avionics Limited Page 29
installation and system attributes for the overall aircraft
installation. AMC20-24 has been adopted as a base standard for
ADS-B surveillance in other parts of the world, and provides
radar-like separation capabilities between participating IFR
aircraft.
Experimental ADS-B
Surveillance
ADS-B trials have taken place in a number of countries which,
because of the experimental nature, have allowed deviations from
the operational standards represented by AMC20-24, but which
have been based on positional information with comparable
integrity.
ADS-B Information
There are a number of non-certified applications of ADS-B
technology such as fleet monitoring and traffic awareness which
can be deployed using position sources of lower integrity which,
because the integrity is reported in the transmissions, are also
compatible with the ADS-B surveillance environment.
There is also a widely publicised fourth level of service in discussion which is the US “Next
Generation” plan, but at the time of publication of this manual a number of key aspects of this are
undefined and no compliance statement against that future plan is practical.
The TT21 is designed to provide all of the required parameters listed in AMC20-24. However, in
order to be used for separation services AMC20-24 sets additional criteria related to the position
accuracy and integrity of the GPS position source used, and the ability for the GPS to communicate
those accuracy and integrity parameters to the ADS-B output system in the transponder. At the
publication date of this manual, the only commercially available GPS receivers we are aware of that
can be used with the TT21 to meet the system requirements of AMC20-24 are the Freeflight 1201 and
the NexNav 3101.
A wide range of other GPS receivers can be used as the position source for ADS-B, including ETSO-
129a receivers and simpler VFR GPS receivers. Whilst these receivers may be suitable for
Informational or Experimental installations, they generally lack one or more of the required integrity
parameters for an operational IFR separation application.
12.3 AMC 20-24 Compliance
For installations seeking certification to AMC20-24 or other applicable standards, additional
compliance information is available on request from Trig Avionics Limited.