TT31 Transponder Installation Manual 1 February 2010
00455-00 Issue AJ
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the aircraft registration using the rotary knob and the ENT button.
Note that the aircraft registration is loaded as letters and numbers only. There are no dashes or other
punctuation marks, and no spaces can be inserted. When you enter a space it finishes the data entry and
moves to the next item.
6.1.4 Airspeed Category
Mode S transponders can transmit their maximum airspeed characteristics to aircraft equipped with
TCAS. This information is used to help identify threats and to plan avoiding action by the TCAS
equipped aircraft. The airspeeds are grouped in ranges; using the rotary knob, select the range that
corresponds to the aircraft.
6.1.5 Aircraft Category
To assist ATC tracking of aircraft, an aircraft category can be transmitted by Mode S transponders.
Using the rotary knob, select the aircraft category that most closely matches the aircraft the transponder
is installed in.
6.1.6 Squat Switch Source
The Squat switch input allows the transponder to automatically switch between Airborne and Ground
modes, and to automatically start and stop the flight timer. The sense of the squat switch input can be
selected using the rotary knob. If the squat switch input is not connected the “Not Connected” option
must be selected.
6.1.7 TIS Output
If the aircraft has a Traffic Information Service (TIS) compatible display connected to the transponder,
select the appropriate interface protocol using the rotary knob.
Note: TIS is a Mode S uplink service that is provided by some US approach radars. TIS
coverage is limited to the coverage areas of those radars; there is no TIS provision outside the
USA.
6.1.8 GPS Input
If a GPS is connected for ADS-B position reporting, select the appropriate interface protocol using the
rotary knob.
6.1.9 GPS/TIS Line Speed
If a GPS input or TIS output has been configured, you should select the appropriate line speed using the
rotary knob. Traffic displays using the Garmin protocol run at 9600 bps. Panel mount GPS units with
Aviation format outputs generally also run at 9600 bps. NMEA GPS units generally run at 4800 bps.
Freeflight 1201 and NexNav 3101 GPS receivers generally run at 19200 bps.
Note: The TIS output and GPS input speeds are not separately controlled on the TT31. Not all
combinations of GPS input and TIS output will be usable if the external devices operate on
fixed bit rates and are different to each other.
6.1.10 GPS System Integrity Level
An important metric for ADS-B ground system behaviour is the SIL or System Integrity Level. It is
intended to reflect the probability that the GPS position source is providing erroneous information. A
detailed analysis of the contribution to system integrity is outside the scope of this manual, and the
installer may need to carry out a system safety analysis to determine the best value to set. However, a
reasonable guideline might be:
Trig Avionics Limited Page 19
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