User's Manual

TT21 Transponder Installation Manual 19 January 2009
00560-00 Issue AA
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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 GPS Input
If a GPS is connected for ADS-B position reporting, select the appropriate interface protocol using the
rotary knob.
6.1.8 GPS Line Speed
If a GPS has been selected at the previous step, you should select the appropriate line speed using the
rotary knob. Panel mount GPS units with Aviation format outputs generally run at 9600 bps. NMEA
GPS units generally run at 4800 bps. Freeflight 1201 and NexNav 3101 GPS receivers generally run at
19200 bps.
6.1.9 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:
Equipment Transmitted Integrity Level
VFR only GPS or uncertified installation Low
GPS installation certified for en-route and terminal IFR
navigation
Medium
GPS installation certified with augmentation, such as
WAAS or LAAS
High
6.1.10 Aircraft Length and Width
On the ground, ADS-B transmits encoded aircraft size information which is used by ATC to identify
taxiing routes and potential conflicts. When configured for ADS-B, the TT21 will ask for the aircraft
length and width (wingspan), in metres, and will calculate the appropriate size code for transmission.
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