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Limitations
3.2.2
Flight Manual Supplement
For GDL 90 installations that do not include the GSL 71 UAT Control Panel, the Flight Manual
Supplement (AFMS or RFMS) for any external control unit interfaced to the GDL 90 must include the
following operational limitation (or similar FAA-approved limitation):
The UAT Datalink is approved for Air Traffic Control (ATC) ADS)-B Surveillance
Services in the United States. For areas where ATC Surveillance Services are provided,
the UAT equipment shall broadcast aircraft position, velocity, barometric altitude
information, flight identification and/or a 4096 squawk code.
UAT datalink is also used to receive Traffic Information Services - Broadcast (TIS-
B) and Flight Information Services - Broadcast (FIS-B) information. A display is
needed to receive TIS-B and FIS-B information.
Not all areas of the United States have the capability to provide ATC services or
TIS-B/FIS-B information on the UAT datalink. Refer to Notice to Airman for areas
of coverage and operational applications, found on the FAA website: www.adsb.gov.
When directed by ATC to turn off the ADS-B transmission, pilot should (...) to stop
ADS-B transmissions while airborne or on the surface.
Replace the (...) with the appropriate instructions for the particular control unit being used in this
installation. For example, if a transponder is being used as the UAT control unit, the instruction would be
"...use the Standby button...".
Check the appropriate box (if present) in the Flight Manual Supplement to indicate which equipment
provides control for the GDL 90.
3.2.3
TCAS Interface
The GDL 90's discrete input TCAS interface is not approved for use. The APM shall not be configured
to use the "TCAS installed and interfaced" configuration (Setting '2'). If TCAS is installed, use Setting
'1'. If TCAS is not installed, use Setting '0'.
3.2.4
GPS Antenna
The GDL 90 is compatible with the Garmin AT A-33 and A-34 antennas. Refer to Section 1.3.7.1 for
specifications.
The GDL 90 GPS receiver is not compatible with SATCOM installations.
3.2.5
UAT Antenna
The GDL 90 requires at least one UHF antenna meeting the following specifications:
Standard 50 vertically polarized antenna with a VSWR < 1.7:1 at 978 Mhz
TSO-C66, C74, C112 or C154 antennas that also meet the VSWR specification. Garmin AT P/N
590-0051 or P/N 590-0052 antenna meet these requirements.
Operating the GDL 90 without RF terminations on the Top or Bottom UAT Antenna
ports can result in equipment damage. Always operate the GDL 90 with the Top and
Bottom UAT Antenna ports terminated with a VSWR ratio of 3.0:1 or less.
3-2
CAUTION
560-1049-02 Rev. B
GDL 90 Installation Manual
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Troubleshooting
