Garmin 010-00861-00 Installationshandbuch - Seite 20
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3 INSTALLATION OVERVIEW
Always follow acceptable avionics installation practices per AC 43.13-1B, AC 43.13-2B, or later FAA
approved revisions of these documents. The GPS/SBAS installation instructions have been prepared to
meet the guidance material contained in AC 20-138A "Airworthiness Approval of Global Navigation
Satellite System (GNSS) Equipment." The communications installation instructions have been prepared
to meet the guidance material defined by AC 20-67B, "Airborne VHF Communications Equipment
Installations."
3.1 Antenna Considerations
This section contains mounting location considerations for the antennas required for the GDL 88 units.
For mounting the GPS/SBAS antenna, refer to FAA approved data. For mounting the NAV antennas,
refer to the aircraft manufacturer‟s data.
3.1.1 GPS Antenna Location
The GPS/SBAS antenna is a key element in the overall system performance and integrity for a
GPS/SBAS navigation system. The mounting location, geometry, and surroundings of the antenna can
affect the system performance and/or availability. The following guidance provides information to aid the
installer in ensuring that the most optimum location is selected for the installation of the GPS antenna.
The installation guidelines presented here meet the intent of AC 20-138A section 16. The greater the
variance from these guidelines, the greater the chance of decreased availability. Approach procedures
with vertical guidance are the most sensitive to these effects. LNAV only approaches, terminal
operations, and en route operations may also be affected. Because meeting all of these installations
guidelines may not be possible on all aircraft, these guidelines are listed in order of importance to achieve
optimum performance. Items 3 below are of equal importance and their significance may depend on the
aircraft installation. The installer should use their best judgment to balance the installation guidelines.
Figure 3-1 shows the recommended placement of antennas.
1. Mount the antenna as close to level as possible with respect to the normal cruise flight attitude
of the aircraft. If the normal flight attitude is not known, substitute the waterline, which is
typically referenced as level while performing a weight and balance check.
2. The GPS antenna should be mounted in a location to minimize the effects of airframe
shadowing during typical maneuvers. Typically mounting farther away from the tail section
reduces signal blockage seen by the GPS antenna.
3a. The GPS antenna should be mounted no closer than two feet from any VHF COM antenna or
any other antenna which may emit harmonic interference at the L1 frequency of
1575.42 MHz. An aircraft EMC check (reference VHF COM interference check in Post
Installation Checkout procedures) can verify the degradation of GPS in the presence of
interference signals. If an EMC check reveals unacceptable interference, insert a GPS notch
filter in line with the offending VHF COM or the (re-radiating) ELT transmitter.
When mounting a combination antenna (ex. GPS and COM, GPS and XM), the
recommended distance of two feet or more is not applicable to the distance between the
antenna elements provided the combination antenna is TSO authorized and has been
tested to meet Garmin‟s minimum performance standards.
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GDL 88 TSO Installation Manual
190-01122-00