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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."

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.

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