Installation Instructions

190-01007-A3 GTN 6XX/7XX AML STC Installation Manual
Rev. 4 Page 2-19
2.4.4.1 GPS Antenna Location
The GPS antenna is a key element in the overall system performance and integrity for a GPS/SBAS
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 the latest revision of AC 20-138A Chapter 12,
Section 12-1. The greater the variance from these guidelines, the greater the chance of decreased signal
availability. Because meeting all of these installation guidelines may not be possible on all aircraft, these
guidelines are listed in order of importance to achieve optimum performance. The sub-items of step 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 2-6 shows the recommended placement of the GPS antenna.
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 with the waterline, which is
typically referenced as level while performing a weight and balance check. A shim may be used
to level the antenna.
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.
3. The GPS antenna should be mounted:
a. 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 (see
Section 5) can verify the degradation of GPS in the presence of interference signals. If an
EMC check reveals unacceptable interference, either insert a GPS notch filter in line with
the offending VHF COM or the (re-radiating) ELT transmitter, or select a different
GPS Antenna location.
NOTE
When mounting a combination antenna, the recommended distance of two feet or more is
not applicable to the distance between the antenna elements in a combination antenna
(e.g. GPS and COM, GPS and SiriusXM) provided the combination antenna is TSO
authorized and has been tested to meet Garmin’s minimum performance standards.
b. no closer than two feet from any antennas emitting more than 25 watts of power. An
aircraft EMC check can verify the degradation of GPS in the presence of interference
signals. If an EMC check reveals unacceptable interference, select a different GPS Antenna
location.
c. no closer than nine inches (center to center) from other antennas, including passive
antennas such as another GPS Antenna or XM antenna. This will minimize the effects of
shadowing at 5° elevation angles.
4. To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting the
antenna closer than 3 inches from the windscreen.
5. For multiple GPS installations, the antennas should not be mounted in a straight line from the
front to the rear of the fuselage (i.e. so that a single lightning strike does not damage all GPS
systems). This is required in non-metallic aircraft, and recommended for all aircraft. Also varying
the mounting location will help minimize any aircraft shading by the wings or tail section (in a
particular azimuth, when one antenna is blocked the other antenna may have a clear view).