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2.2.2 GPS Antenna Location
The GPS antenna is a key element in the overall system performance and integrity for a GPS/WAAS
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 enroute
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 4
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.
For rotorcraft, locate the GPS antenna as far as possible from the main rotor hub. This reduces the
percentage of time the blade blocks the antenna. Also mount it as far below the blade surface as possible
if installing the antenna under the blade. This reduces signal distortion caused by the blades.
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.
3.
The GPS antenna should ideally be located at the opposite end of the aircraft from the
COM unit in order to make the GPS less vulnerable to harmonics radiated from the COM
itself.
4a.
The GPS antenna should be mounted no closer than two feet (edge to edge) and ideally
three feet from any VHF COM antenna or any other antenna which may emit harmonic (or
other) 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.
Note: The separation requirement does not apply to GPS and COM combination antennas,
provided the antenna model is TSO authorized and has been tested to meet Garmin's
minimum performance standards. The separating requirement includes the combination
with an XM antenna element as well.
4b.
The GPS antenna should be mounted no closer than two feet (edge to edge) and ideally
three 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.
4c.
To minimize the effects of shadowing at 5° elevation angles, the GPS antenna should be
mounted no closer than 6 inches (edge to edge) from other antennas, including passive
antennas such as another GPS antenna or XM antenna.
5.
To maintain a constant gain pattern and limit degradation by the windscreen, avoid
mounting the antenna closer than 3 inches from the windscreen.
6.
For multiple GPS installations, the antennas should not be mounted in a straight line from
the front to the rear of the fuselage. 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).
Page 2-6
Revision S
GIA 63/GIA 63W Installation Manual
190-00303-05
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