Installation Planning and Considerations
190-01810-00
GDL 82 TSO Installation Manual
Rev. 5
Page 4-7
4.5.2 GPS (GDL 82 with GPS/SBAS Only)
The following provides guidance for selecting an optimal location for GPS antenna installation. The installation
guidelines meet the intent of AC 20-138D Section 16. The greater the variance from these guidelines, the greater the
chance of decreased signal availability. 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. The installer should use
their best judgment to balance the installation guidelines.
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 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.
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 XM) provided the combination antenna is TSO authorized and has been tested to meet
Garmin’s minimum performance standards.
4.
The GPS antenna should be mounted no closer than two feet from any antennas emitting more than 25 watts
of power.
5.
To minimize the effects of shadowing at 5
°
elevation angles, the GPS antenna must be mounted no closer than
9 inches (center to center) from other antennas, including passive antennas such as another GPS antenna or
XM antenna.
6.
To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting the antenna
closer than 3 inches from the windscreen.
7.
A 12 inch center-to-center spacing between GPS antennas is required to achieve the best possible
low-elevation antenna gain by minimizing pattern degradation due to shadowing and near-field interaction.
When practical, 12 inch center-to-center spacing between GPS antennas must be used. If 12 inch spacing is
not practical, the maximum center-to-center spacing possible must be used, but never less than 9 inch
center-to-center spacing. Spacing less than 9 inches center-to-center results in unacceptable antenna pattern
degradation.
