Garmin GDL 88 Installation Manual page 21

3b. The GPS antenna should be mounted 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.
3c. To achieve the best possible low-elevation antenna gain (by minimizing pattern degradation
due to shadowing and near-field interaction), the GPS antenna must be mounted with clearance
from other antennas, including passive antennas such as another GPS antenna or XM antenna.
When practical, installers will use 12 inch center-to-center spacing between antennas. If 12
inch spacing is not practical, installers will use the maximum center-to-center spacing from
adjacent antennas, but never less than 9 inch center-to-center spacing. Spacing less than 9
inches center-to-center results in unacceptable GPS/SBAS antenna pattern degradation.
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. 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).
GDL 88 TSO Installation Manual
190-01122-00
Page 3-3
Rev. A