Garmin GDU 37X Installation Manual

Also see for GDU 37X: Installation manual Pilot's guide Installation manual

Contents

Garmin GDU 37X Installation Manual Manual pdf 25 page image

GDU 37X Installation Manual Page 3-3190-01054-01 Revision C3.3.2 GPS/XM Antenna Mounting LocationThe GPS antenna is a key element in the overall system performance and integrity for a GPS navigationsystem. The mounting location, geometry, and surroundings of the antenna can affect the systemperformance and/or availability. The following guidance provides information to aid the installer inensuring that the optimum location is selected for the installation of the GPS antenna. The installationguidelines presented here meet the intent of AC 20-138A section 16. The greater the variance from theseguidelines, the greater the chance of decreased availability. Because meeting all of these installationsguidelines may not be possible on all aircraft, these guidelines are listed in order of importance to achieveoptimum performance. Items 4 below are of equal importance and their significance may depend on theaircraft installation. The installer should use their best judgment to balance the installation guidelines.1. Mount the antenna on top of the aircraft in a location with an unobstructed view of the sky, as close tolevel as possible with respect to the normal cruise flight attitude of the aircraft. If the normal flightattitude is not known, substitute the waterline, which is typically referenced as level while performinga weight and balance check.2. The GPS antenna should be mounted in a location to minimize the effects of airframe shadowing dur-ing typical maneuvers. Typically mounting farther away from the tail section reduces signal blockageseen by the GPS antenna.3. The GPS antenna should ideally be located at the opposite end of the aircraft from the COM unit inorder 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 fromany VHF COM antenna or any other antenna which may emit harmonic (or other) interference at theL1 frequency of 1575.42 MHz. An aircraft EMC check (reference VHF COM interference check inPost Installation Checkout procedures) can verify the degradation of GPS in the presence of interfer-ence signals. If an EMC check reveals unacceptable interference, insert a GPS notch filter in line withthe offending VHF COM or the (re-radiating) ELT transmitter.The separation requirement does not apply to GPS and COM combinationantennas, provided the antenna has been tested to meet Garmin’s minimumperformance standards. The separating requirement includes the combinationwith 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 fromany antennas emitting more than 25 watts of power. An aircraft EMC check can verify the degradationof GPS in the presence of interference signals.4c. To minimize the effects of shadowing at 5° elevation angles, the GPS antenna should be mounted nocloser than 6 inches (edge to edge) from other antennas, including passive antennas such as anotherGPS antenna or XM antenna.5. To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting theantenna 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 tothe rear of the fuselage. Also varying the mounting location will help minimize any aircraft shading bythe wings or tail section (in a particular azimuth, when one antenna is blocked the other antenna mayhave a clear view).NOTE