Garmin GPS 400W Installation Manual

Also see for GNC 420AW: Maintenance manual Pilot's guide addendum Quick reference guide Pilot's Guide & Reference Pilot's Guide & Reference

Contents

Garmin GPS 400W Installation Manual Manual pdf 38 page image

Page 2-4 GNS 400W Series Installation ManualRev. G 190-00356-082.4 Antenna ConsiderationsThis section contains mounting location considerations for the antennas required for the 400W Seriesunits.The GPS 400W requires a GPS/WAAS antenna, the GNC 420W/420AW requires a GPS/WAAS andCOM antenna, and the GNS 430W and GNS 430AW require four antennas: a GPS/WAAS antenna, aCOM antenna, a NAV antenna, and a Glideslope antenna.2.4.1 GPS Antenna LocationThe GPS antenna is a key element in the overall system performance and integrity for a GPS/WAASnavigation system. The mounting location, geometry, and surroundings of the antenna can affect thesystem performance and/or availability. The following guidance provides information to aid the installerin ensuring that the most optimum location is selected for the installation of the GPS antenna. Theinstallation guidelines presented here meet the intent of AC 20-138C section 12. The greater the variancefrom these guidelines, the greater the chance of decreased availability. Approach procedures with verticalguidance 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 bepossible on all aircraft, these guidelines are listed in order of importance to achieve optimumperformance. Items 3 below are of equal importance and their significance may depend on the aircraftinstallation. 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 ofthe aircraft. If the normal flight attitude is not known, substitute the waterline, which is typicallyreferenced 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 shadowingduring typical maneuvers. Typically mounting farther away from the tail section reduces signalblockage seen by the GPS antenna.3a. The GPS antenna should be mounted no closer than two feet from any VHF COM antenna or anyother antenna which may emit harmonic interference at the L1 frequency of 1575.42 MHz. Anaircraft EMC check (reference VHF COM interference check in Post Installation Checkoutprocedures) can verify the degradation of GPS in the presence of interference signals. If an EMCcheck reveals unacceptable interference, insert a GPS notch filter in line with the offending VHFCOM or the (re-radiating) ELT transmitter.Note: When mounting a combination antenna (ex. GPS and COM, GPS and XM), therecommended distance of two feet or more is not applicable to the distance between theantenna elements provided the combination antenna is TSO authorized and has beentested to meet Garmin‘s minimum performance standards.3b. The GPS antenna should be mounted no closer than two feet from any antennas emitting morethan 25 watts of power. An aircraft EMC check can verify the degradation of GPS in thepresence of interference signals.3c. To minimize the effects of shadowing at 5 elevation angles, the GPS antenna should be mountedno closer than 6 inches (edge to edge) from other antennas, including passive antennas such asanother GPS antenna or XM antenna.4. To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting theantenna closer than 3 inches from the windscreen.5. For multiple GPS installations, the antennas should not be mounted in a straight line from thefront to the rear of the fuselage. Also varying the mounting location will help minimize any