Garmin GDL 88 Installation Manual

Garmin GDL 88 Installation Manual Manual pdf 20 page image

Page 3-2 GDL 88 TSO Installation ManualRev. A 190-01122-003 INSTALLATION OVERVIEWAlways follow acceptable avionics installation practices per AC 43.13-1B, AC 43.13-2B, or later FAAapproved revisions of these documents. The GPS/SBAS installation instructions have been prepared tomeet the guidance material contained in AC 20-138A “Airworthiness Approval of Global NavigationSatellite System (GNSS) Equipment.” The communications installation instructions have been preparedto meet the guidance material defined by AC 20-67B, “Airborne VHF Communications EquipmentInstallations.”3.1 Antenna ConsiderationsThis section contains mounting location considerations for the antennas required for the GDL 88 units.For mounting the GPS/SBAS antenna, refer to FAA approved data. For mounting the NAV antennas,refer to the aircraft manufacturer‟s data.3.1.1 GPS Antenna LocationThe GPS/SBAS antenna is a key element in the overall system performance and integrity for aGPS/SBAS navigation system. The mounting location, geometry, and surroundings of the antenna canaffect the system performance and/or availability. The following guidance provides information to aid theinstaller 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 thevariance from these guidelines, the greater the chance of decreased availability. Approach procedureswith vertical guidance are the most sensitive to these effects. LNAV only approaches, terminaloperations, and en route operations may also be affected. 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 3 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.Figure 3-1 shows the recommended placement of antennas.1. Mount the antenna as close to level as possible with respect to the normal cruise flight attitudeof the aircraft. If the normal flight attitude is not known, substitute the waterline, which istypically 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 airframeshadowing during typical maneuvers. Typically mounting farther away from the tail sectionreduces signal blockage seen by the GPS antenna.3a. The GPS antenna should be mounted no closer than two feet from any VHF COM antenna orany other antenna which may emit harmonic interference at the L1 frequency of1575.42 MHz. An aircraft EMC check (reference VHF COM interference check in PostInstallation Checkout procedures) can verify the degradation of GPS in the presence ofinterference signals. If an EMC check reveals unacceptable interference, insert a GPS notchfilter in line with the offending VHF COM or the (re-radiating) ELT transmitter.NOTEWhen 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.