Signal Acquisition Check; Vhf Com Interference Check - Garmin GTN 625 Installation Manual

6.11.2

Signal Acquisition Check

NOTE
All other avionics should be turned off at the start of this test, with the GTN powered on in
normal mode.
Ensure the GTN is able to acquire sufficient satellites to compute a GPS position. From the Home page,
touch the 'System' key and then touch the GPS Status key. Under Status, ensure that a 3D Fix, or 3D Diff
Fix is obtained. If the unit is unable to acquire satellites, move the aircraft away from obstructions which
might be shading GPS reception. If the situation does not improve, check the GPS antenna installation.
NOTE
After installation, the initial acquisition of position can take up to 20 minutes. Subsequent
acquisitions will not take that long.
Once GPS position information is available, perform the following steps:
1. On the GPS Status page, verify that the LAT/LON agree with a known reference position.
2. While monitoring the GPS Status Page, turn on other avionics one at a time and check the GPS
signal reception to make sure it is not affected (no significant signal degradation).
3. Before proceeding with the VHF COM interference check, ensure that any connected equipment is
transmitting and/or receiving data from the GTN and is functioning properly.
6.11.3

VHF COM Interference Check

It is known that certain non-aviation radios, including marine transceivers, can interfere with civil aviation
navigation and surveillance equipment including the Garmin GTN. When installing GTN equipment, it is
the responsibility of the installer to ensure that the GTN modification is compatible with all previous
aircraft modifications. Garmin recommends that whenever a GTN is installed in an aircraft that has been
modified with non-aviation radios, particular care should be exercised to verify that these do not interfere
with proper function of the GTN. Special care should also be taken to ensure that there is no interference
with the GTN if non-aviation radios are installed in an aircraft after a GTN has been installed. If
interference is found, it can be addressed by relocating antennas, rerouting cables, using filters to attenuate
unintentional harmonic frequency transmissions, or using various other techniques for elimination of the
interference. It may be necessary to remove or replace the interfering radio with a model that does not
interfere with the proper functioning of the GTN.
If you are testing a transmitter from a non-aviation device, each frequency must be verified by transmitting
for at least 30 seconds on each channel. The interference check must be completed on all IFR installations.
Once the Signal Acquisition Test has been completed successfully, perform the following steps:
1. View the GPS Status Page and verify that at least 7 satellites have been acquired by the GTN.
2. Verify that the GPS "LOI" flag is out of view.
3. Select 121.150 MHz on the COM transceiver to be tested.
4. Transmit for a period of 35 seconds.
5. Verify that the GPS "LOI" flag does not come into view.
6. Repeat steps 4 and 5 for the following frequencies:
GTN 625/635/650 TSO Installation Manual
Page 6-58
190-01004-02
Rev. F