Garmin AT
Rev --
© 2004 Garmin AT
GX Service Manual
5-3
important to mention that the RFI interference coming from the GPS receiver, Comms, and
NAV/Comms is not getting into the case of our GPS receivers. The Apollo GPS units are not
susceptible to RFI interference. The problem comes from the interference reaching the GPS antenna.
To prove this, you can walk around the aircraft with the GPS antenna in hand. When you get far
enough away from the aircraft, normal operation of the GPS receiver will resume.
Remove Comm 1 and repeat the above steps for Comm 2.
Install both Comm 1 and Comm 2. Turn on both Comms and repeat the interference checks. If the
interference returns, then Comm 1 transmitter is causing Comm 2 receiver to re-emit interfering L1
frequencies. To verify inter-coupling, you can turn off one of the Comms and repeat the tests. The
quickest way to solve Comm 1 and 2 inter-coupling is to separate the antennas. Place Comm 1 on
the top side and Comm 2 on the bottom side of the aircraft.
Re-install the ELT and repeat the interference tests. If the ELT causes GPS interference, you can
install a GPS notch filter in line with the antenna. Most of the case enclosures for ELTs are plastic
and the case needs to be wrapped in appropriate metal shielding to ground the case and shield of the
antenna. The ELT is a very common source of L1 interference.
Direction finding equipment used in Search and Rescue operations by the CAP and others can also
be a source of L1 interference. The non-linear properties of pin diodes, used to switch between
antenna elements, will generate harmonics when excited by a strong VHF signal like those from a
nearby Comm radio. Interference on the 12
th
and 13
th
harmonics is re-broadcast from the direction-
finding antenna. This interference on the L1 frequency causes problems with the GPS. To overcome
this interference, make sure the direction-finding antenna is located on the bottom of the aircraft and
that GPS notch filters are installed.
Test one more time. If the interference has been eliminated, permanently mount the GPS antenna
and re-test. These guidelines can be used to minimize GPS interference problems.
Summary of Contents for APOLLO GX SERIES
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