NorthStar 1200-02-01, Installation Manual

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Section Two—Installation Instructions
M3 GPS INSTALLATION MANUAL Revision D PAGE 23
from this interference. Tests shall be conducted by tuning each VHF trans-
mitter to the frequencies listed below and transmitting for 30 seconds,
while observing the signal status of each or all satellites received. Degra-
dation of individual satellite signals or all satellite signals to a point where
navigation using GPS is no longer possible may not be acceptable for use
under IFR, and requires that additional isolation or filter techniques are
included in the aircraft installation or appropriate limitation stipulated in
the Airplane Flight Manual Supplement (AFMS).
The worst-offending equipment is often the VHF comm transmitter and
associated antenna. Additional isolation may be obtained by increasing the
separation between the VHF and GPS antennas or, in some cases, by
re-routing antenna cables. Replacing the comm antenna with an induc-
tive-fed type such as Comant CI-121 or Dorne Margolin DMC 70/11 may
help. Low-pass filtering inserted within the VHF transmitter/receiver
coaxial cable also may be used, provided that the VHF transmission and
reception isn’t significantly degraded.
It has been noted that some ELT transmitters re-radiate harmonics of
comm frequencies in the GPS band. Try disconnecting the ELT antenna if
this is suspected. If the ELT is interfering, try relocating the ELT antenna
or replacing the ELT with another type.
Configuring the M3
CDI centering and
scale adjustment
The interface to an external CDI (or HSI) and annunciators must be cali-
brated for each installation. Be sure to complete the entire proce-
dure—only the fifth step causes the changes to be stored in the unit's
memory. This procedure also verifies correct wiring and operation of the
CDI interface and annunciators.
Table 4: Potential VHF Interferences
121.125 MHz 131.200 MHz
121.150 MHz 131.225 MHz
121.175 MHz 131.250 MHz
121.200 MHz 131.275 MHz
121.225 MHz 131.300 MHz
121.250 MHz 131.325 MHz
131.350 MHz