Garmin GNS 480 Installation Manual page 106

Post-Installation Configuration & Checkout
3. Check the GPS signal reception using the GPS Status page. Typical signal levels are between
60% and 80% of full scale (viewed using the bar graph), although values above and below this
range are normal.
4. Turn on other avionics one at a time and check the GPS signal reception to make sure it is not
affected.
5. Check for VHF com transmitter interference.
The interference check must be completed on all IFR installations.
a) Verify that at least five satellites are displayed in green boxes (with the corresponding
satellite vehicle number displayed in green on the left of the bar graph). Ensure that NO
POSITION FIX is not displayed.
b) Press the COM bezel key and tune the GNS 480 Com to 121.125 MHz. Listen on the
frequency to ensure it is not in use, and then transmit for 30 seconds.
When the mic has been keyed for 35 seconds, a "WARNING Stuck Microphone" message
will pop-up and the com transmitter will stop transmitting. If required to transmit for an
additional 15 seconds, the PTT switch must be momentarily released and then re-keyed to
continue transmitting – the WARNING message does not have to be cleared to continue
transmitting.
c) While transmitting, observe the signal status of each satellite being received. If the satellite
signals are significantly degraded or some satellite signals are lost during the test, continue
transmitting for another 15 seconds. Verify that the GPS position remains valid (if position
is lost, NO POSITION FIX will be displayed in place of the position data). If position is
lost, additional isolation measures will have to be taken.
If required, clear the "WARNING Stuck Microphone" message by pressing the CLR bezel
d)
key. Repeat steps b) and c) for additional frequencies as follows.
121.150 MHz
121.175 MHz
121.185 MHz
121.190 MHz
121.200 MHz
121.225 MHz
121.250 MHz
131.200 MHz
e) Repeat for each com transmitter installed in the aircraft.
f) If aircraft is TCAS-equipped, turn on the TCAS system and verify that GPS position remains
valid (if position is lost, NO POSITION FIX is displayed in place of the position data).
g) If aircraft is SATCOM-equipped, use the SATCOM system and verify that GPS position
remains valid (if position is lost, NO POSITION FIX is displayed in place of the position data).
h) If the GNS 480 is susceptible to VHF Com transmitter interference, then better isolation (or
greater separation) may be required between the GPS and VHF (or other offending system)
antennas. With some com transmitters, a 1575.42 MHz notch filter (such as Garmin AT P/N 162-
3-32
Figure 3-31 – GPS Status Page
[1]
[1]
[1] frequency is only applicable to VHF radios with 8.33 kHz channel spacing
560-0982-01 Rev A
NOTE
NOTE
131.225 MHz
131.250 MHz
131.275 MHz
130.285 MHz
[1]
131.290 MHz
[1]
131.300 MHz
131.325 MHz
131.350 MHz
GNS 480 (CNX80) Installation Manual