GNX 375 GPS dropouts

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Leo Langston

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I got the GNX 375 installed a year ago and I am seeing more and more random GPS dropout failures as I fly around near Houston, TX. I usually notice it when the SYN Vis and magenta line on my Aspen displays goes away. I noticed it again a few days ago shooting approaches into Galveston. Apparently it looses satellites so it can’t get a position. After a few minutes it comes back. The 375 showed it only had 2 satellites. I am not an IFR pilot so when this happened during long cross countries I didn’t really care but since I am doing my instrument training it’s very annoying if not dangerous. Has anyone else experienced similar issues with the 375? When I finally asked my avionics tech he said GPS should never fail unless you know the govt is screwing with it. I have it scheduled for a checkup soon. Is this a antenna, coax or 375 issue? I have a Navion and the GPS antenna is on the upper fuselage just behind the rear of the canopy.
 
Leo Langston said:
I got the GNX 375 installed a year ago and I am seeing more and more random GPS dropout failures as I fly around near Houston, TX. I usually notice it when the SYN Vis and magenta line on my Aspen displays goes away. I noticed it again a few days ago shooting approaches into Galveston. Apparently it looses satellites so it can’t get a position. After a few minutes it comes back. The 375 showed it only had 2 satellites. I am not an IFR pilot so when this happened during long cross countries I didn’t really care but since I am doing my instrument training it’s very annoying if not dangerous. Has anyone else experienced similar issues with the 375? When I finally asked my avionics tech he said GPS should never fail unless you know the govt is screwing with it. I have it scheduled for a checkup soon. Is this a antenna, coax or 375 issue? I have a Navion and the GPS antenna is on the upper fuselage just behind the rear of the canopy.
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I had the same problem with a GNS480. The GA35 antenna was going bad. Replaced it and problem solved.
 
@Kristin mentions one of the common failure points. The GA35 is sensitive to over torquing of the fasteners (there is a specified torque included with the antenna and hardware, which is lower than one might think). If one cranks those down, it causes micro cracking and can be a water intrusion point for future failure of the antenna. Certainly there could be a number of things, including gov't GPS jamming, truckers with GPS jammers, coax issues, antenna issues, issues with the unit, interference from a 2nd COM that is radiating in to GPS spectrum, etc, but that is one of the more common installation oversights that causes a later problem. If you are just parked out on the ramp, and let it acquire a position lock, how many satellites and what relative strength do you see?
 
That’s good to know. Hopefully it is just the antenna. My avionics shop can replace that easy enough.
 
Was this happening while you were transmitting on your Com? I had the issue when I first installed my GNX375 and It was an interference/feedback issue with the ELT when transmitting on the Com at frequencies lower on the band especially between 120.5-121.5. I see Galveston tower is 120.575. Took awhile and a bunch of other troubleshooting before my avionics shop figured it out.

It’s easy to check. Turn on your avionics and pull up the satélite page on the 375. Dial in Different Com frequencies and then hit the transmit button. After a second or two all of the bars for the satélites would drop to near zero. Stop transmitting and it would take a little bit to recover the lock on the satelites. It was only interfering on the lower frequencies. An inexpensive notch filter on one of my coms fixed it.
 
I will try that. Never heard of that before. Thx.
 
Also, try disconnecting the ELT antenna and/or removing the ELT; where's the ELT antenna relative to the VHF/GPS antennas? Any other sources of noise, like a gas station usb charger plugged into the 12v outlet?

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On my Navion the GPS antenna is a few inches aft of the closed canopy on the R side. The VHF com antenna is a few inches farther aft on the L side. The ELT antenna (old school 121.5 only) is slightly behind that on the R side and further aft on the R side are the 2 RSMs for the Aspen 2000 Pro Max displays.
 
This response nailed it. On my Comm2 radio between 120-122 frequencies when I push the PTT it completely kills the GPS. All of them like a light switch. It didn’t occur on Comm 1 but we could see some slight decrease in some of the green bars. So with old Narco MK12D is the solution new radios or something else? I am sure my avionics shop will say new or newer radios!
 
Leo Langston said:
This response nailed it. On my Comm2 radio between 120-122 frequencies when I push the PTT it completely kills the GPS. All of them like a light switch. It didn’t occur on Comm 1 but we could see some slight decrease in some of the green bars. So with old Narco MK12D is the solution new radios or something else? I am sure my avionics shop will say new or newer radios!
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I had the Garmin 175 installed, which is the GPS only version. I also have Narco radios and would lose the GPS upon transmitting. I returned it to my install facility and they installed filters for the Narco radios. This did rectify the problem with no issues since the install.
 
Leo Langston said:
This response nailed it. On my Comm2 radio between 120-122 frequencies when I push the PTT it completely kills the GPS. All of them like a light switch. It didn’t occur on Comm 1 but we could see some slight decrease in some of the green bars. So with old Narco MK12D is the solution new radios or something else? I am sure my avionics shop will say new or newer radios!
Click to expand...
Glad you found the issue. Mine were with Narco 12D+radios. A notch filter fixed it but interestingly mine was happening on com 1 but the notch filter had to be installed on com 2 to solve the problem.
 
My avionics shop will probably tell me to quit Xmitting on #2 or buy a new radio. But I will see if they can install a notch filter. If that just a install on the bayonet fittings or do they have to measure and adjust some sort of a pot to clear up the frequencies?
 
Leo Langston said:
My avionics shop will probably tell me to quit Xmitting on #2 or buy a new radio. But I will see if they can install a notch filter. If that just a install on the bayonet fittings or do they have to measure and adjust some sort of a pot to clear up the frequencies?
Click to expand...
It just connects inline at the radio via BNC connectors, the closer to the radio the better. There is no adjustment.
 
Thx. I will see if they have one they can install.
 
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