GPS intermittent signal loss

[lpaaruule]

I send the below email to GRT Avionics a little while ago, but thought I'd also post it here in case any of you have experienced a similar issue, or have any ideas.

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Hello. While flying I occasionally have GPS signal lose for a few second. Yesterday, I lost GPS signal for for a couple minutes during flight following. The track has me turning the wrong way until I started using the magnetic heading.

I’m not sure if the interference is coming from inside the plane or outside. I was near a military base, but I’ve had it happen other places as well.

What’s even more confusing is that I also occasionally lose GPS signal on my cell phone at he same time. My phone is using an app that communicates wifi with a Stratux. I don’t know if the app uses the Stratux GPS position of it’s own. The Stratux is also serially connected to my SportEX for ADS-B info, which works great.

I fly a Sonex, and I have two GPS antennas on the glare shield. My comm antennas are on the bottom of the plane. However, the GPS antennas are coiled up under the instrument panel, and are close to the comm coax.

I’m considering rerouting the GPS cable locations, but right now I’m just guessing what the problem could be. I’m also considering removing the GPS antenna that goes directly to the EFIS, and just using the one for the SafeFly 2020.

Any suggestions would be appreciated.

[GordonTurner]

Hi Paul

Not my field of expertise, but i have heard that while gps antennas are obviously not transmitting, the modern ones are powered (amplified?) and can interfere with each other. Perhaps the two antennae are too close together.

Let us now what you find out. Gordon

[GraemeSmith]

Not uncommon for GPS to be out or messed with. The fact your phone is also having a simultaneous problem tends to point to an outside / environmental issue. Rather than the plane. At any time I have FOUR GPS antenna running in the plane. One for the EFIS, A WAAS for the ADS-B out, Cell phone and iPad. All within 8 ft of each other. No issues.

Check here:

https://www.gps.gov/support/user/#aviation

[lpaaruule]

My GPS antennas are almost touching each other. I’ll try to separate them a bit, or just remove one of them.

From Garmin GL-82 Manual:
The GPS antenna should be mounted no closer than two feet from any antennas emitting more than 25 watts of power.
5. To minimize the effects of shadowing at 5° elevation angles, the GPS antenna must be mounted no closer than 9 inches (center to center) from other antennas, including passive antennas such as another GPS antenna or XM antenna.
6. To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting the antenna closer than 3 inches from the windscreen.
7. A 12 inch center-to-center spacing between GPS antennas is required to achieve the best possible low-elevation antenna gain by minimizing pattern degradation due to shadowing and near-field interaction. When practical, 12 inch center-to-center spacing between GPS antennas must be used. If 12 inch spacing is not practical, the maximum center-to-center spacing possible must be used, but never less than 9 inch center-to-center spacing. Spacing less than 9 inches center-to-center results in unacceptable antenna pattern degradation.

[lpaaruule]

GRT recommended that I separate the GPS antennas at least 8", and make sure they are at least 12 inches away from any transceiver coax.

I did as they said, and went for a 1/2 hr flight a couple days ago. So far, so good. The issue didn't always occur though, so time will tell. If the problem continues, I'll update this thread.