intermittant GPS

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curts63
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intermittant GPS

Post by curts63 »

I'm hoping someone can shed some light on an issue. I was flying with GPS on a IFR training flight. The GPS quit working. I noticed that in the window that it said there weren't enough satellites in view. I know where I was and have a hard time thinking that was the real problem. Everything else seemed good, radios worked, VOR worked, and so on.

I cycled the master switch off and then back on. The GPS finally came back on. After I landed, got my clearance, and took off for 5 minutes, then it happened again. This time, I didn't cycle power, seeing if it would just "find" the satellites again. No luck.

I'm thinking a loose wire or bad antenna? Any one have this issue or have any suggestions where to start?

Thank you,
Curt
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Rick
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Re: intermittant GPS

Post by Rick »

Do you have WAAS? There is a known problem with a bunch of the Garmin WAAS antennas that causes them to disrupt GPS reception when they go into some strange mode where they transmit. There is a thread about it here. I know this because I had two of these faulty antennas on a previous DA40 I flew...
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curts63
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Re: intermittant GPS

Post by curts63 »

I'm sorry, I should have been more detailed in my equipment listing. I have a 2012 DA40 with the G-1000/GFC 700 with WAAS panels. When the GPS quit, my tablet did not. I was able to look at the moving map to help with situational awareness. In recycling the power, it seemed to fix it. I couldn't cycle the power while trying to talk to ATC.

I looked at the link that was attached above. I'm not really sure this applies? The satellites were there, but the system didn't see them, or find them again without recycling the power. My local avionics shop doesn't really have experience with G1000, so I'm trying to get a good starting point to direct them. Of course, if it's just a loose wire, I'll do that myself.

Curt
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Rick
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Re: intermittant GPS

Post by Rick »

Curt,
I agree - it doesn't sound like the antenna problem. In my case, both GPS receivers as well as my tablet all would lost GPS reception at the same time. But nobody else in the area...

When it fails, have you looked at the AUX screen with the GPS satellites listed? Does it see anything, or not, on either GPS1 or GPS2? Also, on the LRU status screen, are there any red X's, or is everything green?
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Re: intermittant GPS

Post by Rich »

There is an aspect of IFR-approved GPS that does not apply to our tablet apps, auto-GPS, etc. There can be several satellites in view, but they have to be in the right geometry to provide a sufficiently precise solution for the current phase of flight. Remember their positions in orbit are always in flux. WAAS systems should be more resilient to this potential problem, but it remains a possibility.

However, this should be a real rarity. I've never encountered it.
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curts63
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Re: intermittant GPS

Post by curts63 »

Since I don't really know anyway. I thought if the number of satellites weren't sufficient, the system would downgrade, not go offline? I didn't look at the AUX page to see, but you would think that at least two or more satellites would always be present? I know it takes 3 to triangulate and 4 for the vertical portion, how likely is it that in northeast PA. on a sunny afternoon, the satellites would disappear?

I guess I'll have to take the plane out for a flight and see if it does it again, and take note to the AUX page status.

If anyone thinks this may be mechanical/electrical, please let me know.

Curt
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Re: intermittant GPS

Post by Steve »

I don't have the G-1000 system, but my KLN-94 (non-WAAS) does occasionally show a RAIM warning. One of the possible causes of this is an unusable SV (satellite). The SV will show on the satellite (AUX 2) pages, but not indicate that one or more are unusable. If you have that information from another source, you can manually deselect the particular SV:

http://navigationservices.agi.com/Satel ... endar.aspx

This is only for approaches, and the unit will still give a good position enroute.

Not sure you are experiencing an analogous problem, but perhaps you can check the history subheading on the above link to determine if there were any outages around the time you had problems.

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Rich
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Re: intermittant GPS

Post by Rich »

The number of satellites visible is only part of the equation, there must be sufficient number in the right relative positions. You are correct, it should downgrade, not necessarily go offline. But if it downgrades enough, certification rules likely require it to disable itself so as not to render potentially inaccurate information. I do agree this is an unlikely possibility, but not due to sunny day vs. weather. If it occurs on your next flight, it's unlikely to be satellite positioning problems.

I don't have a G1000, instead the 530W/430W setup. So I'm not sure how this might apply: I can check the aux pages of the two independent units simultaneously to see if they both show the same thing. Is that possible in the G1000? I think I still see 2 GPS antennae showing on G1000 planes but does that mean you have 2 GPS receivers that can be compared? Can you evaluate them separately?
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Rich
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Re: intermittant GPS

Post by Rich »

Keep in mind that, other than the WAAS correction birds, which are in geosynchronous orbit, GPS satellites are constantly appearing and disappearing from any given point on the earth. They are not in fixed relative positions.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: intermittant GPS

Post by Rich »

This is an instructive site. This specific link shows the locations of the various birds in near real time. As I type this there are no satellites directly above the "lower 48".

http://www.nstb.tc.faa.gov/RT_WaasSatelliteStatus.htm
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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