Help - Loosing GPS in Mid Flight

I'm really hoping that someone in the community can help a fellow drone hobbyist figure out this GPS mystery.

I have an Ardupilot Mega 2.0 (Ardupilot 2.50) with an external MediaTek MT3329 GPS (3DRobotics) to this point seemed to be working very well together.   Yesterday I nearly avoided a crash when midway in a mission I started loosing my GPS lock and the plane really lost it's mind.   Only by grabbing manual control (timestamp 3:49) was I able to regain some altitude and let it get it's GPS lock again.  There was enough of a lock to resume the mission, but this loss of GPS really makes me worried about attempting some of the more aggressive projects I have planned.  Normally I have a GPS lock of 9-10 satellites, and I'm not used to loosing GPS in the middle of a flight.

I made a video of the event and combined the Mission Planner, Telemetry, and Live View.

Timeline:
YT 01:59 - Cam 17:38:00 - MP 17:29:03 - Loss of GPS to 6 Sat
YT 02:49 - Cam 17:38:50  - MP 17:29:53 - Loss of GPS to 6 Sat
YT 03:14 - Cam 17:39:15 - MP 17:30:18 *** - Loss of GPS to 4 Sat
YT 03:49 - Cam 17:39:49 - MP 17:30:53 *** - Emergency Regain of Control
YT 03:50 - Cam 17:39:51 - MP 17:30:55 *** - Loss of GPS to 4 Sat
YT 04:13 - Cam 17:40:13 - MP 17:31:17 - Resume way-point mission
YT 09:17 - Cam 17:45:18 - MP 17:36:22 *** - Loss of GPS to 7 Sat
YT 09:40 - Cam 17:45:41 - MP 17:36:45 *** - Loss of GPS to 4 Sat
YT 10:36 - Cam 17:46:16 - MP 17:37:40 *** - Loss of GPS to 6 Sat
YT - YouTube / Cam - Camera Timestamp / MP - Mission Planner

Here is an image of my electronics pod:

3690956188?profile=original

After this near-incident I have a few questions:

  1. Is this a potential GPS multipath problem since I way flying over a parking lot and a flat building?
  2. Does Geo-fencing operate when I am in a auto (mission) mode?
  3. Does Geo-fencing operate if I suddenly am outside the fence without the APM "seeing" the fence breach?
  4. Will the new firmware 2.60 with dead-reckoning help prevent these sort of issues?
  5. At what point with dead-reckoning be preferred of a weak or lost GPS signal?
  6. Is my telemetry or radio receiver placement a problem to the operation of the GPS unit?

Here is my telemetry log where the plane appeared to be flying "backwards" on the telemetry screen where I started loosing my satellites:

2012-08-25%2017-23-44.tlog

Any help would be GREATLY appreciated!

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Replies

  • I had similar issues with gps lost on mid flight after installing a 808 key cam near to the mediatek! I think that those cameras produce interferences on gps reception.

  • Just watched you video. You are lucky it didn't end up in the trees. The reason I say this I watched for when you switched to manual and I only saw FBWA and STABILIZE. If the AHRS is giving a bad solution you need to go to full manual mode and fly visually(fpv or los).

  • Ah - I forgot that the Geo-fence has to be re-enabled with every flight so it really wasn't enabled for this fight.   I guess I understand the reason, since it is possible to have the wrong geo-fence set when you are at another location, but I wish there were a more "elegant" solution in Mission Planner than going to the parameter section and changing the value to "1".

    Maybe auto-enable 1 mile radius geo-fence for planes that do not have this feature explicitly configured?  : )

  • To test for electronic interference with your GPS just hook it up to a PC running MiniGPS and look at the SNR bars for the satellites.  Does powering up your other electronics reduce the satellite SNR numbers?  There's no need to guess or hope.

    Note that you can have 9 satellites just barely (with little SNR head room) or 9 satellites with a lot of head room.  The latter is much more robust.

    Also, the way your aircraft seems to bank 45+ degrees you can easily loose several sats.  If your electronics are weakening your GPS reception they may not come back as quickly - or at all - when you level out.

  • 3D Robotics

    Yes, 2.60 fixes a bug that affected ArduPlane when GPS lock is lost.  As for why GPS lock is being lost in the first place, I can only think that putting a huge radiating antenna right next to your GPS module isn't helping ;-)

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