GPS offset error ?!?

Posted by Simon Gibson-Poole on February 15, 2013 at 5:36am

Hi all,

Strange thing happened today when trying another mapping mission (one that has worked fine before) as it seems that the quad was approx 50m west of where it though it was (North/South seemed to be fine however).  Had full 3D fix with 10 satelites, it was a bit windy but not enough to cause the quad any serious issues in completeing the mission.

So I can see on the mission planner where I want it to go, all the waypoints are up-loaded to the APM and off it goes ... I have telemtary so I can see that its saying its getting to those waypoints but looking at it from the ground I can see that its not, and on checking the pictures taken from the quad afterwards, I got some very nice shots of the field 50m odd to the west of the field boundary it should have been flying next to...

Weird, just wondering if anyone has had something similar before and also if there is anyway that I can verify what caused the problem, as I am not sure how I can diagnose what seems to be some sort of GPS offset error.

Regards,

S.

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Replies

  • Simon Gibson-Poole February 27, 2013 at 2:18am

    Genius, was thinking in 2d mode only and hadn't considered moving the apm up ... will give it a go :)

    S.

  • Developer
    Randy February 16, 2013 at 7:03pm

    By the way, when you're ready to upgrade to 2.9.1 you should first look at your accelerometer data.  The frequency of the logging of this data is very low in your tlogs but the vibration looks well out of tolerance for 2.9.1's inertial based alt hold.  i could be wrong of course...the data doesn't' allow me to be sure...but just a word of warning..

  • Developer
    Randy February 16, 2013 at 6:38pm

    Simon,

         To some extent, I think that's just the way it is with the GPS some times.  2.8.1 is so thoroughly tested that it's extremely unlike to be an APM firmware issue.  I graphed your number of visible satellites and it bounces around a bit between 9 and 11.  I'm sure the hdop value is also displayed in the tlogs but I can't find the field.

         Re your compass, I've graphed your throttle vs the mag_field and you do have some significant interference (like many other people actually) but this wouldn't explain an offset in your position.  This would just make it harder for your copter to get to the position 'cuz it would keep heading in slightly the wrong direction.

    3692623634?profile=original

  • Theo Moelich February 16, 2013 at 9:07am
    Simon my very 1st flight ended in a crash because of the exact problems. I only later (through much reading) found out that all my default values regarding GPS and especially compass was out by a mile. I actually set my compass the auto learn fir declination and made all the other compass offsets zero... Now my map flying is great, telemetry also looks much better now when X-quad is pointed north it shows the same on Google satelite, hope it helps...
  • Developer
    Randy February 15, 2013 at 5:55am

    2.9.1 of course?  APM 2.5 with mediatek or Ublox?  a KMZ, dataflash or tlog would be good as a reference.

    My guess is that it's just the GPS acting up but there have been a significant number of complaints about GPS accuracy so I'd like to gather some more data at least.

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