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Altitude from baro or GPS? (APM:Plane)

Posted by Graham Dyer on November 14, 2014 at 10:47am

Is it advisable to give GPS altitude a little weight over baro alt due to baro drift? We are seeing a +10m to -10m altitude variance after a 90 minute flight so auto-landing is out of the question (until we can fit a laser altimeter).

ALT_MIX can be set from 0 to 1 so I presume that 0.9 would be 90% baro and 10% GPS so would this correct the drift or is GPS alt less reliable than baro alt?

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Replies

  • Developer
    Tom Pittenger February 2, 2015 at 2:43pm
    I'm having a similar problem trying to do auto-landing with long flights. @Graham, what mix ended up working best for you?
  • Moderator
    Graham Dyer November 14, 2014 at 9:44pm

    Before takeoff: GPS - 562m / Baro - 562m

    After landing:    GPS - 562m / Baro - 568m

    • Moderator
      Graham Dyer > Graham Dyer November 14, 2014 at 10:49pm

      How does one get baro alt and gps alt in the tuning window while flying?

      • Moderator
        Graham Dyer > Graham Dyer November 15, 2014 at 2:39am

        2nd flight - ALT_MIX - 0.9

        Before takeoff: GPS - 560m / Baro - 570m

        After landing:    GPS - 560m / Baro - 559m

        • iskess > Graham Dyer November 15, 2014 at 11:32am
          So at .9, what altitude error did your HUD show after landing? Was it 90% of the 11m baro error (10m)?
          I still don't understand why the GPS shows no error at all. That just can't be right. Which Gps are you using? Pixhawk with 2 GPS's?
          Give it a try at .5 and see if the baro error is adequately minimized. We can't be 100% dependent on gps as it can glitch.
          I'm looking forward to testing this myself, but I won't be able to fly for a while.
  • iskess November 14, 2014 at 11:32am
    Interesting idea. I hadn't considered this as a work around to the frustrating baro drift issue. I wonder what other side effects it may have. Paul Riseborough is the man to ask since he wrote the TECS.
    • Moderator
      Graham Dyer > iskess November 14, 2014 at 11:55am

      Well, I had set it to 0 earlier after a flight to watch for a change from -11m (100% baro) and then I accidentally left it at 0 for the last flight of 80 minutes, the altitude was a perfect 0 when I landed. Maybe 0.5 is a good compromise?

      • iskess > Graham Dyer November 14, 2014 at 12:21pm
        It's a bit suspicious that there was 0 gps alt error. It might be interesting to graph the baro vs gps alt from that last flight. GPS is notoriously less accurate than baro. However with the Ublox M8 and dual GPS units, it is probably better than when the ALT_MIX default was chosen back in the days of MediaTek.

        I'm looking forward to the results of your testing.
        • Moderator
          Graham Dyer > iskess November 14, 2014 at 12:24pm
          I'll graph the two alts in the morning, it's late here now and I need some sleep.
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