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GPS accuracy at standstill

Posted by simonl on July 27, 2010 at 2:25pm
Hi folks,

Finally getting round to doing something in the UAV world, so this weekend I hooked-up an EM406 GPS to my Parallax Propeller.

Once I'd got the coding right for N/S & E/W hemispheres (!) I stuck it in my garden to collect some data. Whoa! I was expecting the position to be a little inaccurate, but I wasn't expecting the readings to be moving about - is that normal?

Interestingly, if it's moving in a straight line (at least at walking pace) the readings show a straight line, so why the changeable readings when stationary? This surely will play havoc with the quadcopter I'm hoping to build!

Cheers,
Simon

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Replies

  • simonl July 28, 2010 at 2:57pm
    Thanks Tom / Martin, your confirmation of this being "normal" is both what I'd hoped and disappointing - the former because it makes me think it's not my GPS; the latter because I was hoping the GPS would show a stationary reading even if it was out by +/-10m.

    Thanks again to all. Now onward to figuring out the math :-(

    Cheers,
    Simon
  • simonl July 28, 2010 at 10:43am
    @Ken: Thanks for your further thoughts. I think you hit the nail on the head regarding satellites in view - my testing last night was with the GPS just outside my window, so much of the sky would've been obscured by my house. Today's test didn't show the +/-100m inaccuracy; only about +/-10m whilst stationary.

    So do others experience reported "movement" of +/-10m whilst stationary? And how does that play out with autopilots for 'copters?

    Thanks again.

    Cheers,
    Simon
  • simonl July 27, 2010 at 4:12pm
    Thanks for the swift replies.

    @Tom: Loads of movement - in the order of 100m or so. When I was walking it was pretty accurate though.

    @ Ken: Good point about WAAS. I have no idea. I think the lack of accuracy is greater than 2-3x though.

    I'm off on a car journey tomorrow, so I'll take it with me and see how it fairs in a different part of the country.

    Thanks for you r help.

    Cheers,
    Simon
  • Ken July 27, 2010 at 3:43pm
    1. Yes, it will show movement about a few meters. It is a fun project to collect to collect the data for a day or so then plot the points on a 3D graph.

    2. I'm not sure what, if any, WAAS satellites (or their European equivalents if there are any) the EM-406 may be able to use. So your accuracy may be "un-augmented" giving 2x or 3x the error I get over here.

    Probably not affecting you, but a general note to those users seeing less accuracy this summer than they had before, 1 of the 2 WAAS satellites that serve North America went out of service this April. I've not heard about a replacement / repair yet.
  • Tom Yochum July 27, 2010 at 2:57pm
    How much was it moving about? It is expected to move around +-10 meters.

    Did the error distribution drop when you started walking?
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