Blogs

3D Robotics

Testing GPS vs. Pressure Sensor Altitude

Posted by Chris Anderson on August 10, 2010 at 10:00pm


Good data from Doug Weibel today, who was testing the uBlox GPS vs the onboard absolute pressure sensor on ArduPilot Mega with his NexStar Mini EP. In general the uBlox is performing great. And we love the onboard datalogging on APM!


Doug explains the modest data diversion between the two sensors:


"You can see that in the area from roughly 1600 to 2500, there is greater difference between the two altitude measurements. This coincides with when I let my son fly the plane and took it out of fly by wire mode and put it in manual. He was busy trying to get the little NexStar Mini EP to do some aerobatics. Since he was spending a lot of time in very high bank and pitch angles I would guess that the uBlox performance was suffering and the baro altitude was more correct."

E-mail me when people leave their comments –
Follow

You need to be a member of diydrones to add comments!

Join diydrones

Comments

  • Developer
    Ryan Beall August 11, 2010 at 5:22pm
    @John NY

    Good call on the units. I'm pretty sure he's using Meters, and now it makes more sense to me that the Ublox is about where I expected it to be then considering its in meters. With that being the case, GPS is off by like 60 feet at times. To me that's what I would expect from gps and is why I would back it up with more sensors. Baro/accel etc. But good enough is in the eye of the beholder!
  • Developer
    John Arne Birkeland August 11, 2010 at 4:51pm
    If data from two or more systems conform, it is usually a good indication that the data is valid. Especially when the systems use different means of acquiring data like in this case.
  • Peter Seddon August 11, 2010 at 2:37pm
    Is there any surprise at these differences since they are both attempts to measure height by indirect means - air pressure and timing measurements? Both methods are underpinned by data assumptions and hence subject to inaccuarcies (eg relationship between pressure change, altitude, and temperature; orbital data and time meaurement).

    Can we draw any conclusions from these tests other than comfort that two indirect methods of measurement agree reasonably well?

    Peter
  • John_NY August 11, 2010 at 2:10pm
    Units please on the altitude measurement! Are you using inches, feet, meters? Thanks! I've wanted to find the accuracy/precision of the barometric sensor on the APM OilPan, and this looks like a good plot to figure that out.
    -John
  • Patrick Hammer August 11, 2010 at 4:45am
    Doug/ Chris

    For what it worth, I notice that the GPS, other than for a brief span at the start of the graph [run]:

    - consistently measured higher than the baro
    - and that the difference (between the GPS & baro) is more or less consistent throughout the graph run.

    Question:

    - what antenna does the GPS use, and where have you placed it on the airframe?

    Thanx

    Patrick
  • Developer
    Pete Hollands August 11, 2010 at 1:19am
    Doug, lovely graph. Would love to know how you have configured the Ublox in detail to get such good altitude data from it.
  • Developer
    John Arne Birkeland August 10, 2010 at 11:30pm
    Your results are MUCH better then what I usually get. I guess it just shows that the accuracy of GPS altitude will vary greatly depending on where in the world (latitude) you are and how many satellites you receive.
  • Developer
    Ryan Beall August 10, 2010 at 10:32pm
    Yeah, I'd say the Ublox is performing better here than most would ever see. Very interesting data to say the least! I'd like to see up closer to compare the lag of the GPS and noise levels, but from a glance yeah these are pretty great results.

    -Beall
  • Moderator
    Alex August 10, 2010 at 10:26pm
    I am impressed with the ublox, did he do anything differant to get such great peformance from it? Because I dont seem to get my signal this accurate, I guess your flying location plays a part aswell.
This reply was deleted.