Hello, I did this because I noticed that the altitude of GPS is not stable. GPS coordinates are correct however.
They remain well within a radius of ten meters around the real point. But the altitude is completely crazy, it depends on the day, and even the moment of day. This is related to the position of satellites, I am in Europe, Is this the problem?.But, it's may be a problem for the stabilization of our plane in altitude. And increasingly it is not very accurate.
And with two I2C devices it works, cool.
What do you think of this approach?
Comments
I realize I'm being a "bit" pedantic (but that is the whole point of the internet, right?) :-)
The fact that you can hold altitude within 10m of what the gps claims is the current altitude, really speaks more to your altitude hold algorithm than anything. You still don't know what your exact true altitude is and thus don't have a way to measure your true altitude error. (I'm not talking about the error between your sensed altitude and your target altitude, I'm talking about the error between what the gps says your altitude is, and your actual true altitude.)
Anyone who says they've never seen more than 5-10m gps error: would you be willing to setup your gps at a stationary location and collect 30-60 minutes worth of data and plot the altitude for that time. Does your stationary altitude really remain within +/-5m the entire time?
I don't claim to be a gps expert. I've used a few different gps's over the years, and they have all exhibited a lot of wandering in the altitude dimension. I've never seen any non-differentially corrected gps that could always hold correct altitude within +/-10m even if it had 8 or more satellites in view.
And just to belabor the point ... yes, I've seen many times when my gps was right on and was performing really well ... sometimes it's amazing. I have an old plot from a freeway drive that has me dead on in my lane and even shows my lane changes correctly (referencing google maps) but then at other times that same gps wasn't performing so well. But there was another time when I had my airplane climb higher than I dare admit because the gps still thought it was below it's target altitude and the altitude controller thought it needed to climb ... but that was with an old crappy ublox. Not the new ublox5. :-)
Yeah, fair point. What I mean is that we have hours of altitude hold data, based just on GPS, and as best as our eyes can tell it doesn't deviate more than about 10m. If it deviated 200m, we'd know about it ;-)
Thanks,
Curt.
The kalman filter slips during constellation changes are only about 10 meters max (+/- 5m) providing you have reasonable sat geometry. For the sort of jumps you're seeing, it suggests that either
a) You have a bad GPS or
b) There is some interference or shielding happening.
We have hours of data from the sky and have never seen a deviation of more than ~10 meters. It really depends on how many sats you're seeing. Inside or near buildings you may not have enough for good alt resolution, but in the air it's much better.
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