Hi, I've made the same post on apm, however no one thought it was interesting enough to read, but I badly need some answers from knowledgeable people so doubling it here: 

Last Sunday crash-finished my 50+ flight with the current setup. Unfortunately didn't have telemetry as it was a quick FPV fun flight w/o gimbal/gopro so no tlogs. Only APM logs wich I'll be attaching. 
Here is the relevant info: 
Tricopter 120*, 380kv/4s apm 2.6 w/ external GPS MAG 3DR combo.
running AC3.1.3.  (after autotune completed was rock stable, loiter was excellent)

1.2ghz lawmate vTx at 1080mhz channel with a lowpass filter (proven no issues with frsky x8r at 1km)

was flying nearby for 14+ min, flipped RTL just to test -> works awesome, copter comes home, I take over the control and continue in alt hold, switch to loiter, decide to test it again, copter shoots full throttle in the opposite direction it was supposed to go, flipping back to alt hold-> no reaction, continues full throttle "south", flipping to stab, throttle down starts falling, trying to recover unsuccessfully. resulting in 2 broken props (only)

logs show: GPS Err2 right after RTL engaged. (now I know why it shoot "south") but what I do not understand is Err Fligh Mode-5 message right after GPS Err-2, why would a copter fail to enter a flight mode? 

also, if anyone has an idea why GPS can throw an error with 11 satellites and HDop1.6 at the time of error rapidly going to 9sats and HDop 1.8 ? could it indicate a bad GPS? (usually I have 11 sats in the first 60 seconds at that very same spot, this time it was only 6 and than 9 later in flight and than 11sats after 10+min)

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GPS satellites aren't geostationary, so they appear on the horizon, move across the sky, and then disappear over the horizon. A problem occurs as a satellite moves out of view, its signal suddenly degrades and then it disappears. The symptom of this is that the number of satellites will suddenly change and the hdop will increase. Also as the copter flys around, obstacles may interfere with the quality of the GPS signals.

Often there is a sudden shift in calculated GPS position when the GPS device changes satellites or if the signal suddenly degrades. This is usually small but can theoretically be quite large.

I'm very curious why you'd get an error with 9 sats in view.  That should be plenty.

this is what I was wondering about too.

Not if they are all in poor positions in the sky

What you seem to be describing here is a problem with SV geometry. Ideal geometry is at least 4 SVs in view, 3 of them at about 45* elevation from the horizon and spaced 120* apart and the 4th at zenith. This makes me think you may experienced some weird sort of mulitpath error. Multipath can play hell with your elevation data. Were you flying around buildings (bad I know) or perhaps over a body of water? Of course, I am relatively new to the UAV scene, so these GPS errors may not be a problem.

nope, no buildings taller than 2 stories and those were quite at a distance 50-60 meters closest and 100+ meters from all other sides. some trees, but I was flying well above the ground about 40 meters.  clear sky some occasional clouds. 

does anyone know why APM could fail to enter a Flight Mode? 

That response pretty much rules out multipath as the problem.

another "bone": I mentioned that I invoked  RTL second time in a row, could it be that APM didn't know at that time where "home" was? 

GPS glitch will be invoked if the change in location doesn't match what predicted path is. Check out

http://copter.ardupilot.com/wiki/gps-failsafe/ 

Thanks for the link, Rob. That's interesting stuff. I guess I  didn't directly realize that APM was using SBAS/WAAS for position corrections. Our nearest WAAS ground station is about 16km away. My Trimble unit with external receiver and dome antenna picks it up pretty well...after about 10 minutes of collecting position data. I'd be very interested to see how the GPS data logged by APM stacks up against the Trimble stuff.

This sort of begs the question...theoretical of course...

If you were to have an RTK base station on-site, could APM pick that signal up and use it for correction data? Most RTK base stations can provide a stable signal for about 20 miles...or so I have been told. Of course, getting one of these stations ain't cheap, but there ARE places out there that rent them for fairly reasonable rates if one were so inclined and had the need for one.

EDIT: Looks like I just answered my own question. I guess it could be done, at a fairly large expense IF you had a reason for RTK...like precision agriculture or surveying.

http://diydrones.com/profiles/blog/show?id=705844%3ABlogPost%3A1361...

If it can't use them, APM shouldn't count them.  How are we to know how many is good?  Especially if HDop is good too.

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