Flight Mode and GPS Error Codes

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


    • 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.


  • 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? 

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

    • I am having the same problem now. Did you ever figure this out?

  • 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.

    • Moderator

      Not if they are all in poor positions in the sky

    • 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.

    • This may sound like a noob question, but does APM download a new GPS almanac as needed?

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