Loss of GPS signal...


I am trying to figure out what happens when you fly in GPS mode and you suddenly lose GPS signal? I would think nothing really is going to happen?! Isn't the Pixhawk just going to switch to stabilize mode? I looked at the FAQ and other paces, but cannot find the information for the life of me...

Reason why I am asking is, that I hear many people say that beginners rely to much on GPS. When the GPS fails, a crash is coming? Why? Why would your Multicopter crash just because you lose GPS signal? Maybe they mean that the noob suddenly has to fly "himself" and that is the reason? On the other hand, if the FC switches to Stabilize mode, you cant really screw that up either. I mean, its stabilizing itself, how can you crash...?



p.s. Pixhawk on 3.1.5

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  • Panic sets in when one sees their quad suddenly shoot off at high speed in an unexpected direction.  Depending on altitude and obstacles like trees there may not be time to recover.  Not that I have first hand experience with any of that!

    Best things I did was to practice emergency response and buy a better GPS.  The NEO M8 with 17 to 20 Sats in view hasn't let me down yet.

    • Greg,

      Thanks for the reply. After Rob explained a little bit, what happens in the event of losing GPS numbers, it all makes more sense to me.



  • What usually happens is you don't completely loose signal. Normally you will start to loose a number of satellites so the quality of the location degrades. Thus in a very short time your gps will tell the flight controller it is say 100 meters to the south. Then your quad copter will try to fly north to compensate being in the wrong location. If you are using 3.1.5 I would upgrade to 3.2. In 3.2 there are Extended Kalman filters (EKF) that if such a things happens you can set a parameter to land. Determining good GPS is not a trivial program procedure. Yes you should not use gps if HDOP is >2 but I've seen good loiters with HDOP in the range of 2.5-3. 

    I have had degrade of gps signal happen a few times and it was because I had something interfere with the GPS signal (such as ESC and GoPro cameras). The GPS antenna was too close to other electronics. Only one time did I actually crash. Later I learned to put back in stabilize mode very quickly if things don't appear to be flying correctly. I also use telemetry with Droidplanner 2 to monitor the HDOP while hovering in stabilize before I put into loiter or auto mode.

    I've found carefully isolating the GPS antenna and waiting for a good lock give good results. I haven't noticed a GPS signal problem in an open area with my current configurations. I now thoroughly test setups before I trust GPS. This includes cameras and radio systems. So if you add something new to your craft retest the GPS signal for good lock.

    • Rob,

      Thanks for the detailed reply. I am shocked to hear that losing numbers of satellites, will make the controller act this way?! I would think that it would somehow know, if the Multicopter actually moved a 100 meters, or not?! Shouldn't the gyros sense something like this and tell the FC, "Wait a minute! Do not listen to the GPS! We did not move that much to justify an offset of 100m!" Something like that. What do I know?! LOL. Anyhow, I did not know that you can check the HDOP via Droidplanner! Good to know!

      I am trying to isolate my GPS as good as I can. I haven't done the compassmot test yet. I wonder how good my GPS is "shielded" from any interference?



      EDIT: I sound like switching to Stabilize will protect you from having a semi-"fly-away", right?!

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