GPS reject position flight mode



I was setup a standard quadrotors frame with pixhawk 4 FMU and abled to flight without issue . later I decided to do some changes on the frame and because of the FMU location has been changed I re-setup the flight controller unit, format the SD, update the firmware and all other parameters. Unfortunately after that I got a weird problem, despite GPS get 3D lock but I am not able to switch to Position flight mode, system reject this mode, the GPS’s led blinks blue and doesn’t turn green.

I retried and retried to re-setup FMU but every time I got the same issue. QGroundControl doesn’t show any issue, error or warning, just simple doesn’t accept position mode!

Would it be possible for one of you guys give me a hand and help me out to figure out this issue. I really frustrated after spending 10 days and couldn’t flight with my drone. :frowning:

Log file:!AkXkgOK12FhkumWv7QZ5HIZT_lCR

Thanks heaps,



What else did you change to your frame? Could it be that you installed some onboard computer (e.g. with USB 3), or a camera, or PX4Flow? If yes then it’s probably that the new installation creates GPS interference and you no longer get a GPS lock.


Hi Julian,

Thanks for your reply. No, not really, I didn’t installed any extra equipment for this build. I took it to a very open space (a football field) and set it up again then I was able to get the Position mode, as far as I changed the location for further testing it displayed No GPS Signal message on the QGroundControl.
After months of working with Pixhawk PX4 I would say it’s working great just in stabilized mode and it’s an unstable and unreliable product when GPS comes to play.



I disagree that it’s unstable and unreliable with GPS. However, the performance with GPS depends on the GPS receiver as well as the GPS environment, or rather the satellites that are visible where you’re trying to fly.

In your logfile here, we set that you get about 6-8 satellites which is lower than what I’m used. Most of the times we see 10-11 for GPS only receivers and around 20 if the receiver also supports other satellites like GLONASS.

Even though the GPS receiver reported a 3D lock it was quite a “bad” lock, so the estimated horizontal position error was about 6 meters and PX4 considers that not precise enough to fly.

If you share the locations where you tried to fly or a picture of your setup we might be able to find out more.


Hi Julian,

That was my very personal idea about the Pixhawk + GPS based on the time and effort I put for it.
By the way, I decided to disassemble the drone and re-attached all stuffs on a table in the open space for further debugging.
Interestingly GPS get 3D lock with 15 satellites but it refused to switch to the position mode.
I am not sure but I reckon the telemetry device cause the issue, So what I did just detached the Telemetry from the FMU and re-setup it again. Now I’m able to switch to the position mode. I need more testing to be sure what exactly cause the FMU such an odd behavior.
I will let you know and send you more information after I got to an conclusion.



Hi Julian,

I can confirm the telemetry was the source of issue. The interference between telemetry device and GPS receiver cause uncertainty for FMU to get the position flight mode. I use a 915 MHZ device, I am not sure if a 433 MHZ unit may have less interference with FMU.
Can you please let me know if you have any idea about it?



I don’t know. What device is it?


hi @behrang
you go proper location you get the position mode . you do in your home not get the green led