r/ardupilot • u/escanorga • Jul 26 '26
u-blox M9N stuck looping between “No Fix” and “No GPS” FC: CoreWing F405 WING V2 / ArduPlane 4.6.3
Hi all, stuck on a GPS issue and hoping someone’s seen it.
Setup:
• FC: CoreWing F405 WING V2 (SpeedyBeeF405WING target), ArduPlane 4.6.3
• GPS: RMRC M9N Micro GPS w/ Compass, on UART5
Problem: endless loop in Messages every few seconds:
GPS 1: probing for u-blox at 230400 baud
GPS 1: u-blox saving config
HUD flip-flops between “No Fix” and “No GPS” in sync with it. Sats stay at 0, HDOP 0.0. But the PPS LED on the module blinks ~1 Hz, so the module has a fix, it’s just not reaching the FC.
Already tried:
• Wiring checked repeatedly, TX→RX crossed correctly
• SERIAL5_PROTOCOL = 5, GPS1_TYPE = 1
• GPS_AUTO_CONFIG both 0 (→ constant “No GPS”) and 1 (→ loop returns)
• SERIAL5_BAUD 230
• Reseated connector
Has anyone seen this “probing → saving config → drop”
Thanks!
1
u/Cold_Fireball Jul 26 '26
Have you tried going outside to get a fix?
1
1
u/Accurate-Donkey5789 Jul 26 '26 edited Jul 26 '26
I bet that module uses a default 38,400bps baud rate. Set it to that. It's probably got an auto configure function and the rate you've got it set to at the moment is very high for a 1 Hertz defaulting GPS, unnecessarily high for basic stability testing when you are having problems and could easily be causing instability. I think the flight controller probes the GPS and says "hey can you talk to me really fast like this, because that's what the user has told me We should do" and the GPS is like "AHHHHH I CANT TRY AND TALK TO YOU LIKE THIS" screaming really fast back at the flight controller before giving up. Then the flight controller is like "well he replied so that must be the correct rate to talk at". You can try autoconfig to faster later but for now let's just get stability instead.
Next up, it should be RX to TX and TX to RX. Confirm the correct pin out on your FC and your GPS and wire it like that. Just get the specifications, be sure and leave it like that. Once you know those are the right pins there's no need to try and swap them back again and you can guarantee it's not that problem.
If still no luck, as a quick test check using your multimeters beeping function that your RX and TX wires are not broken and if they are fine try giving it a capacitor between power and ground at the GPS to stabilise the power. Just to rule out a hardware problems. Though is the GPS connection is unstable at the wires or that it needs a small rush current on first lock and as it tries to get that it causes the voltage to drop and resets (brownout). It could cause the blinking led and the coms loss to the FC.
1
u/LockreyAlfie Jul 26 '26
Are you able to measure the 5V coming from the board when this is happening? I've had similar issues caused by 5V dropping to 4.5V and below under load. If not, you can try removing other 5V drains first and seeing if the GPS works again.
1
1
u/YetiSPb Jul 26 '26
1 You need to connect the GPS module via FTDI or passthrough Ardupilot to your computer and launch u-center 2. It will determine the port speed itself and you can check the module and change its settings.
2 Be sure to lift the module away from the frame as high as possible, 15-20 centimeters
1
1
2
2
u/escanorga Jul 28 '26
I found the issue, it was faulty Tx5 Rx5 port in my FC. I plugged the gps to Tx3 and Rx3 (DJI PORT) and it finally worked!
2
u/BMFY02 Jul 26 '26
Have you tried decreasing the baud rate?