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Common GPS Errors (u-blox NEO-M8N on Raspberry Pi)

1. Permission denied

  • Cause: User doesn’t have access to /dev/ttyS0 or /dev/ttyAMA0.
  • Fix:
    • Use sudo
    • Add user to dialout group
    • Or create udev rule with MODE="0666"

2. already opened by another process

  • Cause: Another process (e.g., cat, gpsmon, or serial-getty) is using the port.
  • Fix:
    • Check background jobs: jobs -l and kill %1
    • Disable getty: 
      sudo systemctl stop serial-getty@ttyS0.service
sudo systemctl disable serial-getty@ttyS0.service

3. No data / blank output

  • Cause: Wrong wiring, wrong UART, or wrong baud rate.
  • Fix:
    • Cross-check TX ↔ RX
    • Ensure correct port (ttyS0 vs ttyAMA0)
    • Try baud rates 9600, 38400, 115200

4. NMEA shows $GNRMC,,V,... (no coordinates)

  • Cause: GPS has not locked onto satellites yet.
  • Fix: Place module outdoors with clear sky. Wait for blinking LED.

5. stty: /dev/ttyS0: Permission denied

  • Cause: Same as error #1.
  • Fix: Use sudo or adjust permissions.

6. GPS module LED never lights

  • Cause: Module not powered or faulty.
  • Fix: Connect VCC → 5V (with onboard regulator) and GND → Pi GND.

7. Output shows garbage characters

  • Cause: Baud rate mismatch.
  • Fix: Use correct baud (9600 default for most M8N).

8. gpsmon:ERROR: SER: device open of /dev/ttyS0 failed

  • Cause: Missing permissions or busy device.
  • Fix: Combine solutions from #1 and #2.