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Project & Reference

GPS Troubleshooting

No fix, wrong coordinates or a payload with PSN UNKN: diagnose the NEO-6M path

GPS Troubleshooting

Overview

GPS problems usually fall into three buckets: no fix at all, a very slow fix, or coordinates that never reach the payload. All three are diagnosable from the serial console.

Symptom: No Fix After Minutes

Check	How	Fix
Sky view	Move outdoors, away from metal and buildings	Relocate for the cold start
Patch antenna	Facing up, not against metal	Reposition per GPS Antenna Placement
Wiring	GPS TX to GPIO 22, GPS RX unconnected	Verify continuity; only the module TX line matters
Baud rate	Must be 9600	Some clones default differently; configure the module
UART enabled	`gpsen` true in NVS	Enable GPS in the portal

Symptom: Very Slow Fix

Cause	Symptom
Cold start	Normal up to 3 minutes
Almanac lost	First fix after weeks off
Nearby radio TX	GPS patch too close to the SMA

The RTC RAM cache keeps the last fix, so the beacon transmits the previous position while GPS re-acquires.

Symptom: Payload Says PSN UNKN

PSN UNKN means the fix timeout (default 30 s) expired without a fix:

The firmware transmits without coordinates rather than delaying the SOS.
Check the timeout setting (gpstmo, 10-120 s).
Verify the GPS actually fixes by watching [GPS ] Fix acquired on serial.

Symptom: Wrong Coordinates in the Payload

Problem	Likely cause
Transposed digits	Read the DDM format carefully (see Coordinate Plotting)
Old position	Stale RTC cache - force a fresh fix outdoors
Wrong hemisphere	Check the N/S and E/W letters in the payload

Serial Diagnostics

CODE

[      45][GPS  ] Waiting for GPS fix (timeout 30s)...
[   12400][GPS  ] Fix acquired: 45.53124  12.30456  sats=6
[   12401][INFO ] Payload ready: "SOS DE MARIO ROSSI PSN N4553 E01230"

No [GPS ] lines at all = UART wiring or baud problem, not a sky problem.