GPS Testing Indoors
Edit on GitHubWhy GPS fails indoors, what you can still test on the bench, and how to get a fix without going outside.
GPS Testing Indoors
Most GPS modules will not get a fix inside a building. That is normal, not a fault. This page explains what you can and cannot verify on the bench.
Why it fails indoors
The 1.5 GHz GPS signal is weak (about -130 dBm) and does not penetrate roofs and walls well. Concrete and metal framing block it entirely; double glazing attenuates it heavily. A module that worked fine outdoors will sit at “no fix” indefinitely indoors.
What you can test inside
| Test | How | Expect |
|---|---|---|
| Module powers up | Check the module LED | Blinks when searching |
| UART output | Serial monitor at 9600 | NMEA sentences stream |
| Parser works | Feed a recorded GGA line | Coordinates update on OLED |
| Wiring correct | Check TX/RX swap | Sentences appear at all |
Getting a fix near a window
A module placed directly on a windowsill sometimes locks after several minutes, especially if the window faces a clear sky. Elevating it above the sill line helps more than moving it sideways. Do not expect reliability; treat a windowsill fix as a bonus, not a test result.
The recorded-data trick
The most reliable indoor test is to replay a recorded session:
- Go outside with the beacon or a serial recorder and capture 10 minutes of NMEA.
- Feed that recording into the module’s RX pin at 9600 baud.
- The firmware cannot tell the difference between a recorded satellite stream and a live one, so the full parse-and-display path is exercised.
When to test for real
The definitive GPS test is outdoors with a clear view. Fifteen minutes before your first field trip, power the beacon outside and confirm the satellite count climbs and the coordinates settle within a few meters of your map position.
Related pages
- GPS Troubleshooting for fault diagnosis
- GPS Sky View for why the view matters
- Outdoor Testing for the real test procedure