GPS Coordinate Accuracy

Overview

The beacon encodes position to ~0.1 arcminute, which is about 185 m at mid latitudes. Understanding where that error comes from separates “good enough to search” from “good enough to walk to”.

The Encoding Precision

Component	Example	Resolution
Latitude	N4553 = 45 deg 53.0 min	0.1 min ~ 185 m
Longitude	E01230 = 12 deg 30.0 min	0.1 min ~ 130 m at 45N

The DDM truncation alone gives roughly 185 x 130 m of quantization - the payload cannot express finer positions by design.

Error Budget

Source	Magnitude
DDM truncation	~185 m
Consumer GPS accuracy	2.5-5 m (with clear sky)
Stale fix (RTC cache)	Unbounded - could be hours old
Multipath near cliffs	10-50 m

The truncation dominates. The message defines a search area, not a point.

What This Means for Rescue

Task	Accuracy needed	Beacon sufficient?
Define search area	500 m	Yes
Walk to the survivor	20 m	No - use SEARCH audio to home in
Confirm identity	n/a	The name field helps

Improving the Outcome

1. Get a fresh fix before deploying (avoid the RTC cache).
2. Let the payload include the name so rescuers confirm identity.
3. Use SEARCH mode’s audio for the final tens of metres.
4. Transmit on a known frequency plan so the base knows what to listen for.

Related Pages

• GPS Integration
• Coordinate Plotting and Maps
• RTC RAM State
• Search Patterns and Procedure