Work out gps accuracy estimate instantly with clear inputs, formula shown and shareable results.
Position error is the product of geometry and ranging error: sigma equals HDOP times UERE. Corrections attack the UERE term — SBAS removes most ionospheric error, DGPS removes local common-mode error, and RTK uses carrier phase to reach centimetres. Vertical error is typically 1.5-2 times horizontal.
GNSS accuracy
sigma_horizontal = HDOP x UERE; 95% accuracy is about 2 sigma
A geometry factor: how well the tracked satellites are spread across the sky. Below 1 is excellent, 1-2 good, above 5 poor and usually caused by obstructions.
Satellites are only ever above the horizon, so there is no geometry below the receiver to constrain height as well as horizontal position.