GPS Accuracy: What Affects Your Speed Reading?

What the Accuracy Figure Means

The GPS accuracy shown in TrainSpeedTest.net (e.g., "±8 m") is the 68% confidence radius reported by your device's GPS receiver. This means there is a 68% probability that your actual position is within 8 metres of the reported position. Speed uncertainty is approximately proportional.

Factors That Improve Accuracy

Open Sky Visibility

A clear, unobstructed view of the sky allows your device to track more satellites simultaneously. Under ideal conditions (open countryside, 8+ satellites visible), horizontal accuracy of ±3–8 metres is typical.

Multi-Constellation Receivers

Modern smartphones receive signals from multiple satellite systems: GPS (USA), GLONASS (Russia), Galileo (EU), and BeiDou (China). More systems means more satellites and better geometry.

High Speed

At 150 km/h, a position error of ±10 metres represents a speed error of roughly ±1.3 km/h — about 0.9% of the reading. At 15 km/h, the same position error represents ±13 km/h — 87%. Higher speeds produce proportionally more accurate readings.

Doppler Velocity

Many devices report Doppler-derived speed (independent of position accuracy). This can achieve ±0.2–0.5 km/h accuracy at typical train speeds.

Factors That Reduce Accuracy

Tunnels

GPS signals are blocked by tunnel walls. Speed readings freeze or drop to zero inside tunnels. This is an unavoidable limitation of satellite-based positioning.

Tall Buildings and Urban Canyons

In dense city environments, GPS signals reflect off buildings (multipath effect). This can add position errors of 10–50 metres and cause speed to jump momentarily.

Dense Tree Canopy

Forest cover attenuates GPS signals, reducing accuracy from ±5 m (open sky) to ±20–50 m.

Cold Start

A GPS receiver that has not been used recently needs 30–60 seconds to download satellite almanac data and acquire multiple satellites. Readings during this period are less reliable.

Device Hardware Variation

Entry-level smartphones may use single-frequency GPS receivers, which are more susceptible to multipath and have lower baseline accuracy than the multi-frequency receivers found in mid-range and premium devices.

TrainSpeedTest.net's Mitigation Approach

• GPS samples with accuracy worse than 200 m are discarded entirely
• The exponential moving average filter reduces noise from individual bad samples
• Physically impossible acceleration values are dampened
• The stationary threshold prevents GPS drift from appearing as movement

Expected Accuracy Under Typical Conditions

See also: How It Works · GPS Accuracy Article