Why Does GPS Speed Sometimes Jump Suddenly?
GPS speed can jump, freeze, or give unexpected readings. Here is why this happens and what TrainSpeedTest.net does to prevent noisy data from reaching the display.
GPS jumps happen when a position fix is briefly off by tens of metres — multipath reflection, satellite switching, or tunnel exits are the usual causes. TrainSpeedTest.net filters these out with accuracy rejection, acceleration caps, and EMA smoothing.
- Signal multipath in cities can create phantom positions
- Satellite handovers cause brief position anomalies
- Tunnel exits produce a burst of low-confidence fixes
- Filtering keeps the display smooth without hiding real changes
Typical jump error
Apparent position offset
Accuracy reject threshold
Worse fixes are discarded
For a reliable fix
Below this, accuracy degrades
Protective layers
Accuracy, acceleration, EMA
A Position Error Can Look Like a Speed Spike
GPS speed can jump, freeze, or give unexpected readings. Understanding why this happens — and what TrainSpeedTest.net does about it — helps you interpret occasional odd readings correctly instead of assuming the tool is broken.
How GPS Position Errors Happen
GPS calculates your position by measuring the time it takes for signals to travel from satellites to your device. Any interference with this measurement introduces position error. When GPS reports a position that is off by 20-50 metres from your actual position, and the error corrects on the next sample, the apparent movement between two readings can correspond to an extremely high speed — even though you haven't actually moved that fast.
Cause 1: Signal Multipath
In urban environments, GPS signals reflect off buildings. Your device may receive both the direct signal and reflected copies, making it appear to be in a slightly different location than it actually is.
Cause 2: Satellite Switches
GPS receivers track multiple satellites simultaneously. When a satellite goes below the horizon or becomes obstructed, the receiver switches to a different one. This transition can cause a brief position anomaly.
Cause 3: Tunnel Exits
Emerging from a tunnel, the GPS receiver has lost all signals for the tunnel duration. On exit, it rushes to re-acquire satellites, and the first few fixes may have higher uncertainty than normal.
Cause 4: Low Satellite Count
With fewer than 4 visible satellites, position accuracy degrades significantly. In open countryside this is rare, but it can happen in steep valleys or dense tree cover.
How TrainSpeedTest.net Handles Jumps
Several protective filters run on every incoming sample before it reaches the display.
Accuracy filter
Samples with reported accuracy worse than 200m are discarded entirely.
Acceleration cap
If a speed change would imply acceleration beyond physical railway limits, the sample is dampened using a weighted blend.
EMA smoothing
Exponential moving average reduces the impact of any single noisy sample.
Stationary threshold
Prevents GPS drift while stationary from registering as real speed.
The Result: A Smooth, Realistic Display
Combined, these filters mean the vast majority of jumps never reach the screen — the display shows a curve that tracks real acceleration and deceleration rather than GPS noise.
What You Can Do to Minimise Jumps
A few habits reduce how often jumps occur in the first place.
Sit near a window
Better satellite visibility means fewer multipath reflections.
Wait after starting
Give it 10-20 seconds before trusting readings.
Discount tunnel-adjacent readings
Avoid relying on readings in or immediately after tunnels.
Frequently Asked Questions
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