Lap timers and GPS accuracy: 1 Hz vs 25 Hz
A GPS lap timer records your position many times per second and works out when you crossed the timing line. At 1 Hz (one fix per second) you move about 150 ft (45 m) between fixes at 100 mph, so the timer must interpolate between points. At 25 Hz the gap is about 6 ft (1.8 m), which gives more precise lap and sector times and much better data in corners.
- Sample rate sets the spacing between position fixes; interpolation estimates the moment you crossed the line.
- Accuracy is closeness to the true time; precision is how repeatable it is lap to lap.
- Mount the phone or receiver with a clear view of the sky, away from metal and heat.
- An external 25 Hz GNSS receiver helps most for corner analysis, short tracks and close comparisons.
What sample rate means
Sample rate is how many position fixes the receiver produces each second, measured in hertz (Hz). Most phones give location at about 1 Hz. Dedicated motorsport receivers commonly provide 10 or 25 Hz.
The faster you go, the further apart those fixes are. At 100 mph (161 km/h) you cover about 147 ft (45 m) per second. At 1 Hz that’s the gap between points. At 25 Hz it’s about 6 ft (1.8 m).
Gaps that big don’t just affect timing. A corner taken in two or three seconds is only two or three points at 1 Hz, so the shape of the line, the braking point and the minimum speed are all estimates.
Interpolation at the timing line
A good timer doesn’t wait for the next fix after the line and call that the crossing. It takes the last fix before the line and the first fix after, and works out where along that segment the line was crossed, using speed and position.
If the line is 60% of the way between two fixes a second apart, the crossing is estimated at 0.6 seconds after the first fix. That turns a possible error of up to a whole second into something much smaller.
Interpolation assumes you were travelling in a straight line at steady speed between the two fixes. On a straight that’s close to true. If the timing line sits in a braking zone or a corner, the error grows, especially at 1 Hz.
| Lap | S1 | S2 | S3 | Time |
|---|---|---|---|---|
| 1 | 44.48 | 37.28 | 45.57 | 2:07.330 |
| 2 | 43.71 | 37.12 | 45.54 | 2:06.370 |
| 3 | 43.54 | 37.32 | 45.44 | 2:06.300 |
| 4 | 43.83 | 37.06 | 45.61 | 2:06.500 |
Purple = best of the session in that sector. Add up your best sectors and you get the ideal lap: the time already in your hands, just never in one lap.
Accuracy vs precision
Accuracy is how close a lap time is to the truth. Precision is how consistent the measurement is from lap to lap. A timer can be precise but slightly inaccurate, for example if its line is a few feet off the official one, and that’s fine for comparing your own laps.
Position errors matter too. Consumer GPS is typically accurate to a few metres in good conditions. Because nearby fixes tend to share the same error, lap-to-lap precision is usually better than the raw position accuracy suggests.
Phones may also smooth location data, which can make the reported track lag slightly behind the car in direction changes. That smoothing is one reason a phone’s line through a tight corner can look rounder than the one you drove.
Mounting and multipath
Let the receiver get a solid fix before you leave the paddock, ideally a minute or two with a clear sky view.
- 1Give the antenna a clear view of the sky: high on the windshield or on the dash, not low in a footwell or console.
- 2Keep it away from heated or metallised windshields, roll-cage tubes directly overhead and other electronics where possible.
- 3Mount it securely so it can’t fall or move under braking, and keep it out of the airbag deployment area.
- 4Keep phones out of direct sun; overheating can make them throttle or stop recording.
- 5Watch for multipath: signals bouncing off grandstands, walls, buildings or trees, and drop-outs under bridges, which can shift positions briefly.
When an external receiver helps
A phone at 1 Hz with good interpolation is fine for lap times and for spotting big trends. An external receiver with a higher sample rate, a better antenna and support for several satellite systems is worth it when you want reliable corner-by-corner analysis, accurate sector times on short tracks, or to compare laps separated by a tenth.
It also takes the GPS job away from the phone, which can then be mounted wherever is convenient, and usually holds a more stable fix in tricky surroundings.
FAQ
Is phone GPS good enough for lap timing?
For lap times and broad comparisons, often yes, if the timer interpolates at the line and the phone has a clear sky view. For detailed corner analysis, a higher-rate receiver is much better.
Why does my lap timer disagree with the official timing?
The timing line may be in a slightly different place, the official system uses a fixed beacon or loop, and GPS has small position errors. Your own lap-to-lap comparisons stay valid.
Does a 25 Hz receiver make me faster?
Not by itself. It gives better data, which helps you and your coach find where the time is.
- Delta time explainedDelta time shows if you’re ahead of or behind a reference lap right now. Predictive vs reference delta, reading the bar, distance alignment and limits.
- How to read your lap dataPlain-language motorsport data analysis: speed traces, braking points, minimum speed, throttle, the g-g diagram, and comparing laps one change at a time.
- What is HPDE?HPDE is coached, non-competitive driving on a real race track in your own car. How run groups, instructors and classroom work, and how it differs from racing.