iPhone GPS on circuit: how accurate is a lap time?
An iPhone is not a dedicated 25 Hz logger. It can still time a track-day session usefully, if we stay honest about what it measures.
What iPhone GPS can do
Modern iPhone satellite positioning is more than enough to detect a line crossing on a closed circuit, compute a lap time and split sectors. That is the ST1 Track use case.
Accuracy depends on sky view, surroundings, how the phone is held and signal quality. An open circuit, a firmly mounted iPhone, a session started a few minutes before the first lap: those details matter more than a spec sheet.
What it does not replace
A dedicated GPS box, a competition IMU or an ECU connection still matter if you want engine telemetry, a high sample rate, or team use. ST1 Track does not claim to replace that equipment.
For a track day, a club session or a first day on circuit, the phone often answers the real question: what is my time, where do I lose, am I improving.
How ST1 Track uses it
ST1 Track relies on location during the session, not to track you outside the app. Coordinates are used for timing, layouts and analysis.
Transparency is part of the product: if you need an external GPS, you will know. If you just want to start a session with what is already in your pocket, the iPhone is the starting point.