How accurate is a GPS lap timer?

Chrono lands times to the millisecond from a GPS that updates 25 times a second. With coded triggers at the line, timing is more accurate than GPS-only, because it no longer depends on position or reception.

Do I need triggers to time laps?

No. Chrono times a lap on GPS alone. Triggers are optional: one for a closed lap, two for an open lap, and each extra one times a split.

Can a GPS lap timer time a downhill run or an enduro stage?

Yes. Both are open laps, with the start and finish in different places, timed on GPS alone or with two triggers, one at each end. It works on a mountain bike and on an enduro motorcycle.

Do riders on the same track interfere with each other's triggers?

No. Each trigger carries its own code, so every rider's timing stays their own.

How GPS lap timers work for motocross and MTB

A GPS lap timer records the bike's position many times a second and times each lap when that position crosses a start-finish line set on a map. Chrono updates its multi-constellation GPS 25 times a second and lands times to the millisecond; coded triggers beside the track mark the line exactly where GPS reception is weak.

Published 2026-11-11

A rider on a red mountain bike passing a small red trigger box on a rock beside the trail, with a phone screen overlaid showing the timed lap — A coded trigger beside the trail timed this lap at 00:51.384 over 519 m: to the millisecond, and without depending on GPS reception at the line.

How a GPS lap timer works

GPS lap timer: A device that logs its own position from satellites and times a lap or a run each time it crosses a line you set, with nothing required at the track.

The update rate decides how finely the crossing is placed. Chrono's GPS updates 25 times a second, a fresh position every 40 ms, and it reports times to the millisecond: 00:51.384, not 00:51.4.

GPS-only timing versus trigger timing

Chrono's four configurations. With triggers, timing is more accurate than GPS-only, because the trigger marks the line itself instead of depending on GPS position or reception.
Configuration	Placed at the track	What it adds
GPS only	Nothing	Lap times, line and speed from satellites
GPS and speed sensor	Nothing	Distance and comparison under dense tree cover
GPS and external triggers	Coded triggers	Timing marked at the line itself
GPS, speed sensor and triggers	Coded triggers	Triggered timing, distance from the speed sensor and position for line analysis

A trigger is a small red coded box placed beside the track, and it works like a transponder. Each carries its own code, so riders sharing a track never trigger each other's laps.

How many triggers a lap needs

Closed lap, start and finish in one place: 1 trigger, timing every lap after the first
Open lap, such as a downhill run or an enduro stage: 2 triggers, one at each end
GPS waypoint lap: 0 triggers, with splits dropped on the map
Every trigger beyond the minimum times a split, to a maximum of 10

OpenOpen lapClosedClosed lap

TriggersWith triggersGPSGPS only

A run from A to B. The start and the finish are different places, so each needs its own trigger, and so does every split between them. The triggers time the run themselves, which keeps the timing independent of GPS reception under dense tree cover.

The same run from A to B with nothing at the track. Chrono times it from GPS alone and the split lines are drawn on the map afterwards — no hardware to carry, and nothing to set up before a session.

A circuit — the route returns to where it started. It crosses the same line twice, so one trigger is both start and finish and times every lap after the first. The trigger does the timing, so it holds under dense tree cover.

The same circuit with nothing at the track. Each lap is timed as the GPS trace crosses the line drawn on the map — nothing to carry, and nothing to set up before a session.

Splits drawn on the map need no hardware at all. A trigger added later times a split alongside them in the same session.

Why tree cover changes the answer

A wooded descent is where GPS reception degrades and where accurate distance matters most, because comparing two laps means comparing them at the same point on the track. Chrono's built-in speed sensor carries distance and comparison through those sections.

The lap table in the Chrono app: two laps, each with its time, average speed and distance, followed by three split times — Two laps 0.199 s apart: lap 2 lost 0.250 s in the first sector and 0.551 s in the second, then won 0.602 s back in the last. The recorded distance of the two laps differs by 18 m.

Splits are what turn a lap time into a decision. Without them, the two laps above are 0.199 s apart; with them, most of the time is visibly lost in the middle sector and partly recovered at the end.

What a lap timer measures and what it does not

Chrono reports lap and run times with total and moving time kept apart, splits, the delta between two runs, the ridden line with entry and exit speed, and a braking and acceleration map. Its app runs on iOS and Android phones and tablets with no extra fees.

A lap timer does not measure the suspension. What the fork and shock did through a run is suspension data acquisition, recorded by Telemetry V3 and Telemetry V3 Pro, and many riders run both.

Common questions

How accurate is a GPS lap timer?: Chrono lands times to the millisecond from a GPS that updates 25 times a second. With coded triggers at the line, timing is more accurate than GPS-only, because it no longer depends on position or reception.
Do I need triggers to time laps?: No. Chrono times a lap on GPS alone. Triggers are optional: one for a closed lap, two for an open lap, and each extra one times a split.
Can a GPS lap timer time a downhill run or an enduro stage?: Yes. Both are open laps, with the start and finish in different places, timed on GPS alone or with two triggers, one at each end. It works on a mountain bike and on an enduro motorcycle.
Do riders on the same track interfere with each other's triggers?: No. Each trigger carries its own code, so every rider's timing stays their own.