Academy · Timing

How do you time a race when the start is two kilometres away?

No cable, no radio drama, no stopwatch — just two clocks that agree to the microsecond, courtesy of atomic clocks in space.

Imagine a rowing race. The start is two kilometres up the lake. No cable connects the gun to the finish camera, and nothing sends a “go” down the course. Yet at the finish, the system produces each crew’s time to the thousandth of a second. The trick starts by abandoning the most familiar object in the sport: the stopwatch.

Two timestamps instead of one stopwatch

Suppose the starting device writes down, in absolute time, Race started at 14:00:00.250000, and the finish camera writes This boat crossed at 14:01:35.632415. Subtract: 95.382415 seconds. No clock had to “run” between start and finish; no signal travelled during the race. Two events were each recorded where they happened, and the race time is the difference. One condition, and it’s hard: both clocks must agree extremely closely.

GPS is not about “where.” For timing, it’s about “when.”

Every navigation satellite carries atomic clocks and broadcasts, in effect, “this signal left me at exactly this time.” A receiver on the ground disciplines its own clock to match — so the start and the finish can borrow atomic-clock time without owning an atomic clock. The gun never talks to a satellite; its receiver has simply been listening, and at the bang the device just writes the time down. Start and finish don’t even need the same satellite — what they share is the time system itself.

The consequence nobody expects: the start time can arrive late

Because every slice of the finish image already carries an absolute timestamp, the finish can be recorded before the start time has even arrived. The image is on screen, then the phone rings: “official start was 14:00:00.250000.” Enter it, and every elapsed time appears instantly. The start had to be recorded accurately — never delivered instantly. That single property frees the finish line from the cable.

What if the signal disappears?

Neither device uses the satellites as its clock. Each has a highly stable internal clock — a temperature-compensated oscillator (TCXO) — that the satellite time regularly checks. Lose reception and the TCXO carries on from the last confirmed time. One sentence holds the architecture: the satellites set the clocks; the local clocks carry the race. In Lion Timing, the camera’s clock is a TCXO, and Lion Timing Advanced adds a GPS module for satellite-disciplined absolute time.

1. Does the starting gun send a message to a satellite?

No — the device just writes down the time from its own satellite-corrected clock. Nothing at the race transmits to space; nothing needs to travel at all.

2. The start timestamp reaches the finish twenty minutes late. What happens to the results?

Nothing is lost — every finish slice already has an absolute timestamp, so the subtraction just happens late. Accuracy of recording matters; speed of delivery doesn’t.

3. The camera loses reception mid-race. What keeps timing valid?

The TCXO carries on from the last synchronised time, then re-synchronises when the sky returns. GNSS is the reference, not the heartbeat.

The explanation worth remembering

A stopwatch says: start this clock and let it run. A satellite-synchronised system says: keep both clocks agreeing, and when something happens, write down the time. The start doesn’t have to reach the finish — the two only need to agree on what time it is.

Next in the Academy: The timing box: the translator you may not need →