Four sensors, explained
Including what each one gets wrong, because everything does.
Heart rate, and when not to trust it
The optical sensor reads blood volume through the skin at 1 Hz during activity and 0.1 Hz at rest. That is accurate to within three beats per minute against a chest strap for steady efforts, and it is measurably worse during intervals, because optical sensing lags a sudden change by four to eight seconds. We say so in the app rather than smoothing the graph until the problem disappears. If you are doing 30/30s, wear a strap and pair it — we support ANT+ and Bluetooth straps for exactly this reason.
Dual-band GPS, no phone
Gen 3 receives L1 and L5 simultaneously. L5 is the newer civilian band and it is far harder to bounce off a building, which is the single biggest source of the wandering routes you get from a single-band watch in a city. In practice a run down a street with towers on both sides holds the pavement instead of crossing it. It costs battery: eleven days of ordinary use becomes about four if you record an hour of GPS every day, and we would rather print that than the eleven.
Sleep, without the mysticism
Staging comes from movement plus heart-rate variability, which is the same input every wrist device uses and it is genuinely good at separating awake from asleep and mediocre at separating deep from REM. The nightly score is built from four numbers you can see — time asleep, time awake after first falling asleep, resting heart rate against your own baseline, and HRV against your own baseline. Tapping the score shows all four. No wearable can stage sleep to laboratory standard, and one that implies otherwise is selling you a number rather than a measurement.
Eleven days, measured honestly
Eleven days means: continuous heart rate on, sleep tracking on, the display at automatic brightness, notifications on, and no GPS. That is how most people wear it and it is the number we print. Record an hour of dual-band GPS daily and it is four. Turn continuous heart rate off and it is nineteen. All three figures are in the app under Battery, alongside what your own last fortnight actually drew, so you can check our claim against your wrist rather than take it.
