Your watch is only as good as the numbers it feeds you. Here's how the five watches on our board handled satellite tracking and heart rate, and what you can do to make yours more trustworthy.
Our baseline for lap-by-lap distance checks.
Two signal bands per satellite to reject reflections.
Polar's ECG-style chest strap as heart rate truth.
Wait for the lock before you press start.
Track testOn the track, the four dual-frequency watches (Forerunner 265, Ultra 2, Vantage V3, Pace 3) stayed close to the painted line, usually finishing a 5K within a few dozen metres. The Pixel Watch 3 tended to cut corners and add short zig-zags on the straight. For outdoor 400 m work, every watch was better used in its dedicated track mode where one exists.
Gym testGripping a barbell flexes the wrist tendons and disturbs optical readings. The Polar Vantage V3 kept the closest match to the chest strap during squats and rows; the Pixel Watch 3 sometimes read a beat count that lagged badly after heavy sets. If strength is your focus, pair a strap.
Your watch doesn't "see" you on a map. It listens to timing signals from satellite constellations, including the American GPS, Europe's Galileo, Russia's GLONASS and China's BeiDou, and calculates where you are from tiny differences in arrival time. Every second or so it drops a point, and the line between points becomes your route and your distance.
The trouble starts when signals bounce. Glass office blocks, deep valleys and dense tree cover reflect signals before they reach your wrist, which makes the watch think you are a few metres off to one side. Join enough of those mistakes together and your 5K becomes 5.2K. Dual-frequency receivers listen on two bands, L1 and L5, and the newer L5 signal is easier to tell apart from its reflections. That's the single biggest reason our city testers preferred the four watches that have it.
The little green lights on the back of your watch shine into your skin. Blood absorbs green light, so as each heartbeat pushes blood through your wrist, the amount of light bouncing back changes. The watch counts those pulses. It's clever, but it is easily confused by movement, cold skin, tattoos in the sensor area, and a strap that's too loose.
For steady efforts, such as an easy run, a long walk or a gentle ride, all five watches tracked within a few beats per minute of our chest strap most of the time. The cracks show in fast changes: a 200 m sprint, the first minute of a hill rep, or the jump in heart rate when you rack a heavy squat. Wrist sensors often take several seconds to catch up, and occasionally lock onto your cadence instead of your heartbeat.
Once you trust your heart rate, you can train by zones. A simple starting point is the Karvonen method, which uses both your resting and maximum heart rate. Pair it with how a session feels on the Borg RPE scale. If the watch says zone 2 but it feels like a 7 out of 10, believe your body and check your strap. Data is a coach's assistant, not the coach.
Next up: see how each watch handles specific sports or where your heart rate data ends up.
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