Multiband GPS: when it matters and when it is marketing

Satellite positioning assumes the signal travelled in a straight line. When it has bounced off a glass tower, a cliff or a wet road first, it arrives late, and the watch reads that delay as distance. That is multipath error, and it is why a run through a city centre draws a track that crosses buildings.

A multiband receiver listens to the same satellite on two frequencies. The two are delayed differently by the atmosphere and by reflections, so comparing them lets the watch identify and discard a signal that did not come straight. That is the whole trick.

• City streets with tall buildings and glass — the case it was designed for.
• Dense tree cover, where signals scatter through canopy.
• Steep valleys and gorges, where half the sky is rock.
• Anywhere you care about the exact distance rather than the approximate one — a measured track session, or a course record.

And where it does not: open road, fields, moorland, a track in the open. On those, a single-frequency watch from five years ago is already about as accurate as the satellites allow.

The cost is battery, and it is not small. Across the catalogue the multiband figure runs from roughly a fifth lower to two thirds lower than the GPS-only one — see battery claims for what that does to a long day.

Two things fall out of that table. The gap is not consistent between brands, so it is worth checking rather than assuming. And the watches with the largest absolute multiband figures are the physically large ones — which brings the wrist-size question back into a decision that looked like it was about chips.

1. 1

   Picture your three most common runs

   Not your most exciting one. If two of the three are open road, this feature is not your deciding factor.

2. 2

   Check whether your existing tracks look wrong

   If you already run with GPS and the lines follow the road, you do not have a problem to fix.

3. 3

   Weigh it against the battery you need

   Multiband on a watch with no headroom is a choice between accuracy and finishing the day with data.

The Garmin range is where this gets confusing fastest, because multiband appears and disappears across models at similar prices — which Garmin untangles that.

It is worth being concrete about the size of the problem, because "more accurate" is doing a lot of work in most reviews. Single-frequency GPS in the open is typically good to a handful of metres. In a city centre with tall glass, multipath can push that to tens of metres, and the errors are not random — they pile up in the same direction as you run the same street.

Over an hour that shows up as a distance that is wrong by a few percent, consistently. Which matters in three specific ways and almost no others.

• Your pace reads wrong in real time, which ruins a session run to pace rather than to feel.
• Your distance totals drift, so training load and weekly mileage are built on a slightly wrong number.
• A route you recorded to follow later has kinks in it that were never there.