GPS and running watches: how it works

How does GPS work?

Twenty thousand kilometres above you, there are a bunch of about 30 GPS satellites. They’re spread out, so that wherever you go, the sky above you contains at least four of them, and they pump out regular messages saying exactly where they are. A GPS monitor picks up these messages, and by looking at the time they were sent, it can work out how far the messages have travelled.

If there’s a satellite over Manchester, for example, and it sends a message that your watch receives 0.001 seconds later (light travels at around 186,000 miles per second), your watch knows its somewhere within a 186-mile radius of Manchester.

Related: The best GPS running watches for beginners 

Related: Where to buy a cheap Garmin GPS running watch 

That's no use by itself, but at the same time, your clever watch is getting messages confirming that it's 53 miles from London, 175 miles from Bristol and so on. Draw all those circles on a map, and I’ll be standing in the bit where they overlap.

Amazingly, this all fits in something you can wear on your wrist and can pinpoint you to within 10 metres. Buildings, trees and soupy atmosphere can mess things up a bit, but generally speaking, it works well.

What’s not to like?

Well, don’t forget there have been examples of eejits who drove into rivers or along train tracks because their GPS said so. But science is working on medication for these people. We’re talking about the more important stuff for runners. Like how far you ran, your average pace or whether the race you just did was accurately measured.

Say your stock run is around nine minutes per mile pace, which means you cover about 15 metres every five seconds. That's how often GPS watches typically check where you are. There you are in the diagram (below), running from point A to point B, along the red line.

And there's the first problem. Even if my watch is deadly accurate, pinpointing me exactly at points A and B, it doesn’t know what I did in between – and so it assumes I ran in a straight line, underestimating my distance slightly.

But unless you hit a sharp bend, it won’t be far off, right? Well yes, but if the GPS misses a yard every five seconds, that translates 12 yards a minute, or 540 yards over a 45-minute run. If at the end of the run the GPS reports that you ran five miles, that would work out at an average pace of nine minutes per mile. But if you actually covered an additional 540 yards, you actually run faster than that - your true pace would actually be 8:29 per mile. Quite a difference if you're aiming for a specific pace.

RELATED: Run on effort rather than pace

So it’s a best estimate?

Exactly. And even if you feel like you’re running at a consistent pace, your watch can record quite a variation. In the diagram, the big circles around A and B represent the 10-metre error margin around each point. The watch could place me at points C and D - adding another 20m to the distance I covered in one five-second burst, turning nine-minute-mile-ing into something a lot faster. But fortunately, it's a problem that evens out, rather than adds up – the points recorded are just as likely to be too close together.

So those laps of the car park to get my distance up to five miles are unnecessary?

While there’s no doubt that GPS is a marvellous invention that has made runners’ lives a whole lot easier, it’s not completely infallible in measurement terms – something worth remembering next time you're giving yourself (or someone else) a hard time because the race you've just done is short or long according to the measurement on your wrist. If you use it as a tool to help track your running rather than becoming a slave to it you won’t go far wrong – and you won’t end up running into any rivers.