Questions about GPS watches for runners often crop up in the RW forum. The following article tries to answer some of those questions and provide some interesting information about how GPS works. I don’t refer to any particular model of GPS, and questions relating to features other than the recording of distance and speed are not covered.

### 1 How does it work?

It’s quite complicated! To start with something that’s a bit easier, imagine you want to locate your position in a room, accurately. You could measure your distance from two corners. If they were opposite corners you would get a non-unique answer, so let’s measure our distance from three corners. Each measurement has a small degree of uncertainty or inaccuracy about it. This will be reflected in the answer we get. The more points from which we measure our distance, the smaller the likely error in our calculated position.

Now imagine we can, from anywhere in the world, measure our distance from a number of different satellites in space. This is how GPS works. The GPS receiver picks up radio signals from the satellites. The signals contain information including exactly where the satellite is in space relative to the Earth, and exactly what time it sent the signal. Since we know how fast the radio signals travel (the speed of light – it does 10K in 1/30000th of a second) the GPS receiver can work out how far it is from each satellite. The receiver then plugs those numbers into some very complicated calculations to work out its position on the Earth.

### 2 How accurate is it?

Normally you can expect accuracy to be within a few feet.

### 3 Why does the accuracy vary?

There are a number of reasons why the accuracy varies:

• It depends on the number of satellites from which it is receiving a good signal.
• It depends where those satellites are in the sky. If they are all in the same part of the sky, then the result will not be as accurate.
• The satellite signals will not pass through buildings (or mountains!), so if you are out of the direct view of a satellite you might get a signal from it, but reflected off another building or mountain. In this case the signal will have travelled further and so your calculated position will be wrong. This is called multipath error. It’s a particular problem in cities. Most modern GPS units can recognise and discard really bad reflected signals, and it’s not nearly so much of a problem in moving vehicles.

### 4 Does the weather affect GPS?

Yes, it does. In poor reception conditions, clouds and rain can weaken signals to the point where they are unusable from particular satellites. Also, although we think of the speed of light as being constant, this is only true in a vacuum. The speed of light in the atmosphere depends on the pressure, humidity and what is going on in the ionosphere, so small position errors of up to 15 feet or so can result.

The very weak signals we get from the satellites can be further weakened by dense foliage, especially if it’s wet. Different makes of receiver vary as to their sensitivity. The more sensitive the receiver, the less it will be affected. In general you can expect newer designs to be better than older ones here. Given that the satellites orbit at a height of 12,600 miles it’s pretty amazing that it works at all!

### 6 Why is the pace readout GPS so erratic when I am running?

From the answers given above you can see that your calculated position contains errors. Some of these errors are relatively constant. Weather effects are a good example of slowly changing errors. Some, though, are relatively random and if you plotted out your raw calculated position it would look as if you were staggering about drunkenly.

 This is the track of an out-and-back run recorded on a GPS watch,then exported to a PC and overlaid onto a satellite image. The actual run was entirely on the road, and the runner was sober!

Of course, you may actually be staggering about drunkenly, but the GPS will assume that you are not and attempt to do some mathematical smoothing of the raw positions. Even so, there will still be significant random position errors.

The GPS takes the distance and time between two consecutive position readings and uses these to calculate your pace. The random errors in the position readings cause the fluctuations in pace readouts. One way round this, that is available on many units, is to set a lap distance, for example 1K, and have the unit display your average pace over the lap.

### 7 I ran a 10-mile race, but the GPS said I had run 10.4 miles. Why?

There are two reasons for this:

• You didn’t actually follow the optimum line of the course, but had to run around other runners, sometimes on one side of the road/track/trail, sometimes on the other.
• Random errors, such as multipath errors, always work to increase the calculated distance you travelled.
A GPS will nearly always over-estimate the distance run.

### 8 What about altitude and total ascent/descent? The numbers I get from my GPS are rubbish!

It is true, GPS receivers are much better at calculating horizontal position than vertical position. To a large extent this is due to problems of geometry and the relative positions of the satellites. The GPS receiver has an internal mathematical model of the shape of the earth that represents our planet as a sphere (flattened at the poles). It takes no account of the actual contours. To do so would require much more memory and processing power than is currently available to fit on a human wrist.

### 9 Why does it take my GPS so long to lock on to the satellites?

Newer receivers are better than older ones in this respect. Even so, the way the system works means that some delay is inevitable. Remember that the receiver has to know exactly where each satellite is in space. This is calculated from what is called a set of orbital elements. This is a bunch of numbers that accurately describes the satellite’s orbit. If we have this, and know what time it is, we can calculate the satellite’s position. The satellites only transmit the information every 12.5 minutes, so the time to lock on to a reasonable number of satellites will always be some fraction of this. Most receivers can also do a 'warm start', where they assume at the beginning that the orbital elements have not changed since last usage. If it’s more than one day since you last used it, though, it will most likely do the full 'cold start', collecting the orbital elements from scratch.

### 10 Who operates the satellites?

The GPS satellites are run by the US military. Other systems, some operational, some in development, are run by Russia, China, and the EU. When the EU system called Galileo becomes operational, sometime around 2010, receivers may be available that can use both systems.

### 11 Can the US military tell where I am?

Not unless the CIA has bugged your trainers! Your GPS receiver does not transmit any information back to the satellites.

### 12 Can I use my runner’s GPS as a satnav in a car?

Not really. Car satnav units contain both GPS and sophisticated mapping and route planning software. Runner’s GPS units lack the latter two essential items. Besides, you should keep your eyes on the road rather than peering at a small LCD display on your wrist!