Garmin's Edge 705 is a bike-specific GPS receiver

What is GPS?

The Global Positioning System allows a suitably equipped receiver to pinpoint its position anywhere on the planet's surface. While originally designed for (and still operated by) the US military, over the last 12 years it has had a major impact in navigational systems for everything from walkers to pilots. Bike-specific GPS receivers are a relatively recent development.

How GPS works

A constellation of satellites orbit the Earth at a height of around 12,000 miles and send out a stream of messages by radio. Those messages contain the time the message was sent and precise orbital information about the satellite in question (how high it is, what angle it's orbiting at, how fast it's going and so on). The receiver measures how long the signals take to travel from the satellite and thus computes the difference. Once it knows how far it is from four satellites, it can use trilateration to determine its position.

(If you're wondering why it needs four rather than just three, the reason is that the satellite signals travel at the speed of light, which is a very big number. Even a very small error in the receiver's clock would, therefore, result in a big error in the location. With four satellites, the receiver can do its sums not just for x, y and z coordinates (with z being altitude) but also correct its own clock.)

The satellite network is arranged to cover the whole planet. The original design called for 24 satellites, although there are actually 31 at the moment. Because of the arrangement of the satellite orbits, different numbers of satellites will be in view from a particular point on the surface of the globe at different times, although at least six should usually be within range. Most receivers will be able to tell you how many satellites they can see and how strong the signals are, as well as letting you know the accuracy of your position.

Limitations of GPS

All GPS itself does is give you a position. What you can do with that information depends on the capabilities of your GPS receiver and any other software you're using. Most receivers will log your position at time intervals to give you a track of where you've been, and allow you to mark important locations or landmarks. Some have built-in mapping capability, but pretty much all will connect to a PC running mapping software.

GPS is not a replacement for the traditional navigation skills of map and compass use. For a start, unless you're actually moving it doesn't know which way you're facing. It's an additional resource that may be used either on its own or in combination with these traditional skills. One of the big attractions for MTBing is the ability to see where you've been on a PC after the ride, with accompanying speed information. The ability to upload tracks to the GPS receiver to follow is also an increasingly-popular feature.

It is important to remember that your personal safety is your own responsibility at all times, and whilst GPS is a useful tool it should not be relied on as the sole means of navigation in the outdoors.

Does GPS work everywhere?

Because the system uses radio waves, the ability to receive a signal is partially dependent upon the surrounding environment, although increases in the sensitivity of the receivers are improving this situation. Dense woodland, tall buildings, and canyons in particular can make reception difficult or reflect the signal giving a false reading.

The recent SiRF III chipsets have gone a long way to reducing this problem and a GPS receiver based around this chipset should be suitable in almost any outdoor location - and even indoors within reasonable distance of a window or line of sight. Despite the advances in chipset sensitivity, ionospheric distortion, caused by high solar activity, can cause dramatic malfunctions by slowing the progress of the radio signals through the ionosphere.

This is one of the reasons why GPS isn't a total replacement for map and compass skills.