Yet despite its apparent robust simplicity, satellite positioning remains a sensitive and complex system vulnerable to many factors. It's a technology that relies on extremely high precision where billionths of a second really do matter. The tiniest errors can lead to discrepancies on the ground of hundreds of metres.

Positioning equipment is now big business. And that means the usual competitive marketing claims, hype and confusion. The hardware alone can be the biggest part of a field project budget. And if you're going to invest that much money, you really need to understand for yourself the technology, its language, limitations and how to get the best from it.

This is the first of a series of articles aimed at helping our customers sort the hype from the reality, understand the real potential of GPS, see though the sales jargon and hopefully make the right investment decisions.

And the core messages are these:

§       be very wary of 'accuracy' promises on GPS adverts

§       products may be getting cheaper but you still (usually) get what you pay for

§       and be very clear on your usage requirements or your shiny new GPS may never be any use to you at all.

We'll explain each of these in turn, but to do that we need to start at the beginning.

So how does GPS actually work? Put simply, there's a network 24 satellites covering the Earth with a web of roughly 12 hour orbits. Each has a synchronised atomic clock and constantly broadcasts that time down to the surface. Your GPS receiver also has a very accurate clock. If your receiver can 'see' three or more satellites, it can measure the time delays from each satellite, and knowing the speed of light, can calculate the distance and thus triangulate its position. In practice, at least four satellites are used and generally the more the better.

In part two we'll explain why that is, why GPS fails and what you can do about it.