How to play hide-and-seek by satellite

Ah, the child-like pleasure of playing hide-and-seek, rediscovered by internet reporter Mark Ward

Consider a vast sphere some 40,075 kilometres (24,900 miles) in circumference, hanging in space.

It is your task to find a small container, about the size of a large lunch box, concealed somewhere on its surface.

You don't have to look everywhere, because oceans cover almost two-thirds of the sphere's surface. However, the rest of it is covered in the kind of complicated, knotty landscape that would make even the most seasoned treasure seeker consider a change in career.

But if you are a technology lover with a taste for the open air, this doesn't describe a seemingly impossible task. Instead, it describes a hobby rapidly gaining fans among the gadget friendly.

Handheld hobby

The hobby goes by the name of geo-caching. And it is the latest outgrowth of a specialised form of leisure activity known as hacker tourism.

Geo-caching involves using a GPS handset to travel to specific latitude and longitude co-ordinates that mark the location of hidden goodies put there by a fellow "cacher".

You're getting close

Those finding the treasure in this way are supposed to take something out, put a new item in its place, sign the visitors' book and move on. Items left in caches include CDs, soft toys, sweets, money, pictures, and tools.

The handset uses signals from the constellation of 24 satellites, which make up the Global Positioning System, to work out where you are on the planet.

Over the years, the US Department of Defense has spent $12 billion building and maintaining the GPS satellites.

Indirect approach

Finding a location on Earth is done by working out very accurately how far someone is from at least four satellites.

N 52° 43.6 W 001° 14.82 (WGS-84) - Leicestershire country park, 760 feet (230 metres), nice views from the top. Secreted between large rocks, this cache will require long arms and a little stone rolling to find.

Details of a new geo-cache - see internet links

The distance between the satellites and the person on the ground is measured by timing how long it takes a signal to travel to the GPS receiver that person is holding. By triangulation, it is possible to pinpoint a location to within six metres (20 ft).

The distance is enough to make a hunt for an exact location of a small box frustrating, especially if there is a big geographical feature in the way that means a direct approach to the cache is impossible.

"You could find yourself a few hundred metres from it, yet have to take a many-kilometre detour and approach from the other side because a river is in the way," pioneering geo-cacher Steven Langford told BBC News Online.

Goodies galore

Some caches are hidden at the bottom of cliffs that have to be reached by very circuitous routes. One is even in the rusted wreck of a car lying at the base of a cliff in California's Mount Tamalpais State Park.

Most caches are hidden out in the countryside, where the reception of signals from satellites is better, and involve a walk of several miles to reach. On the geo-caching website, caches are scored according to how difficult they are to get to.

The hardest, level 5, can only be reached using special equipment such as rock climbing gear and typically have expensive items inside to reward the finders.

Geo-caching only became possible in May 2000 when the US Government ended its practice of degrading the GPS signals to make them less effective. The first geo-cache was placed in Portland, Oregon, two days after and GPS enthusiasts were told via the sci.geo.satellite-nav newsgroup. Three days after this first cache was placed, it was found. Twice.

Walking and thinking

Now, there are hundreds of caches in the US and they are starting to appear in other countries too. The UK's first cache was hidden in December last year and now another eight have been placed.

I borrowed a Magellan GPS 315 handset and sought out the "View from Coombe Hill" cache at N 51° 45.044 W 000° 46.37, which is somewhere between Wendover and Great Missenden in the Chilterns. It is near a monument set on the top of Coombe Hill but not so close that casual searching will find it.

Like orienteering, geo-caching is something of a "thought sport". Once I'd got used to the handset, turned it to use the right set of co-ordinates and worked out which direction the cache was in, it took quite a bit of aimless wandering before I got to the exact location. Even then, I had to crawl into a bush to retrieve the box.

The hard part was trusting the technology to lead me to the prize. Next time I try it, I'll probably plunge over a cliff while I'm reading co-ordinates.

Geo-caching is an offshoot of a geek pastime called hacker tourism which essentially involves visiting sites of interest to technology or gadget lovers.

The Government Code and Cipher School in Bletchley, where Alan Turing did pioneering work on early computers, is a popular site of pilgrimage.

Gotcha - the result of Mark Ward's hunt