GPS, or Global Positioning System, is the technology behind 'SatNav' that we are now familiar with seeing - not only directing dirt-free urban 4x4s to the nearest car wash but also in modest family cars guiding owners to a new holiday destination or the fastest route to granny's house.

GPS works with the 24 satellites orbiting this globe(Earth), each constantly sending out electronic messages. A GPS receiver is a small computer device that looks for and listens to these messages, from which it can use trilateration(similar to triangulation) to work out where the receiver is relative to all the satellites.
GPS needs a line of sight with at least 4 satellites to get enough messages to identify a location. Providing you aren't standing under dense tree cover or in a canyon there will always be 4 satellites visible to a receiver but normally 7-8 to give good accuracy, and even your altitude. GPS receivers only receive the messages from which they work out your location, they don't send messages so you are not actually being tracked or spied on when you use GPS.

Add onto a GPS receiver a nice screen, some maps and directions software, plus the voice of a calm woman telling you to “make a U-turn at the next opportunity” when you get spontaneous, and the result of the mix is SatNav.

GPS has been around since 1978 for military guidance and navigation anywhere in the world, and it's been available for public use for some time, but only in the last 2-3 years have we seen SatNav really take off. It will be to no surprise to me to see lots of nominations in the New Media Awards this year that make use of GPS in ways other than SatNav. Because GPS can give your precise location in a few seconds, it can be used to report something for someone else to find at the same place, or to record information about an area. There are also leisure time uses for GPS, such as geocaching.

Geocaching started days after the GPS satellite messages were first made available for public use in 2000. A small box, or 'cache', is hidden in a public place and the co-ordinates noted down from the hiders GPS receiver. The co-ordinates are then passed onto other people who can use their GPS devices to get to the hiding place and discover the cache. At their simplest a cache just contains a log book where each finder records their visit with a message. Other caches are small boxes containing items such as key rings and small toys, finders being allowed to add something then take something. Certain rules apply such as no food, weapons, etc. and nothing damaging to animals/people that may find it.

So this weekend I decided to go out geocaching with my friend Emily who has heard my stories but never experienced all the fun. First off was to check a Geocache listings site, the most well-known being Geocaching.com where you can type in your postcode or location to be presented with a listing of caches and its rough distance away. The Geocaching.com database currently lists around 390 thousand caches in the world, and over 17 thousand of them in the U.K. that are hidden in public places undiscovered by those not in the know.

And what luck, a new geocache had been hidden that was a nice walk from home. So I printed off the cache's page which includes the longitude and latitude co-ordinates of the cache, a description, and logs posted by other users who found it or not.

The description told us that the cache was by one of the entrances to a walled garden that I pass fairly often but haven't seen for about 10 years, it also told me there is now a museum which I didn't know of. I met up with Emily and we turned on the GPS device that gave our location, knowing where the walled garden was we walked across the park to be on the outside of one of the corners. My PDA showed us the longitude and latitude values, which didn't quite match the printed sheet so we walked to the nearest gate to see if that was right.

Longitude was spot on, meaning on a north/south aspect we were in the right place. The latitude number was more west than the print out so we needed to head through the garden and directly east. Being a formal walled garden we walked straight down a path, round a fountain, and there was another side gate directly opposite the one we came in.

Now our current location according to the GPS matched the ones on the printed cache description. An additional clue helped us out by saying the cache was under a fallen tree. We spotted the tree, but I will admit we were helped by the trampled grass that seemed to be a recent mark of others searching for this new cache. We started peeping into the ivy wondering how much we will have to rummage, then I walked round the side to see before me a big litre plastic box marked 'geocaching' on the lid. I walked back round to Emily with her ankles now attacked by stinging nettles. This is my 7th successful geocache find, and definitely the biggest one yet, we signed the log book together but didn't take anything out.

Geocaching is a great activity not least because it inspires you to get out of your car for an enjoyable walk during which you can learn something you wouldn't otherwise have known. One geocache taught me about London coal posts as I had to see and use the numerical date written on one to get co-ordinates for the actual cache. While visiting Brighton I went to a cache on the pier and signed a log book hidden next to hundreds of un-suspecting passer-bys. Geocaching is also being used to promote cleaning up rubbish and increasing awareness of diabetes, for more information you can read this pdf explanation or the Geocaching.com website