Initial Experiences with a G.P.S.
GPS stands for global positioning system and a GPS is a small radio receiver designed to listen to signals from navigational satellites. The United States Government maintains 22 of these orbiting around the world and the success of the Gulf War was greatly aided by the precise measurement of position made possible by receiving and comparing the signals from them. For commercial use the signals are degraded (and in the event of war are liable to further degradation). The accuracy claimed is within 15 meters (i.e. it will tell you its position to within 15m, wherever you are in the world). It is possible to improve this to 5m by comparing the signal with that at a base station in a fixed position and a computer program is available to do this.
Such systems are particularly useful to yachtsmen and they include a computer which tells you how to navigate to a position fed into the device (e.g. the next port). Since they are mainly used for navigation they give position in latitude and longitude to the nearest second of arc and height to the nearest foot. More expensive versions have the same accuracy, but have extra navigational information and may be more sensitive, finding the satellites more quickly. They will not work indoors, but an external aerial is available to mount on a car.
These devices have been slowly coming down in price and I felt that one ought to be useful in archaeological work. I had hoped to be able to use one in the preparation of the Kent Historic Buildings Index, hoping to stand at the front door of a house, press the button and read off the National Grid Reference. It will not do this as the frequencies in use demand 'line of sight' to the satellites and to work properly it must be able to receive signals from four satellites. A few years ago I had a demonstration from a yacht chandlers off Piccadilly and it was not possible to use it nearer than the centre of Green Park. It is basically for use in the open and the instruction book warns yachtsmen that it will not work too close to a line of cliffs. In spite of this, I fell for one when I saw that the price had fallen below £200. I bought it in Brighton. It would not work in the grounds of the Royal Pavilion, but did so along the Marine Parade. To start it one has to set it to the latitude and longitude of the nearest town in a world table in the instruction book - in my case LONDON. This is so that it goes straight to the appropriate satellites. If you don't do this it will try all 22 in sequence, taking over half an hour to do so. It works best when held at arm's length above the head, but still takes five minutes to read. If it can only find three satellites it reads latitude and longitude, but not height. You know that it is working when it tells the time (in hours, minutes and seconds), in the same way as a radio-controlled clock. Most models bought in this country provide the option (coded OSGB) of the National Grid, although this is in the form of two letters and six figures (i.e. to 100m.), the latitude and longitude being more precise. It gives good readings in open country and usually works on station overbridges. I lent it to a friend who is a keen hill walker and he said that it was very good and tried some of the navigation programs, which he found useful. Another friend used one effectively while excavating in the Middle East, where a layer of sand obliterated the site each morning and he was able to relocate it.
However, we hope that Kent may be chosen for one of the pilot recording schemes for portable antiquities under the new legislation and the County Archaeologist, John Williams, has asked that finds should be located to 10m, requiring 2 letters and 8 figures. To do this with my equipment would require the ability to translate from latitude and longitude (just possible, but difficult on a 1:25000 Pathfinder Map), and really requiring the base station.
Hence the GPS available doesn't quite do the job we require, but the technology should be watched as improvements are being made and it is so nearly there.
Kenneth Gravett