On 22 february 1978, the american goverment started a revolution in navigation by launching the first satellite of a system that would eventually provide accurate,continuously update position information to anyone, anywhere on the earth, by night or day and regardless of weather.

By the end of that year, they had an experimental constellation of four navstar GPS satellites in operation, and by 1985, there were another half dozen. The system wasn’t in fully operational until 1995, but by that time the price of civilian receivers had already fallen low enough for them to be becoming commonplace on small craft, with hand-held sets selling for as little as a couple of hundred pounds.

GPS use a constellation of about two dozen satellites in orbit  some 14,000km above the surface of the earth. The number of satellites varies slightly, as old ones are taken out of service and as new ones are launched, but the general idea is that there are enough to ensure that a receiver anywhere on the surface of the earth should be able to receive the radio signal sent by at least four of them at a time.

Operating on microwave frequencies, ten times higher than marine VHF(1575.42MHZ), each satellite transmits a signal which says, in effect, I am here …and   The time is now….

Radio waves travel at a more or less constant speed of approximately 300,00km per second, so if a receiver detects the signal 0.07 seconds after it was sent, the receiver must be 21,000km from the satellite.

GPS time is not quite the same as universal time. Although most GPS receivers show the time and date in a conventional format such as 30 apr 2010 16:12:22, the system time is measured in weeks and seconds.

Another difference arises because there aren’t really 24 hours in a day!  The earth is graduaiiy slowing down so one complete spin now takes 24 hours and 2 milli-seconds. To cope with this , we have a leap year to put our clock time back in step with astronomical time.

best buy sat nav prices have fallen, accuracy has improved, and the system has a proven track record of accuracy and reliability.