NIKOLAS: TECHNICAL SERVICES

Hi. I'm Nikolas, and I work in Technical Services at Talking Technologies. This is not a full explanation of GPS technology, but it does tell you enough so that you can understand what it is and what it can do.

The Global Positioning System is a navigational technology which uses radio transmissions from twenty four satellites in distributed stable orbits about 12 600 miles above the Earth.

A twelve-channel receiver locks on to any radio-visible satellite. For each one, it measures the transmission travel time and calculates the transmission distance. The data from three satellites provides a two-dimensional fix: as latitude and longitude. The data from a fourth provides a three-dimensional fix: as latitude, longitude, and altitude.

The receiver tracks whichever four satellites are in the best positions, but includes data from other satellites to create a more accurate position.

Each satellite has very accurate atomic clocks on board, and a regularly updated almanac of the current and expected positions for the other satellites. When a receiver locates a satellite, it downloads the almanac and can find the other satellites quickly.

GPS is free, and it works 24 hours a day in all weather conditions. It's used worldwide for road-route guidance, touring, off-road driving, search and rescue, vehicle tracking, hiking, camping, boating, rafting, pony-trekking, ballooning, general aviation, skiing, surveying, mapping ...

GPS is controlled by the US Department of Defense. Although initially designed for the military, the Government soon realised that there were other applications, so a two-level accuracy system was created.

The more accurate system was only available to the military, to prevent attacks against the US using its own national navigational system.

However, civilian receiver technologies quickly improved, making real-time adjustments and proving to be unexpectedly accurate. To counter this, the military developed a random-degradation system to reduce the accuracy.
You could see the effects when you were stationary, with random changes in latitude, longitude, altitude, and speed, and slow position-wandering: when your boat was moored, the speed regularly changed to a few knots, even though you were docked.

From May 2000, the two-level accuracy was dropped, although the military can still localise the control system to deny GPS signals to select areas.