The global positioning system is a satellite-based navigation system consisting of a network of 24 orbiting satellites that are eleven thousand nautical miles in space and in six different orbital paths. The satellites are constantly moving, making two complete orbits around the Earth in just under 24 hours (that’s about 1.8 miles per second!). The GPS satellites are referred to as NAVSTAR satellites. Of course, no GPS introduction would be complete without learning a bit more about the satellites:

• The first GPS satellite was launched in February, 1978.

• Each satellite weighs approximately 900 kilos and is about 5 metres across with the solar panels extended

• Each satellite transmits on three frequencies. Civilian GPS uses the ‘L1’ frequency of 1575.42 MHz

• Each satellite is expected to last approximately 10 years. Replacements are constantly being built and launched into orbit. The GPS programme is currently funded with replacements through to 2006

Once distance measurements are added from a few more satellites, your position can be triangulated. This is exactly what a GPS receiver does. With a minimum of three or more satellites, your GPS receiver can determine a latitude/longitude position - a 2D position fix. With four or more satellites, a GPS receiver can determine a 3D position which includes latitude, longitude, and altitude. By continuously updating your position, a GPS receiver can also accurately provide speed and direction of travel (referred to as ‘ground speed’ and ‘ground track’).



Previously, the largest source of position error was Selective Availability (SA). SA was an intentionally imposed degradation in the accuracy of civilian GPS by the US Department of Defence. This was in place because GPS was originally designed and built for military applications and it was felt that without SA there would be a threat to national security. Under SA, GPS accuracy could be degraded to a maximum of 100 metres (328 feet). During this time, errors of 30 metres or more were not unusual. However, on the 1st May 2000, the President of the United States issued a statement outlining his intention to discontinue the degradation of global positioning system accuracy from midnight that night.

The orbital paths of these satellites take them between roughly 60 degrees North and 60 degrees South latitudes. This means that you can receive satellite signals anywhere in the world, at any time. As you move closer to the poles you will still pick up the GPS satellites, they just won’t be directly overhead anymore. This may affect the satellite geometry and accuracy, but only slightly. One of the biggest benefits over previous land-based navigation systems is that GPS works in all weather conditions.



Now, to determine your position, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received by the GPS receiver. The time difference tells the GPS receiver how far away that particular satellite is.

This means that civilian users of GPS are now able to pinpoint locations up to ten times more accurately than they previously could.

Unfortunately, there are still several issues which could prevent you from achieving the best possible accuracy in your GPS position. One of these is satellite geometry. In simple terms, satellite geometry refers to where the satellites are located relative to each other (from the perspective of the GPS receiver). If a GPS receiver is locked onto four satellites which are all in the sky to the north and west of the receiver, satellite geometry, and therefore accuracy, is rather poor. In fact, the GPS receiver may be unable to provide a position reading at all because all the distance measurements are from the same general direction (i.e. poor triangulation). With those same four satellites, if we spread them out in all directions equally at approx. 90 degree intervals (N, S, E, W), our position