We use GPS in our everyday life but do we even know how the GPS works?
The Global Positioning System, also called NavStar, that we use was built by the US military and has been fully operational since 1995.
The GPS system currently has 31 active satellites in orbits inclined 55 degrees to the equator. The satellites orbit about 20,000km from the earth's surface and make two orbits per day. The orbits are designed so that there are always 6 satellites in view, from most places on the earth.
The GPS receiver gets a signal from each GPS satellite. The satellites transmit the exact time the signals are sent. By subtracting the time the signal was transmitted from the time it was received, the GPS can tell how far it is from each satellite. The GPS receiver also knows the exact position in the sky of the satellites, at the moment they sent their signals. So given the travel time of the GPS signals from three satellites and their exact position in the sky, the GPS receiver can determine your position in three dimensions - east, north and altitude.
There is a complication. To calculate the time the GPS signals took to arrive, the GPS receiver needs to know the time very accurately. The GPS satellites have atomic clocks that keep very precise time, but it's not feasible to equip a GPS receiver with an atomic clock. However, if the GPS receiver uses the signal from a fourth satellite it can solve an equation that lets it determine the exact time, without needing an atomic clock.
If the GPS receiver is only able to get signals from 3 satellites, you can still get your position, but it will be less accurate. As we noted above, the GPS receiver needs 4 satellites to work out your position in 3-dimensions. If only 3 satellites are available, the GPS receiver can get an approximate position by making the assumption that you are at mean sea level. If you really are at mean sea level, the position will be reasonably accurate. However if you are in the mountains, the 2-D fix could be hundreds of meters off.
A modern GPS receiver will typically track all of the available satellites simultaneously, but only a selection of them will be used to calculate your position.
GPS uses Complex technology but it has a simple idea of locating something.
Accuracy
The accuracy of the position your GPS reports is influenced by a number of factors, such as the positions of the satellites in the sky, atmospheric effects, satellite clock errors and ephemeris errors etc.
GPS units often show on the screen an accuracy figure, e.g. EPE on Garmin units. Under ideal conditions, this may be 5, or even 3 metres. Manufacturers are vague on exactly how this figure is determined and it would be unwise to take this figure literally.
You'll get a more realistic figure by looking in the specification section of your GPS receiver's user-manual. Typically, with a handheld GPS, 95% of horizontal GPS positions will be within 10 metres of their true location. The error in altitude will probably be at least twice the horizontal error.
The accuracy of a GPS can be improved by using secondary data from external reference stations.
Many consumer GPS units have a WAAS option. WAAS uses a network of ground-based reference stations. Readings from the reference stations are used to correct for some of the sources of error mentioned above. The correction data is sent to geostationary WAAS satellites which transmit them back to WAAS-enabled GPS receivers to improve position accuracy. WAAS is not available in New Zealand.
Differential Global Positioning System (DGPS) is a similar system. The data from ground reference stations is transmitted to the GPS using longwave radio, FM radio, or even cellphones.
Best GPS Technologies -
- Tomtom
- Garmin Inc.
- GeoTab
- Spireon
- Motorola Solutions Inc.
- Johnson Outdoors Inc.
- OmniTracs LLC