@mappingtoolsThe term ‘GPS’ is commonly misunderstood and misused. GPS stands for Global Positioning System, and is usually used in reference to the US Department of Defense Global Navigation Satellite System. This system uses satellites to triangulate the location of a receiver on the Earth’s surface. Originally developed for military applications, there has been a gradual opening up of the service to civilian operators.
In the US, the term ‘GPS’ is mistakenly used for personal navigation devices (PNDs)- i.e. the in-car devices which can give you real time driving directions. These incorporate a GPS receiver, but they also include a small computer, map information, etc. The colloquial British term is ‘satnav‘, which is more accurate than ‘GPS’ and less clumsy than ‘PND’.
Microsoft MapPoint, Streets & Trips, and AutoRoute can all work directly with most types of GPS receiver to provide their own driving directions. Although they can transfer information to/from many types of satnav / PND, this ability depends on the capabilities of the specific device.
Basics of Satellite Positioning Systems
GPS works using a constellation of 24+ NavStar satellites. Each of these satellites carries an atomic clock, and constantly transmits its own time and position. The receiver uses this information to triangulate its own position. With four unknowns (three dimensions and time), signals from a minimum of four satellites are required, although ideally many more are used for an accurate location.
The timing signal includes a sophisticated correlation signal. Different signals allow different levels of accuracy with the most accurate being encrypted for military use only. Standard receivers incorporate corrections for Special Relativity and some global atmospheric effects.
Local atmospheric effects (i.e. weather) can have a significant effect on the accuracy of the receiver, and can only be corrected using external ground based signals and/or base station signals. These corrections are required for high resolution surveying applications, but not for consumer navigation receivers. Therefore consumer receivers typically have an accuracy limited to about 6m. Note that this is good enough for road navigation. Mobile receivers can often appear to be more accurate due to their ability to measure velocity and extrapolate accordingly.
Russia has had the similar GLONASS system in development for a number of years. Although not fully deployed, it was opened to civilian users in 2007. The European Galileo system is currently at a research & development stage, and China plans its own Compass system. Although virtually all civilian receivers only work with the US GPS system, it is likely that future receivers will be capable of working with multiple systems for increased availability and accuracy.
