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GPS - How it works and how to speak it
GPS and Satellites
Did you know that every satellite has an atomic clock which is accurate to 1 second every 70,000 years? And did you know that we have so many up there now that there are always 8-10 satellites on the horizon - regardless of your location?
Well, GPS’s (Global Positioning Systems) make use of these two facts to provide a service that can tell you where you are on the face of the Earth to within 100 metres.
A GPS receiver chooses either 3 satellites with a known altitude and calculates your horizontal position (ie latitude and longitude, known as a 2D fix) or 4 satellites, which gives you both a horizontal position and an altitude (known as a 3D fix).
GPS receivers know the exact location of all satellites and they automatically choose the best satellites to give you a fix. These are usually the ones which are the furthest apart. This is what is called ’good geometry.’ It then calculates the position using a method known as triangulation.
However, in order to work at its best your GPS receiver must have a clear view of the sky to avoid poor signal strength. The physical environment can also affect the positional fix and navigation information. So if you use one, it will work best outdoors.
Common GPS Acronyms
As you can imagine, GPS receivers are complex bits of machinery so there’s a lot of "tech talk" associated with using them. So here’s a cut-out-&-keep list of the most common GPS terms:
EPE - Estimated Position Error
ETA - Estimated Time of Arrival
ETE - Estimated Time En route
UT - Universal Time
UTM - Universal Transverse Mercator
VMG - Velocity Made Good
CTS - Course To Steer
Active Leg - The segment of a route currently being travelled
Backtrack - Retraces the position fixes (up to 21 stored automatically).
Bearing - The compass direction from your position to your destination.
Track - The actual path travelled.
Track History - The track over a selected period of time.
Heading - The direction in which your GPS receiver is moving.
Position Fix - Your position co-ordinates as computed by the receiver.
Reverse Route - Reverses the order of waypoints in the existing route.
Waypoint/Landmark - A location saved in the unit’s memory.
Unlike any other radio navigation technology, GPS is the most accurate technology available. It is not affected by the weather, it can be used anywhere in the world, and works 24 hours a day. So that’s why I carry one with me whenever I go outback - and why you probably should too.
If you run into trouble, the first thing the rescue people will want to know is where you are. If you have a GPS, you can tell them with pinpoint accuracy to within 5 metres. If you don’t, well...
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