GPS - How it
works and how to speak it |
Richard Oliver |
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... |