Network of 24 Satellites
When you make use of GPS you are actually making use of a network of at least 24 satellites (with another couple as spares) that were placed into orbit by the US Department of Defence in the late 70s and 1980s. These were originally intended for military use, but were also opened up for commercial/public use in the 90s.
Does it cost anything for us to use these satellites? Fortunately for us we don't have to pay a fee to the US government whenever we make use of their satellites. There are NO GPS detector vans! Strictly speaking we are not really using these satellites anyway we are just making use of the signal they are constantly sending out. Think of it more in terms of a series of 21st century lighthouses in space, that are on all the time.
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Its only a GPS receiver
All these satellites are set up so that each one sends out its own unique signal at exactly the same time. The thing to remember is that your In car satellite navigation system or handheld gps is only a GPS receiver - it does not communicate with the satellites neither are you being tracked by the satellites - you are infact just recieving the signal that each satellite is pumping out continuously.
Your TV set is another example of a receiver - which is recieving signals which are continuously being pumped out by TV transmitters. Your mobile phone is little different in that it is both a mini transmitter and a receiver, thats why you'll see mobile phone masts all over the place, often on top of buildings - this is the mobile phone companies trying to provide full coverage for their service.
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So how big is a GPS receiver?
Do I have to mount a radar dish on the top of my car for all this technology to work? Good question! with the advances of new technology the answer is an emphatic NO, the actual receiver is very small and often built into the unit - as with the Tom Tom Go (see pictures below left).
External antennae's are optional - for example - more expensive cars with micromesh heated windscreens can block the signal and may require one.
How does it work out your location from these signals?
The clever thing about GPS is that your signal is coming from 24 satellites that are in orbit above the earth. They have been positioned exactly so that there will always be at least 4 satellites broadcasting their information to any location on the surface of the world.
Your gps device is pre programmed to identify and know the exact orbit location of each satellite at any time. When it locks onto the four nearest satellites it then calculates your position by recording the time it takes for the signals to travel from the satellite to your gps device.
From this information through whats known as 'triangulation' it can then calculate your location in longitude and latitude anywhere in the world as well as your altidude and even your speed! |
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GPS needs maps
The trouble with longitude and latitude is that it really isn't that much use to you unless you have a map!
So for example thats why your handheld GPS devices are really useful for sailors or climbers or anyone walking outdoors. These devices can give you your exact location - in longitude and latitude and in conjunction with an accurate map or sea chart you can pinpoint your actual location and avoid walking off a cliff in fog, or running aground at sea.
For dedicated in-car GPS systems, also known as in-car Satnav (short for Satellite Navigation) - the GPS receiver is built in as part of the unit, the maps are also part of the system - which you view on screen. So as you are driving along the device shows the map of exactly where you are and seemlessly feeds up the next map as you move. And if you type in where you want to go - some systems will actually talk to you as you drive along "Left turn in 40 metres.... 20 metres" in a selection of different voices! You can also program in any points of interest along the way - such as pubs or hotels!
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With the Palm and Mobile phone options, you are basically buying a separate dedicated GPS receiver and software (and probably additional memory) to make these devices GPS enabled.
Line of sight - if you are in a tunnel or in a mine underground chances are your gps receiver isn't going to receive any signal and will not work.
The Future
It is expected that in the future all mobile phones will be GPS enabled. Especially useful for an emergency 999 call that will enable the emergency services to pinpoint the exact location of the emergency or person in trouble. A potentially life saving device!
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