What exactly is GPS

Asks Miranda

A. GPS stands for Global Positioning System and refers to a worldwide navigation system of 24 satellites that orbit the earth and coordinate with ground receivers to pinpoint exact geographic locations.

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Q. Who owns or controls GPS

A. The satellites are owned, operated and maintained by the US Government (Department of Defence) but can be used by anyone in the world equipped with a GPS receiver.

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Q. Who built the satellites and how heavy are they

A. The constellation of 24 GPS satellites, known technically as Navstar, were built by Rockwell International and each weigh 1900lbs in orbit and measure 17ft wide with their solar panels extended. Their lifespan is 7.5 years. 21 replacement satellites have already been built to replace these.

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Q. How much did it cost to set up GPS

A. It cost US$ 12 billion.

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Q. How accurate are GPS receivers

A. Incredibly accurate - with the rate of accuracy dependent on the type of equipment you are using - for example most GPS equipment used by consumers today can pinpoint a location anywhere within a 10-100 metre radius, whereas equipment used by the military can pinpoint a location within one metre (usually a couple of centimetres).

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Q. How does GPS work

A. The constellation of 24 satellites (21 are in constant use, with three spare) orbit the earth (10,600 miles from the earth's surface) and are accurately positioned so that at every point on the earth there will be four satellites above the horizon from which to take a reading.

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Your GPS receiver will then triangulate its position by taking readings from three of the four satellites above the horizon. When measuring triangulation the GPS receiver measures distance using the travel time of radio signals - this basically means that the three satellites used will give measurements of your geographical location on earth and these will be calibrated together to give an accurate reading on where you are exactly - in other words it gives every square metre of the earth's surface an address - which is relayed back to your GPS receiver in the form of a geographic position, either in latitude or longitude.

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Q. Why can four satellites be seen from any point on the earth, but only three are used to give a geographic reading

A. The fourth satellite is used to give an altitude reading. All GPS receivers will be able to pick up the signals of three of the four satellites to give a geographic reading in latitude or longitude - only more powerful machines can actually take readings from the fourth satellite on the horizon and if they can they can pick up the altitude of their position too.

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Q. What equipment does each satellite carry so that it can send back navigational information to a GPS receiver

A. Each satellite carries a computer, a radio and an atomic clock - so that it is continually aware of its changing position and time. Technically, a GPS receiver measures the distance to each satellite by calculating how long a radio signal takes to reach it.

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To actually make this measurement it assumes that both the satellite and the receiver are transmitting the same pseudo random codes at the same time and then compares the time each arrive i.e. by comparing how late the satellite's pseudo random code appears compared to the receivers code it can calculate how long the signal took to reach you. Your receiver will then multiply this time, known as travel time, by the speed of light and give you your exact distance - and your geographical position.

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The atomic clock is a near-perfect timing device that gives precise times, if it is inaccurate it can give inaccurate readings on your GPS receiver e.g. it is believed that if the atomic clock on a GPS satellite is a thousandth of a second out it can result in a GPS reading being about 200 miles out - so it is vital to the whole workings of the GPS system.

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Q. Why are consumers buying GPS systems

A. There are usually used for outdoor use e.g. sailing, flying, ballooning, hiking and skiing - for pursuits where you can easily get lost (GPS was originally designed to provide navigational information for ships and planes). In addition, they are continually being installed in cars, for example, as they can relate a driver's location to traffic and weather information and if you're on the move the GPS receiver can also calculate your speed and direction of travel and give you an estimated time of arrival.

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By Karen Anderson