An Introduction to GPS Operation - New Generation Video

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An Introduction to GPS Operation

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What is the nature of the GPS system?     Back to top

"GPS" stands for "Global Positioning System." This system uses earth-orbiting satellites to determine position -- latitude, longitude, and altitude. Unlike TV broadcast and other communications satellites which remain in fixed positions over the equator at an altitude of approximately 23,000 miles, the GPS satellites orbit at an altitude of approximately 100 miles. There are 24 active satellites. At a particular location and a particular time, only a few satellites are used to determine position, and the satellites being used are constantly changing as they orbit.

The GPS system was created by the United States military for accurate location of ground forces and for weapons guidance. However, it is freely available for civilian use, at least during peacetime. (The system has remained available during the wars in Afghanistan and Iraq.) GPS is available worldwide. Other countries have their own systems., which in general are compatible with the GPS system.

How are the satellites used to determine position?     Back to top

Each satellite contains an accurate time reference based on an atomic clock. Each satellite transmits its exact position and the exact time. The GPS receiver also has a time reference. By comparing the time each signal is received with the time it was sent, the receiver can determine the distance to the satellite. The distances to and positions of multiple satellites gives the position of the receiver. Ideally, three satellites would determine position: latitude, longitude, and altitude. However, the time reference in the receiver is much less accurate that that in the satellites, so the position may be in error. Measurements from a fourth satellite will also permit the receiver to determine the correct time, and hence an accurate position. If only three satellites are in view, but the receiver knows the approximate altitude (perhaps from manual input), the receiver will find latitude, longitude and time, but not exact altitude.

Why does it take so long for a GPS receiver to find a position?    Back to top

The time required for a GPS receiver to find an initial position depends on the satellites which are in view, the distance that the receiver has moved since its last position measurement, and the GPS receiver being used.

What are the "channels" in a GPS receiver?    Back to top

Each channel allows the receiver to receive the signal from one satellite. The particular satellites that a receiver uses are constantly changing as the satellites orbit, but the number of channels in the receiver indicates the maximum number of satellites from which signals may be received at any one time. It is common for a modern GPS receiver to have 12 channels.

What determines the accuracy of a GPS position?    Back to top

The accuracy is determined partially by the strength of the satellite signals and the positions of the satellites in the sky. For highest accuracy, the satellites must be located in different directions from the receiver. Accuracy is also affected by the distortions to the radio signals caused by passage through the atmosphere, which is again affected by the position of each satellite -- the signal from a satellite near the horizon must pass through more atmosphere than the signal from a satellite overhead. The GPS system used to use what was known as "selective availability." This deliberately distorted the signal to degrade the accuracy for civilian users. Selective availably was removed in the 1990's, but military users are still able to obtain much more accurate positions than civilian users. Most receivers will indicate the estimated position error, or "EPE."

What affects GPS reception?    Back to top

The GPS system uses high frequencies that are line-of-sight and are easily blocked. For good reception, the receiver must be outdoors (or near a window if in a vehicle) and not blocked by trees. In cities, even tall buildings can hinder GPS reception.

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