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About gps, What is gps, How does gps work – DeLorme Topo North America 9.0 User Manual

Page 180: Gps position accuracy

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DeLorme Topo North America 9.0 User Guide

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shrinks and changes direction as the Sun moves across the compass (similar

to a sun dial).

• Sun rise/set and Moon rise/set information.
• Moon cycle information (such as the day of the moon cycle and the phase).

About GPS

What is GPS?

The Global Positioning System (GPS) applies modern technology to the ancient basics of

navigation. The U.S. Department of Defense has developed and launched a series of

positioning satellites in an orbiting constellation. These satellites are used as reference

points much the same way stars have been used in conventional navigation.
Using these satellites, a GPS receiver can determine your position anywhere on the globe.

GPS provides accurate information about coordinate position, elevation, speed, and

direction of travel. Many people have discovered the benefits of GPS for tracking vehicles,

recording routes, and much more.

How Does GPS Work?

The GPS constellation consists of 24 satellites, each carrying several atomic clocks to ensure

the most accurate time possible. The satellites broadcast low-power radio waves containing

the satellite's identity code and the exact time (to the nanosecond) that the message was

sent.
When a GPS receiver picks up a satellite signal, it identifies the satellite and compares the

signal time with its own clock. The time difference represents the time it has taken for that

radio wave to travel from the satellite to the receiver. Since radio waves travel at the speed

of light, the time difference can be used to calculate the distance from the satellite to the

GPS receiver.
The satellite's identity code provides the location of the satellite, and the distance to the

receiver creates a sphere of possible locations for the GPS receiver. Without more

information, the receiver only knows that it is located somewhere on that sphere.
Two additional satellites are necessary to narrow down the receiver's possible position. Each

of these satellites sends a similar radio message containing time and identity information.

The GPS receiver checks the orbital location of each satellite and uses the elapsed time to

create two additional spheres of possible locations. These three spheres intersect at two

points; however, one of these points is eliminated because it is far away from the earth's

surface. Therefore, the second point is assumed to be correct. The data from these three

satellites provides the receiver with a two-dimensional location.
Data acquired from a fourth satellite pinpoints the receiver's exact location. This additional

positioning information allows the GPS receiver to calculate its elevation, which is

particularly important for GPS users in mountainous locations.

GPS Position Accuracy

The accuracy of the data your GPS receiver provides is dependent upon many factors,

including the quality of your equipment. A low-quality clock within the receiver decreases

the accuracy of your location. The atmosphere, the ionosphere, and the number of channels

your receiver can handle all affect the accuracy of your system. Consult your GPS hardware

manuals for information on how your receiver adjusts for errors.
Any buildings, natural structures, or heavy foliage that obstruct the GPS antenna's view of

the sky prevent satellite signals from reaching the receiver and decreases the accuracy of

your position.