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GPS
GPS Overview
Street Atlas USA 2005 allows you to take advantage of the Global Positioning System (GPS) through an interface
with most GPS receivers, such as the DeLorme Earthmate
®
or GpsTripmate
®
.
Using Street Atlas USA 2005, a laptop computer, and your GPS receiver, you can:
•
Display a "bread crumb trail" to track your progress as you travel.
•
Send and receive route information, draw objects, tracks, and waypoints to/from your GPS device using the
Exchange Wizard.
Safety Warnings
•
Bring a passenger along to serve as GPS operator while you are driving a vehicle.
•
Street Atlas USA 2005 should not be used in automatic navigation, guidance
systems, or for any purpose requiring precise measurement of distance or direction.
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 therefore decreases the accuracy of your position.
