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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
knows only that it is located somewhere on that sphere.
Two additional satellites are necessary to narrow down the receiver’s position. Each of these satel-
lites 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 elimi-
nated 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
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 decrease the accuracy of your position.
Frequently Asked Questions
Can I install non-DeLorme maps on the Earthmate GPS PN-20?
DeLorme’s professional GIS software, XMap
®
, allows you to register non-DeLorme maps and
transfer them to the Earthmate GPS PN-20.
How can I improve the life of my batteries?
When the backlight is on, it consumes a large part of the battery. Changing the backlight timer to turn
off after 15 seconds could double the life of your batteries. For information on changing the backlight
timer, see ToModifytheBacklightTimer on page 68.
Other ways to increase the life of the batteries include:
• Removingyourbatteriesforlong-termstorage
Getting Started
Note: Placing the Earthmate
GPSPN-20inabackpackpock-
etorjacketshouldnot result
inproblemswithGPSposition
accuracy.