Frequently Asked Questions - DeLorme Earthmate PN-40 User Manual

Note: Placing the Earthmate
PN-40 in a jacket or backpack
pocket should not affect GPS
position accuracy.
8
How Does GPS Work?
The GPS constellation consists of at least 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
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
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
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 PN-40?
DeLorme's professional GIS software, XMap
transfer them to the Earthmate PN-40.
, allows you to register non-DeLorme maps and
®