Appendix B — What Is Gps - Garmin eTrex Summit Owner's Manual And Reference Manual

Appendicies
Appendix B
What is GPS?
58
By now the overall picture of how GPS works should
be getting much clearer. Each satellite transmits a message
which essentially says, "I'm satellite #X, my position is
currently Y, and this message was sent at time Z." Of
course, this is a gross oversimplifi cation, but you get the
idea. Your GPS receiver reads the message and saves the
ephemeris and almanac data for continual use. Now, to
determine your position, the GPS receiver compares the
time a satellite signal was transmitted with the time it was
received by the GPS. The time difference tells the GPS
receiver how far away that particular satellite is. If we
add distance measurements from a few more satellites,
we can triangulate our position. This is exactly what a
GPS receiver does. With a minimum of three or more
satellites, your GPS can determine a latitude/longitude
position—what' s called a 2D position fi x. With four or
more satellites, a GPS receiver can determine a 3D posi-
tion fi x which includes latitude, longitude, and altitude.
By continuously updating your position, a GPS receiver
can also accurately provide speed and direction of travel
(referred to as 'ground speed' and 'ground track').
How accurate is your GPS? There are several things
that can have an affect on GPS accuracy, satellite geom-
etry, multipath errors, atmospheric delay, and the United
States Department of Defense just to mention a few. The
descriptions of these items are covered in depth on our
web site www.garmin.com. In general you can expect
your GPS to be accurate from 60 to 255 feet depending
on the number of satellites available and their geometry.
Appendices