Garmin GPS 12MAP - Manual (Page 13)

3
The Global Positioning System (GPS) is a
system of 24 satellites which circle the earth
twice a day in a very precise orbit and transmit
information to earth. The GPS 12 MAP must
continuously “see” at least three of these
satellites to calculate your position and track
your movement. At times, additional satellites
may be needed to determine a position.
By using an almanac (a timetable of satellite
numbers and their orbits) stored in the receiver’s
memory, the GPS 12 MAP can determine the
distance and position of any GPS satellite and use
this information to compute your position.
Your GPS receiver can only see satellites above
the horizon, so it needs to know what satellites to
look for. To use this almanac data, your GPS needs
to be told its general location (“initialized”) or
given the opportunity to find itself.
Initialization is necessary under these conditions:
• The first time you use your receiver.
• After the receiver has been moved over
500 miles from the last time you used it.
• If the receiver’s memory has been cleared
and all internally stored data has been lost.
Because the GPS 12 MAP relies on satellite signals
to provide you with navigation guidance, the GPS
receiver’s view of the sky will generally determine
how fast you get a position fix—or if you get a fix at
all. GPS signals do not travel through rocks,
buildings, people, metal, or heavy tree cover, so keep
a clear view of the sky for best performance.
Once the GPS 12 MAP has calculated a position
fix, you’ll have anywhere from five to twelve
satellites in view. The receiver will select the
satellites in view to update your position. If some
of the satellites get blocked or “shaded,” the
receiver uses an alternate satellite to maintain the
position fix. A GPS receiver needs four satellites to
provide a three-dimensional (3D) fix, however, it
can maintain a two-dimensional (2D) fix with
only three satellites. A three-dimensional fix
means the unit knows its latitude, longitude, and
altitude, a two-dimensional fix means the unit
knows only its latitude and longitude.
GETTING STARTED
What is GPS?
When new (or if a posi-
tion cannot be deter-
mined after 5 minutes),
the GPS 12 MAP will
prompt you to initialize
the receiver. The unit
needs a starting position
to determine which sat-
ellites are in view.
Check the Satellite Sta-
tus Page for ‘2D Naviga-
tion’ or ‘3D Navigation’
to verify a position fix.