Both almanac and ephemeris data are required for a GPS receiver to locate
and acquire satellites quickly and compute a position fix.
ACCURACY
GPS positioning with an SPS receiver that is intended for general use will
produce accuracies of 25 meters or better.
In fact, SPS receivers have proven to be far more accurate than anyone
anticipated. DoD has decided that 25-meter accuracy is a potential risk, and
has introduced Selective Availability (SA) to maintain a military advantage. SA
is a random error that is introduced to the SPS code ephemeris data and
reduces the accuracy of any SPS receiver. The size of the error changes, but
rarely exceeds 100 meters.
The DoD civil GPS user policy is that GPS accuracy as affected by SA is
sufficient for general navigation. In an open environment, it usually is. Even
with SA, a GPS receiver will bring you within visual range of a destination or
target, and GPS remains the best available source of accurate, repeatable
navigation and positioning information.
MORE INFORMATION
For information relating to the operation of your Magellan GPS recevier, call
Magellan at (909) 394-5000 and ask for Customer Service.
General information on the Global Positioning System and satellite status is
available from the Civil GPS Information Center (GPSIC) in Virginia. lt is
operated by the United States Coast Guard for the Department of Transpor-
tation, and was established to provide information and to serve as a point of
contact. There are three ways to telephone the GPSIC:
1.
24-hour recorded message at 703-313-5905
2.
computer bulletin board at 703-313-5910
(parameters: 8 data bits, 1 stop bit, no parity)
3.
24-hour live Operator at 703-313-5900
Navtech provides seminars (for a fee) and books on GPS and navigation. The
Navtech bookstore can be reached at 800-NAV-0885 or 703-931-0500, or
at FAX 703-931-0503.
A variety of newspaper and magazine articles have been written on GPS in
the lastfew years. One is "The GPS Navigation System" in the June
1992 issue
of
Popular Electronics.
Check your local library.
There are other sources for GPS information, ranging from free, governmen-
tally produced literature to purchased professional literature and seminars.
The geography department of your local college or the local office of the
National Geodetic Survey may be able to help you locate additional sources
of general or technical information.
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