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Magellan NAV 6500/NAV 651096
curacy is a potential risk, and has introduced Selective Avail-
ability (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 af-
fected 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.
If you feel that you really need 25-meter accuracy, the effects
of SA can be overcome with a technique called broadcast dif-
ferential to produce highly accurate position fixes.
DGPS
Differential GPS (DGPS) computes the size of the error and
applies it to positioning information. There are several ways
to perform DGPS, one of which is broadcast differential.
Broadcast differential uses GPS receivers at control sites to
measure the range errors for all visible satellites and deter-
mines a correction for each satellite. These corrections are
broadcast in the RTCM SC-104 format by a radio beacon at
the control site to any differential beacon receiver that is within
range of the signal.
The differential beacon receiver receives and demodulates the
signal, then relays it to the user’s differential-ready GPS re-
ceiver. The user’s GPS receiver applies the corrections to the
positioning information it collects to compute differentially
corrected position and navigation data.
This technique requires that your GPS receiver be connected
to a compatible differential beacon receiver (such as the
Magellan DBR™, which is compatible will all differential-
ready Magellan receivers). You must also be within range of a
differential radio beacon.
More Information
For information relating to the operation of your Magellan
GPS receiver, 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. It is operated by the United States
Coast Guard for the Department of Transportation, and was
established to provide information and to serve as a point of
contact. There are three ways to telephone t
he GPSIC:
1. 24-hour recorded message at 703-313-5907
2. Web site at www.navcn.uscg.mil
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 FAX 703-931-0503.