Magellan GPS Tracker - Manual (Page 91)

Magellan GPS Tracker 75
Ephemeris data is the precise satellite positioning information that is used for
ranging. Each satellite transmits its own ephemeris data.
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 anticipated.
DoD has decided that 25-meter accuracy is a potential risk, and has intro-
duced 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 under 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 for accurate, repeatable
navigation and positioning information.
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 of errors for all visible satellites and
determines 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.