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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.
If you feel that you really need 25-meter accuracy, the effects of SA can be overcome
with a technique called broadcast differential 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 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.
The differential beacon receiver receives and demodulates the signal, then relays
it to the user’s differential-ready GPS receiver. 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.