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Reference manual (78 pages)

Table Of Contents

Table of Contents

Both almanac and ephemeris data are required for a GPS receiver to locate and ac-

quire satellites quickly and compute a location 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 Selec-

tive 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 re-

ceiver. 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 re-

ceiver will bring you within visual range of a destination or target, and GPS remains

the best available source of accurate, repeatable navigation and positioning informa-

tion.

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 location 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 correc-

tions to the positioning information it collects to compute differentially corrected Po-

sition and navigation data.

This technique requires that your GPS receiver be connected to a compatible differen-

tial beacon receiver (such as the Magellan DBR

ential-ready Magellan receivers). You must also be within range of a differential radio

beacon.