Precision - Magellan RoadMate 300 Reference Manual

Information

Precision

On average, a satellite-navigation receiver is accurate to within 15 meters. Thales Navigation employs several
technologies to increase the accuracy of their professional and Magellan®-branded receivers. An accuracy of 3
meters or better is achieved using correction signals from satellite navigation-augmentation systems. In the U.S., an
accuracy of 3 meters is achieved using signal corrections from a network of ground stations and fixed-position
satellites known as WAAS (Wide-Area Augmentation System). Throughout Europe a similar system, EGNOS
(European Geostationary Navigation Overlay System), provides the same accuracy. In Asia, satellite navigation-
signal correction is provided by MSAS (Multifunctional Transport Satellite-based Augmentation System). Other ways
to increase the accuracy of satellite navigation include the use of DGPS (Differential Global-Positioning System);
ground relay stations, set at known positions, transmit corrected satellite-navigation signals. Various methods and
applications of DGPS can increase satellite navigation accuracy from a few meters to within a few millimeters. Using
DGPS requires a differential-beacon receiver and antennae in addition to a satellite-navigation device. Accuracy can
also be increased using an RTK (Real-Time Kinematic) satellite-navigation system. This is a receiver capable of
transmitting a phase-corrected signal from a known position to one or more rover receivers.
A number of positioning errors can occur, limiting accuracy to within 15 to 25 meters. These errors are monitored
and compensated for in a number of ways:
Orbiting errors -- Occasionally a satellite's reported position does not match its actual trajectory. In the U.S., the
Department of Defense continuously monitors each satellite, making orbital corrections with onboard booster
rockets.
Poor geometry -- If all of the satellites within line of sight of a receiver are clustered closely together, or lined up
relative to the position of the receiver, the geometric calculations necessary for triangulating a position become
difficult and less reliable. The use of differential-correction signals from satellite-based augmentation systems or
DGPS can compensate for both orbital errors and poor geometry.
Multipath signals -- Signals may be reflected off tall buildings or other obstructions before reaching the receiver,
increasing the distance a signal travels and reducing accuracy.
Thales Navigation receivers make a number of complex mathematical calculations to effectively compensate for
other potential errors in positioning:
Atmospheric delay -- Satellite-navigation signals slow as they pass through the Earth's atmosphere. Thales
Navigation receivers calculate the average delay in nanoseconds to compensate.
Clock errors -- The clock built into a receiver is not as accurate as the atomic clock on a navigation satellite, which
is accurate to one second every million years. Each Thales Navigation receiver compensates for time differentials by
comparing the time signals of several satellites and adjusting its calculations and its clock to match.
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