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USER’S MANUAL XNAV3550
Glossary - 83 -
9 Glossary
GMT GMT (Greenwich Mean Time) is the mean solar time at the Royal
Greenwich Observatory in Greenwich near London, England,
which by convention is at 0 degrees geographic longitude.
Theoretically, noon Greenwich Mean Time is the moment when
the Sun crosses the Greenwich meridian (and reaches its highest
point in the sky in Greenwich). Up to 1972, GMT was the global
time standard. Because of the Earth's uneven speed in its elliptic
orbit, GMT has been replaced by UTC (Universal Time
Coordinated) which is an ultra stable time standard based on
atomic clocks.
GPS The GPS (Global Positioning System) is based on 24 satellites
which are in orbit round the earth. They are permanently
emitting the time and their current position. The GPS receiver
receives this information and calculates the longitude and the
latitude of its own current position.
The signals of at least three satellites are needed to determine
the longitude and the latitude. With the signals of at least four
satellites the elevation may be calculated, too. The determination
has an accuracy of about 3 yards.
HDOP The Horizontal Dilution of Precision (HDOP) indicates the quality
of position determination. Theoretically any value from 0 to 50 is
possible. The smaller the value is, the more accurate is the
position determination (value 0 = no deviation from the actual
position). Values up to 8 are convenient for street navigation.
POI Point of Interest (POI). See ÎSpecial destination.
Special destination Special destinations, also called POI (Points of Interest), are
covered by the map and may be displayed on it. Harbors,
airports, restaurants, hotels, gas stations, public buildings, and
others belong to the special destinations. You may determine
special destinations as route points for navigation purposes.