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USER’S MANUAL XNAV3550 

Glossary 

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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orbit, GMT has been replaced by UTC (Universal Time 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. 

  

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.  

  

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.  

  

Point of Interest (POI). See ÎSpecial 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.