Haiyang AQUA HGP-660, User Manual, Page: 61 / 67

GPS
For centuries, sailors have een searching for a reliable and precise method of travelling the worl
d’s waterways. From celestial navigating to the modern navigation techniques as Loran, Decca
navigator, Omega or Transit Satnav, each system has had its problems with weather, range and
reliability. Without doubt, the “Global Positioning System”, or GPS for short, is the most significa
nt advance in navigation: it fives the navigator a position 24 hours a day, 365 days a year in an
y weather condition.
GPS is a satellite based navigation system which provides suitably equipped users with accurate
position, velocity and time data. Originally the GPS, developed by the U.S. Department of Defe
nse, was conceived for military purposes, but now it is used in a host of civilian applications.
GPS navigation uses satellite signals to determine your position in relation to a set of satellites o
rbiting the earth. The GPS constellation of satellites continuously send radio signals, containing
the precise position for each satellite back to earth. By knowing the position of 3 or 4 satellites
and calculating various time differences between transmitted signals, the GPS receiver can dete
rmine its present position anywhere on earth, and thanks to continuous updates, calculate spee
d and course information.
HOW GPS WORKS
Currently, the GPS constellation consists of 26 orbiting satellites (including 3 spares), but this n
umber will increase in the future.
The GPS receiver computes an accurate position by calculating the distance to the GPS satellite
s that orbit the earth. Signals are required from 3 satellites for two dimensional (2D) position cal
culation whilst 4 satellites are required for three dimensional (3D) position calculation.
As mentioned earlier, GPS satellites are not geostationary, but they are orbiting the earth as illus
trated on the following figure:
[The GPS constellation]
Note that position is repeatedly fixed through the following three steps while any 3 satellites are i
n line of sight.
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