Gps; How Gps Works; Position Fixing Accuracy: Hdop - Si-tex SNS-1100 User Manual

6. GPS

For centuries, sailors have been searching for a reliable and precise method of travelling the
world'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 significant advance in navigation: it gives the navigator a position 24 hours a day, 365 days
a year in any 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 Defense, 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
orbiting 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
determine its present position anywhere on earth, and thanks to continuous updates, calculate
speed and course information.
6.1 HOW GPS WORKS
Currently, the GPS constellation consists of 26 orbiting satellites (including 3 spares), but this
number will increase in the future.
The GPS receiver computes an accurate position by calculating the distance to the GPS satellites
that orbit the earth. Signals are required from 3 satellites for two dimensional (2D) position
calculation 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
illustrated on the following figure:
Fig. 6.1 - The GPS constellation
Note that position is repeatedly fixed through the following three steps while any 3 satellites are
in line of sight.
The position calculation procedure is indicated in the following three steps:
1. GPS satellites continuously transmit their own precise orbital data and the GPS receiver
computes their locations by receiving this data.
2. In this receiving process, the GPS receiver measures very accurate distances to the
satellites, using the "Spread Spectrum Modulation" method. Excellence in GPS's position-fixing
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