Operation Manual

User Manual

up to 100m (GPS has a natural precision of 15 meters).

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 differ-

ences between transmitted signals, the GPS receiver can determine its present

position anywhere on earth, and thanks to continuous updates, calculate speed

and course information.

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. 1.3 - 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 accuracy is mainly due to

this technology.

3. When the satellite locations and their distances are known, the GPS

receiver fixes its own position by triangulation:

Fig. 1.3a - The GPS position calculation

As illustrated in the previous figure, the position is calculated as the meeting point

of three spheres which are drawn around the three satellites with diameters d1,

d2 and d3.

Position Fixing Accuracy: HDOP

The GPS fix accuracy is due to the locations of 3 satellites in the sky. High accu-