VDO MAP 11csE Plus GPS, Руководство пользователя

Страница: 16 / 120

17
User Manual
MAX and NT/NT
+
C-CARD coexistence
♦
When NT
+
data and MAX data cover different areas, the chart plotter
gets data from both charts (depending on the current position).
♦
When NT
+
data and MAX data cover the same area, the chart plotter
gets data only from MAX chart.
1.3 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 Defence, 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.
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:
16
13
11
14
9
7
2
4
18
19
20
17 3
8
6
1
10
5
15
21
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.