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09/25/02 GPS_field_guide 8:19 A9/P9
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CHAPTER 1: INTRODUCTION TO GPS
1.1 What is GPS?
The Global Positioning System (GPS) is a system allowing to precisely identify locations
on the earth's surface.
The GPS system has 3 parts (Figure 1):
· The Space segment: a network of 24 satellites placed into orbit (Figure 2). The first
GPS satellite was launched in 1978 and a full constellation of 24 satellites was
achieved in 1994. Each satellite is built to last about 10 years. Replacements are
constantly being built and launched into orbit.
· The Control segment which consists of ground stations, located around the world that
make sure the satellites are working properly.
· The User segment: the GPS receivers used by the community (eg. ETrex device).
Figure 1 : The 3 GPS segments
This satellite-based system offers highly precise location data for any point on the planet,
in any weather conditions, 24 hours a day. It is mainly used for navigation, positioning
and other research applications.