GPS Field Guide
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).
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.
How does GPS work
GPS satellites circle the Earth twice a day in a very precise orbit at an altitude of around 19 000 Kilometres (Figure 2). This constellation allows any user to access between five and eight satellites from any point on the Earth.
Each satellite transmits radio signal information which is tracked and used by the GPS receiver to calculate the user’s exact location.
To determinate precise latitude and longitude/position, the receiver measures the travelling time of the signal between the satellites and itself and transform it into a distance.
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