Leica Geosystems GPS Basics - Manual (Page 6)

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GPS Basics -1.0.0en System Overview
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2. System Overview
The total GPS configuration is com-
prised of three distinct segments:
• The Space Segment – Satellites
orbiting the earth.
• The Control Segment – Stations
positioned on the earth’s equator to
control the satellites
• The User Segment – Anybody that
receives and uses the GPS signal.
2.1 The Space Segment
The Space Segment is designed to
consist of 24 satellites orbiting the earth
at approximately 20200km every 12
hours. At time of writing there are 26
operational satellites orbiting the earth.
GPS Satellite Constellation
The space segment is so designed that
there will be a minimum of 4 satellites
visible above a 15° cut-off angle at any
point of the earth’s surface at any one
time. Four satellites are the minimum
that must be visible for most applica-
tions. Experience shows that there are
usually at least 5 satellites visible above
15° for most of the time and quite often
there are 6 or 7 satellites visible.
GPS satellite
Each GPS satellite has several very
accurate atomic clocks on board. The
clocks operate at a fundamental fre-
quency of 10.23MHz. This is used to
generate the signals that are broadcast
from the satellite.