Understanding Gps - Suunto M9 Quick Manual

UNDERSTANDING GPS

OVERVIEW
Suunto M9 uses Global Positioning System (GPS) in determining the user's position.
GPS incorporates a set of satellites orbiting the Earth at an altitude of 20 000 km at the
speed of 4 km/s. The satellites send relatively weak radio signals at power levels
comparable to normal household light bulbs. So, the GPS reception gets disturbed much
more easily than, for example, cellular phone signal reception because the GPS signals
that reach the Earth are thousands of times weaker than cell phone signals. However, due
to the extreme sensitivity of the built-in GPS radio receiver, even the faintest signals can
be picked up.
The GPS system is run by the United States Department of Defense and has been fully
operational since 1995. At least 24 operational satellites circle the globe every 12 hours
in 6 orbital planes with 4 satellites per plane to provide a true global coverage. A
worldwide ground control network monitors the health and status of the satellites. The
system works 24 hours a day regardless of the weather. It can provide service to an
unlimited number of users and it is free of charge.
LINE-OF-SIGHT NEEDED
To ensure a good position measurement, there must be a line-of-sight connection between
the satellites and the receiver. This means that the objects or materials between the
satellites and the GPS receiver must not excessively attenuate the signal. In practice,
water-containing objects, such as people, trees or other dense vegetation, can cause
problems. The signals will not penetrate rocks or buildings or any metallic materials.
However, the receiver usually works well under relatively thin plastic, fabric or wooden
covers, e.g. inside boats.
ACCURACY
To calculate a position, a GPS receiver usually requires simultaneous reception from at
least four satellites. The position accuracy typically increases with the number of satellites
being tracked but the most important factor is actually the relative geometric position of
the satellites. The best position estimate is obtained when satellites from different directions
and angles can be tracked. In other words, the less obstructed the receiver's view of the
sky, the better the accuracy.
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