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FREQUENTLY ASKED QUESTIONS
8 Frequently Asked Questions
Q: What is GPS?
A:
GPS stands for Global Positioning System. Officially named NAVSTAR GPS, It is a satellite-based
radio navigation system developed and operated by the U.S. Dept of Defence. GPS is operated and
controlled by the Government of the United States of America, who are responsible for the
availability and accuracy of the system. The system is allowed to be used free of charge.
GPS signals are available to an unlimited number of users simultaneously, and allow users to find
their position on land, sea and in the air, the correct time, and velocity 24 hours a day anywhere on
earth. The first GPS satellite was launched in 1978 and the full constellation of 24 satellites was
achieved in 1994. A GPS satellite weighs in at about 900 kilograms and lasts about 10 years before it
is required to be replaced.
Q: How does GPS work?
A:
The GPS Navigation system is made up of three segments
The Space Segment:
This is currently made up of a network of 31 satellites orbiting the earth at an
altitude of approximately 19,000 kilometres and travels at about 11,000 kilometres per hour. They
orbit the earth twice per day along 6 circular orbital planes. They are positioned so that every inch of
the surface of the earth can receive signals from at least 6 of these satellites. GPS satellites transmit
signals constantly. These signals are aimed at earth based receivers such as GPS navigation systems.
Each satellite transmits data about its location and height of orbit and the time that the signal was
sent.
The Control Segment:
Is the ground based tracking stations around the world that track and monitor
each satellite in the NAVSTAR system. The tracking information is monitored by the United States Air
Force 2d Space Operations Squadron (2SOPS) at the Schriever Air Force Base in Colorado Springs.
2SOPS regularly contact each satellite with navigational updates. These updates also synchronise the
automatic clock on each satellite to within one microsecond and update the orbital data.
The User Segment:
is the GPS receiver. This is commonly referred to as a Satnav unit or simply a
GPS. The receiver will obtain information from at least three satellites and calculate the location and
the time it took to receive the signal from each satellite. The receiver then calculates latitude,
longitude, direction and velocity. In conjunction with the map software loaded on to the GPS unit, it
can calculate your exact location, heading, speed and correct time and overlay the results on a map
of the area. The software in the system can also calculate the best route to other locations and give
instructions to the driver on the best route giving fair warning of impending turns. These warnings
can be given visually or audibly depending on the model of the system.
Содержание GPS unit
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