About Gps; What Is Gps - Sony MSW-900 Supplemental Operation Manual

2-2 About GPS

2-2-1 What is GPS?

The GPS (Global Positioning System) is a system that allows you to
know exact location of any place, anywhere, all over the world, using
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high-precision satellites belonging to the United States of America.
The GPS consists of 24 earth-orbiting satellites. Four satellites each are
placed in six orbits the slant angle of which is 55 degrees.
One cycle around the earth is 11 hours 58 minutes and two seconds. The
satellites orbit the earth synchronizing with the sun. At least 3 satellites
can be located from in any place on the earth.
The GPS receivers receive radio waves from satellites and calculate the
exact location using almanac data from satellites and data based on the
transmission time of radio waves.
Calculating a location is called "Geodetic survey". You can calculate a
location by receiving radio waves from three satellites. However, it is
necessary to utilize four satellites for calculating a location because
interpolation should be done by synchronizing the clock of the receiver
with the satellite clock.
Note
Since the positions of GPS satellites are always changing, it may take
time to finish calculating a location or calculating a location may not be
possible depending on the time or the place where you use your GPS
receiver.
Even if signals from enough satellites for calculating a location are
received, it may take time to finish calculating a location according to
the positions of the satellites.
When calculating a location cannot be carried out
Try to receive radio waves from the satellites for a period of time longer
than 15 minutes in a location free from obstructions, such as buildings.
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Chapter 2 Using GPS Data (When Using an HKDW-704)