Principles of Operation
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GPS. At any one time, with a standard 15 degree angle, up to
10 or 12 GPS satellites are visible from any point on earth.
When a receiver can also track GLONASS satellites, between
10 and 16 satellites are visible.
The GPS and GLONASS network has three components:
• Space – GPS and GLONASS satellites orbiting
approximately 12,000 nautical miles above Earth and are
equipped with a clock and radio. These satellites
broadcast digital information (ephemerides, almanacs,
time frequency corrections, etc.).
• User – The community and military that use GPS/
GLONASS receivers and the corresponding satellites to
calculate positions.
• Control – Ground stations located around the Earth that
upload data, including clock corrections and new
ephemerides (satellite positions as a function of time), to
ensure the satellites transmit data properly.
GPS receivers use ephemeris and almanac data to calculate
accurate positions, and the positions of your survey points.
Calculating Positions
Once the receiver locks on to a satellite, it starts
recording measurements and receiving the various digital
information (ephemeris, almanac, and so on) the
satellites broadcast. To calculate a position, receivers use
the following basic formula:
Velocity x Time = Distance
Where Velocity is the speed at which radio waves travel
(i.e., the speed of light) and Time is the difference
between the signal transmission time and signal
reception time.
To calculate absolute 3-D positions—latitude, longitude,
altitude—the receiver must lock on to four satellites. In a
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