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How does GPS work?
GPS is based on precise time and position information. Using atomic clocks (accurate
to within one second every 70,000 years) and location data, each satellite continuously
broadcasts the time and its position in space. A GPS device receives these signals,
listening to three or more satellites at once, to determine the user's position on Earth.
By measuring the time interval between the transmission and the reception of a satellite
signal, the GPS receiver calculates the distance between the user and each satellite.
Using the distance measurements of at least three satellites in an algorithm computation,
the GPS receiver arrives at an accurate position fix. Three satellites will provide a two-
dimensional (latitude and longitude) position, while receiving signals from four or more
satellites will result in a three-dimensional position (latitude, longitude, and altitude).
Each satellite continuously broadcasts two signals, designated L1 and L2. The L1
frequency contains the C/A code that provides Standard Positioning Service (SPS) for
worldwide civilian use. The encrypted P-code is broadcast on both the L1 and L2
frequency, resulting in the Precise Positioning Service (PPS) for military use. The SPS
signal will provide a civilian user with a raw accuracy of about 17 meters
(approximately 50 feet). Through position averaging and other techniques, Pharos'
GPS receivers are able to obtain 3-5 meter accuracy (10-15 feet) on a consistent basis,
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