Many portable electronic devices, such as laptop computers, cellular mobile phones, and personal digital assistants (PDAs) are now equipped with global positioning system (GPS) receivers. GPS receivers have been formed integrally with personal computers and in addition to their map navigation function, their use as portable communication tools provided with, a PDA function, a camera function, a communication function with a linkage to a portable telephone etc., is spreading. GPS is a space-based radio triangulation system using a constellation of satellites in orbit around the earth. GPS signals that are broadcast from satellites in stationary orbits are used by GPS receivers for generating location data to locate the receiver. More accurate location information can be generated as more satellites are detected. A GPS receiver receives radio signals from various ones of the satellites and determines the times that the radio signals take to travel from the GPS satellites to the GPS receiver. By multiplying the travel time by the speed of light, the GPS receiver can calculate a range for each satellite in view. The spread spectrum signals continuously transmitted from each satellite utilize a highly accurate frequency standard accomplished with an extremely accurate atomic clock. Each satellite, as part of its data signal transmission, transmits a data stream indicative of that particular satellite. The GPS receiver device acquires spread spectrum GPS satellite signals from at least three satellites to calculate its two-dimensional position by triangulation.
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