1. Field of the Invention
The invention relates to time keeping and more particularly to a timepiece which automatically adjusts time as time zone boundaries are crossed.
2. Description of Related Art
People who travel frequently find it necessary to frequently adjust their watches to reflect the correct time for the time zone in which their destination is located. Multiple time zone watches are known which attempt to deal with this problem by simultaneously displaying the correct time for a plurality of time zones. This has the disadvantage that the multiple time zone displays must be independently set in advance. It is possible to set them incorrectly with undesirable results such as missed flights and appointments.
The global positioning system (GPS) is a constellation of twenty-four satellites that orbit the earth twice a day, transmitting precise time and positioning information to anywhere on the globe, twenty-four hours a day. The system was designed and deployed by the U.S. Department of Defense to provide continuous, worldwide position and a navigation data for the use of the United States and allied military forces. The potential for commercial applications of GPS were recognized early in the system's development and a determination made to allow free access to GPS signals with certain constraints applied.
Each GPS satellite broadcast two signals, PPS (Precise Positioning Service) and SPS (Standard Positioning Service). The PPS signal is an encrypted military-access code. The SPS is an unencrypted, spread-spectrum signal broadcast at 1,575.42 MHz. Unlike signals from Land-base navigation systems, the SPS signal is virtually resistant to multi-path and nighttime interference, it is unaffected by weather and electrical noise.
GPS receivers listen to signals from either three or four satellites at a time and triangulate a position fix using the interval between the transmission and reception of the satellite signal. Any particular receiver tracks more satellites than are actually needed for a position fix. The reason for this is that if one satellite becomes unavailable, the receiver knows exactly where to find the best possible replacement. Three satellites are required for two-dimension positioning (i.e. position only). Four satellites are required for three-dimension positioning (i.e. position and elevation). In general, an SPS receiver can provide position information with an error of less than twenty-five meters and velocity information with an error of less than five meters per second. A PPS receiver permits much greater accuracy. The higher accuracy is obtainable with the GPS make it suitable as a precision survey instrument.