part3 - Part III GPS MAJOR COMPONENTS AND THE SIGNAL...

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Civil and Environmental Engineering and Geodetic Science Part III GPS: MAJOR COMPONENTS AND THE SIGNAL STRUCTURE GS608
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Civil and Environmental Engineering and Geodetic Science GPS Satellite System GPS Satellite System 24 satellites altitude ~20,000 km 12-hour period 6 orbital planes inclination 55 o
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Civil and Environmental Engineering and Geodetic Science Precise time measurement is behind the success of GPS GPS uses its own time system that is based on the atomic time scale Basic units : second of the week (second since the beginning of the week) and a week number The initial GPS epoch (week 0) is 0h UTC of January 6, 1980 Universal Coordinated Time (UTC) is the time scale based on atomic second that corresponds to Greenwich time, and is the basis for most radio time signals and legal time systems GPS Time System GPS Time System
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Civil and Environmental Engineering and Geodetic Science Since TAI (atomic time) is independent of the Earth’s rotation, the concept of Coordinated Universal Time (UTC), that is in some prescribed way connected to the rotational time, was introduced in 1961, taking advantage of the stability, predictability and almost immediate accessibility of TAI. UTC is based on the atomic second, thus its rate is uniform. Also, its epoch is manipulated accordingly so that the difference between the time based on Earth diurnal rotation and UTC is maintained on a level less than or equal to 0.7 s. For that purpose UTC is modified by introducing a leap second, when required, e.g., on December 31 and/or June 30. As a result, UTC and TAI always differ by an integer number of seconds that can change only every year or one-half year Time Systems Time Systems
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Civil and Environmental Engineering and Geodetic Science Since UTC is altered to keep it synchronized with the rotational time (based on Earth rotation rate), the difference (in seconds) between UTC and GPS time grows Consequently, what you see on most of GPS receiver displays is the GPS time, which is close to UTC (Greenwich time), which is 5 hours ahead from our time zone One can usually set up the receiver to display local time if needed GPS Time System GPS Time System
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Civil and Environmental Engineering and Geodetic Science The Global Positioning System (GPS) experienced the first rollover of its internal clock, termed the End of Week (EOW) Rollover , on August 21, 1999 The EOW rollover exists because the largest increment for counting GPS system time is one week, and weeks are accumulated in a 10-bit register GPS time started Jan. 6, 1980 with week "0000" and continued until 23:59:47 Universal Time Coordinated (UTC), Aug. 21. After the rollover, the GPS clock reset itself to "0000." This was
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part3 - Part III GPS MAJOR COMPONENTS AND THE SIGNAL...

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