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  Solar Time, Universal Coordinated Time, and GPS Time

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Author Topic:   Solar Time, Universal Coordinated Time, and GPS Time
jimh posted 02-04-2012 10:14 AM ET (US)   Profile for jimh   Send Email to jimh  
I came across a rather fascinating explanation of various standards for time on the Naval Observatory website at

http://tycho.usno.navy.mil/leapsec.html

The standard time interval of one second is based on the notion of the Earth's mean solar day, the time for the Earth to make one revolution, with the unit of the second being an interval of 1/86,400 of the mean solar day. The figure 86,400 comes from 60-seconds/1-minute x 60-minutes/1-hour x 24-hours/1-day. Atomic clocks based on Cesium 133 provide a time base standard with extreme precision at that frequency. However, the actual rotation of the Earth is subject to variation. (Or, to put it another way, as a precision time keeping device--a chronometer--the Cesium clock is more accurate than the Earth.) The keeping of time known as universal coordinated time or UTC is generally done with the Cesium atomic clocks. About every 500 days the difference between UTC and the actual astronomical time of the Earth will be a variance of one second, with the astronomical time tending to run slow due to the inconsistent rotational speed of the Earth which is lately tending to rotate slightly slower than normal. Since there is no possibility of adjusting the speed of the Earth's rotation, the steady and unvarying time of Atomic clocks has to be compensated to match the variable astronomical time. To accomplish this, an extra second--known as a positive leap second--is added to UTC.

The process of adding leap seconds has been going on since 1961. A listing of the leap seconds can be found at

http://maia.usno.navy.mil/ser7/tai-utc.dat

To connect this directly to boat electronics, it should be noted that the time used in the U.S. Air Force NAVSTAR GPS is also measured with atomic clocks. Epoch for timing in the GPS was 0000h January 6, 1980. The GPS system time is not modified by any leap seconds, and, as a result, GPS time is not retarded by one second every 500 days or so, as occurs with UTC. As a result the time used by GPS is now running 15-seconds advanced from UTC.

You can find more about the various time standards here:

http://leapsecond.com/java/gpsclock.htm

The GPS broadcasts data about the offset of its time from UTC, and GPS receivers use this information to display UTC correctly. See:

http://en.wikipedia.org/wiki/Global_Positioning_System#Timekeeping

The GPS also measures time by noting the number of seconds into each week, with the weeks measured in sets of 1024 weeks. The epoch for week measurement was January 6, 1980. The first rollover at week 1024 occurred in August 1999. At that time there were some GPS receivers whose software became confused. The 1024 week rollover occurs every 19.6-years. The next rollover will be on April 6, 2019. See

http://tycho.usno.navy.mil/gps_week.html

for current week rollover information.

jimh posted 02-05-2012 04:29 PM ET (US)     Profile for jimh  Send Email to jimh     
The publication of my article about time triggered a notice to Steve Allen, a scientist who monitors such things. He sent me an email containing links to articles he has authored on the topic. You may find these interesting:


http://www.ucolick.org/~sla/leapsecs/amsci.html

and

http://www.ucolick.org/~sla/leapsecs/timescales.html

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