Leap Of Faith

[+cachew nut]

I was reading this article dealing with the leap second being added on to this year through the atomic clocks around the world. I'm pretty sure that the clocks in our GPS are atomic ones as well, but located on the satellites themselves. Anyone know more about this?

[+El Diablo]

The satallites get their time data from control stations located on the ground. You can read more about it here. GPS

 

El Diablo

[+Cardinal Red]

Found by way of a Web search:

 

GPS has become the world's principal supplier of accurate time. It is used extensively both as a source of time and as a means of transferring time from one location to another. There are three kinds of time available from GPS: GPS time, UTC as estimated and produced by the United States Naval Observatory, and the times from each free-running GPS satellite's atomic clock. The Master Control Station (MCS) at Falcon Air Force Base near Colorado Springs, Colorado gathers the GPS satellites' data from five monitor stations around the globe. A Kalman filter software program estimates the time error, frequency error, frequency drift and Keplerian orbit parameters for each of the satellites and its operating clock. This information is uploaded to each satellite so that it can be broadcasted in real time. This process provides GPS time consistency across the constellation to within a small number of nanoseconds and accurate position determination of the satellites to within a few meters.

 

Because of this process, GPS cannot tolerate the introduction of leap seconds. Hence, in 1980, when the Department of Defense started keeping time on the GPS satellites, its system time and frequency were set to agree with UTC(USNO MC). At that time, TAI minus UTC was 19 seconds. Since then, UTC has been delayed many leap seconds and GPS time has not. Hence, GPS time is still very close to TAI minus 19 seconds. The specification on GPS time is that it is to be kept within one microsecond of UTC(USNO MC) modulo one second. In other words, as a leap second is introduced into UTC(USNO MC) time, no such step occurs in GPS time. But GPS time is still steered to agree as well as possible with UTC(USNO MC), as if no leap seconds had occurred since 1980. In practice, the steering performance is much better than the one-microsecond specification; typically, it is well within 40 nanoseconds.

[+fizzymagic]

The satallites get their time data from control stations located on the ground. You can read more about it here. GPS

Your statement is a little misleading. Each GPS satellite has multiple atomic clocks on it that combine to give a single clock; corrections between that clock and the main standard are sent from the ground.

 

Of course, the leap second really has nothing fundamentally to do with atomic clocks in the first place. Leap seconds are required because the Earth's rotation is slowing down, mostly a result of tidal forces with the Moon. Clocks don't slow down, so in order to keep the solar time in sync with highly accurate clocks, an extra second has to be added every now and then.

[+briansnat]

Leap seconds are required because the Earth's rotation is slowing down, mostly a result of tidal forces with the Moon.

 

Good thing that changed. I was getting worried for a while there. The days were getting shorter and shorter and if it kept up at that rate there wouldn't be any daylight in year or so. I'm glad the world slowed down so the days could start getting longer again.

[+cachew nut]

Leap seconds are required because the Earth's rotation is slowing down, mostly a result of tidal forces with the Moon.

 

Good thing that changed. I was getting worried for a while there. The days were getting shorter and shorter and if it kept up at that rate there wouldn't be any daylight in year or so. I'm glad the world slowed down so the days could start getting longer again.

Anyone want to buy my Petzl stock?

Edited December 25, 2005 by cachew nut

[+rjb43nh]

One web site describes the GPS system thusly:

"Each Block II satellite weighs 844 kilograms and has a design life of 7.5 years. Each satellite is about the size of a large van with each solar panel covering a surface area of 7.2square metres. The satellites contain two rubidium and two cesium atomic clocks, and three nickel-cadmium batteries which provide energy during eclipse periods. The satellites have an orbital period of 12 hours such that they complete 2 orbital revolutions within a 24 hour period while the earth rotates 360 degrees. This results in a trace of the satellite orbit on the earth's surface which will repeat itself daily. Thus, the positions of the satellites in the sky at any location can be defined for any particular period of time."

 

These 4 onboard clocks are highly accurate (parts in 10 to the -12 or -13) and are fine tuned from the master control ground station. Specialized receivers can use the frequency derived from the GPS system to control ground-based 10Mhz frequency standards. I use the signals to control my rubidium standards using a receiver built by HP to be used in cell phone sites to bill you for the last microsecond.

 

I believe the GPS time is ahead of UTC by 13 seconds currently. International Atomic Time (TAI in French) is 19 seconds ahead.

[+blindleader]

The satellites have an orbital period of 12 hours such that they complete 2 orbital revolutions within a 24 hour period while the earth rotates 360 degrees. This results in a trace of the satellite orbit on the earth's surface which will repeat itself daily.

A little knowledge can be dangerous . The writer of that content assumes, incorrectly that the earth rotates 360° in 24 hours (what we call 24 hours). The conclusion that the trace of the orbit on the surface of the earth repeats daily is wrong, as can be observed by anyone who looks as his satellite screen on successive days or looks at the results of satellite prediction software on successive days.

 

For further study: sidereal time.

[+reveritt]

The satellites have an orbital period of 12 hours such that they complete 2 orbital revolutions within a 24 hour period while the earth rotates 360 degrees. This results in a trace of the satellite orbit on the earth's surface which will repeat itself daily.

A little knowledge can be dangerous . The writer of that content assumes, incorrectly that the earth rotates 360° in 24 hours (what we call 24 hours). The conclusion that the trace of the orbit on the surface of the earth repeats daily is wrong, as can be observed by anyone who looks as his satellite screen on successive days or looks at the results of satellite prediction software on successive days.

 

For further study: sidereal time.

Quite correct, but all quite irrelevant to the discussion of leap seconds.

[+El Santo]

I am just going to keep using the clock with the double stick tape on the dashboard of the pickup

 

I will update it though - what time is it?

[+reveritt]

I am just going to keep using the clock with the double stick tape on the dashboard of the pickup

 

I will update it though - what time is it?

Wednesday.