[ntp:questions] Leap second functional question

Unruh unruh-spam at physics.ubc.ca
Fri Feb 22 10:31:57 UTC 2008

David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:

>Unruh wrote:

>>> High precision orbital dynamics, such as the JPL's DE405, are done in 
>>> Barycentric Dynamic Time, which runs at a different rate than TAI, but
>>> is "TAI like" in that it has no leap seconds.
>> It does? The second was defined to make the speed of light a consant. Are
>> you saying they use a system in which the speed of light changes from time
>> to time? That sounds horrible. 

>I believe he is talking about the general relativity correction.  Even 
>GPS would be wildly wrong if you used the earthbound definition of a 
>second for the clocks in the satellites.

A common misconception. The GPS people actually dynaically track the time
delivered by the sattelites and adjust their scales accordingly. Even if
they know nothing of GR, they would have discovered that the clocks were
running a bit fast and applied a correction fudgefactor. The problem is
that the clocks on the board could be running at the wrong rate doe to
other environmental problems and they have to be able to correct for those
or your GPS will cease to work.

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