[ntp:questions] Leap second functional question

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sat Feb 23 22:20:03 UTC 2008

Unruh wrote:

> 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

SPECIFICATION <http://www.navcen.uscg.gov/pubs/gps/sigspec/gpssps1.pdf>. 
on page 18, explicitly refers to relativistic corrections:

<<The L-band SPS ranging signal is contained within a 2.046 MHz band 
centered about L1. The carrier frequency for the L1 signal is coherently 
derived from a frequency source within the satellite. The nominal 
frequency of this source -- as it appears to an observer on the ground 
-- is 1.023 MHz. To compensate for relativistic effects, the output 
frequency of the satellite's frequency standard -- as it would appear to 
an observer located at the satellite -- is 10.23 MHz offset by a
∆f/f = -4.4647 x 10-18 or a ∆f = -4.567 x 10-3 Hz. This frequency offset 
results in an output of 10.22999999543 MHz, which is frequency divided 
to obtain the appropriate carrier modulation signal (1.022999999543 
MHz). The same output frequency source is also used to generate the
nominal L1 carrier frequency (fo) of 1575.42 MHz.>>

Also, on page 39, it puts an explicit requirement on receivers to apply 
a relativistic correction.  I'm not sure if this is SR, GR, or a mix.  I 
think this is referring to relativistic effects along the whole signal 
propagation path.

<<The coefficients transmitted in subframe 1 describe the offset 
apparent to the control segment two-frequency receivers for the interval 
of time in which the parameters are transmitted. This estimated 
correction accounts for the deterministic satellite clock error 
characteristics of bias, drift and aging, as well as for the satellite 
implementation characteristics of group delay bias and mean differential 
group delay. Since these coefficients do not include corrections for 
relativistic effects, the user's equipment must determine the requisite 
relativistic correction. Accordingly, the offset given below includes a 
term to perform this function.>>

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