[ntp:questions] Leap second functional question

Danny Mayer mayer at ntp.isc.org
Mon Feb 25 02:58:20 UTC 2008


David Woolley wrote:
> 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
> 
> The 1995 GLOBAL POSITIONING SYSTEM STANDARD POSITIONING SERVICE SIGNAL 
> 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.>>

Bill Unruh has a more than passing knowledge of General Relativity and 
time is more than just a general interest to him: 
http://www.physics.ubc.ca/php/directory/research/fac-1p.phtml?entnum=181

However, that reference to relavistic effects is almost certainly 
special relativistic effects. GR would be almost unnoticeable.

Danny



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