[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

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.>>

```