[IGSMAIL-4175]: sp3 files with more stable clocks

[Jim Ray (NGS 301-713-2850 x112)]

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IGS Electronic Mail      29 Nov 05:53:32 PST 2002      Message Number 4175
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Author: Jim Ray <jimr at ngs.noaa.gov> & Ken Senior <Ken.Senior at nrl.navy.mil>

This is to bring your attention to the availability of sp3 satellite
orbit-clock files wherein the clock values have been realigned to the
new IGS timescale, which is currently in a test mode.  The sp3 files
are posted together with the associated clock rinex, clock summary,
and related plot files at <http://timescales.nrl.navy.mil>.

The ephemerides in these sp3 files are unchanged from those in the IGS
standard products.  This is also true for the clock differences between
satellites.  Thus, the geodetic performance for point positioning and
other applications should be identical to the standard IGS sp3 files.

However, the underlying timescale for the satellite clocks is much more
stable than in the standard sp3 files, so the behavior of point-positioned
clocks will also be much more stable.  Over intervals of 1 day, clocks in
the new sp3 files are referenced to a timescale with an estimated
stablility of about 1 part in 10^15 whereas the timescale of standard sp3
clocks is about 20 times poorer.  This difference is only relevant for
stations equipped with frequency standards at least as stable as H-masers
or high-quality cesium clocks.  Using these test sp3 files in a point-
positioning mode with data from such a station, the observed station clock
performance should be very close to that obtained in a very good global
network solution but with better frequency stability than is otherwise
accessible.  In this way, point positioning can be used to determine
station clocks fully consistent with the new IGS timescale for those
stations not included in the IGS products.  (Of course, it is necessary
for the point positioning solution to follow all the usual IGS conventions
and standards.)

The test sp3 files are available since GPS week 1086 (29 Oct 2000) for the
IGS Final and Rapid products.  Results are posted soon after the release 
of the official IGS products.  Reactions and feedback from users of these
files would be appreciated.  It is expected that these new products will
become official in the near future.

For further information on the new clocks products please refer to our
article in the current issue of GPS World, pp. 45-51, November 2002.