[IGSMAIL-6384] REMINDER: switch to IGS08/igs08.atx on 17 April 2011

[Jim.Ray]

IGS users are reminded of the changes that will be made for
product files starting 17 April 2011 = GPS wk 1632_0 =
MJD 55668:

* reference frame IGS08 will replace IGS05 -- see IGSMAIL
#6354

* antenna calibrations igs08.atx will replace igs05.atx --
see IGSMAILs #6355 and #6374

A presentation describing these changes is available at
   http://acc.igs.org/trf/igs08_egu11.pdf
and advice on accounting for station-specific position corrections
due to the new antenna calibrations is given in IGSMAIL #6356.

The impact on user terrestrial frame results will be mostly
a scale/height shift of about 6 mm and a Z origin translation
of a few mm, with only minor changes in the other Helmert
components.  The IGS orbits will be unchanged within a few mm.


Associated with these changes, the Analysis Centers have been
asked to implement two new IERS Conventions (2010) models:

* EGM2008 geopotential model -- including updated values for
the time-variations of low-degree coefficients (see Conventions
Chapter 6)

* mean pole model -- see Conventions section 7.1.4, which is
used for both the geopotential and for station displacements
due to the pole tide; see eqn (7.25) & Table 7.7


In addition, Ken Senior will implement a new generation time
scale ensemble, which will cause a phase discontinuity between
IGS Rapid and Final clocks published before and after
2011-04-17T00:00Z.  For each station and satellite clock, the
new Kalman filter ensemble stochastically models its phase
(time), frequency, and drift (frequency rate), rather than just
a single frequency state till now.  An added phase state is
included to model a pure white phase noise process, as well as
12-hr and 6-hr harmonic terms for the satellite clocks.  The
intrinsic stability of this new time scale ensemble is
significantly improved over past performance, mainly for
intervals longer than a day, which permits softer steering to
UTC.

A poster presentation giving further information on the new
time scale algorithm is available at
   http://acc.igs.org/clocks/igst-v2_IGS10_poster.ppt

--Jim Ray
   IGS Analysis Coordinator