Dear Colleagues,

Considering the planned meeting of the CPP on 27 April at the EGU General
Assembly, it seems advisable to prepare topics for discussion there.  I
recently discovered some materials from the Napa meeting on 11 Dec 2004
are posted by the IERS under "News" (but, oddly, not at the CPP site) at:
  http://www.iers.org/workshop_2004_napa/

As can be seen in the Napa presentations, almost nothing new has yet been
achieved within the CPP context.  This can be attributed primarily to an
invalid framework for the CPP work (based on weekly inter-technique
combinations) and to a lack of solutions of the proper type (namely, long
time series).  Zuheir and I have previously shown that a proper inter-
technique combination must be formed as a long-term solution of time series
of intra-technique solutions; see
  http://www.iers.org/iers/about/wg/wg3/docs/Annex_5_Altamimi_Ray.pdf
Once formed, it can be maintained operationally in one of two ways.  Lacking
the essential ingredients, the CPP has been fundamentally handicapped from
its start.

Now, with the call for ITRF2004 submissions, the lack of long-term time
series solutions is finally resolved.  Presumably, this process will produce
a long-term multi-technique reference solution for station positions,
velocities, and some EOP(t) components, complete thru the end of 2004.  In
my view, this step achieves a critical level to which the CPP should have
aimed but did not.  While sufficient for a new ITRF realization, this 
process will not fully satisfy other IERS needs, however, namely for
maintaining consistent EOP series into the future.

Given the ITRF2004 activities, which finally achieve the first step of what
the CPP could not, the CPP should now be redefined to satisfy these remaining
objectives for the IERS:

1) Validation and completion of ITRF2004 products

Once formed by the ITRS Center, the ITRF2004 system (including EOPs) must be
validated.  In addition to past tests, this process must be extended to
include the combined polar motion series and VLBI-based UT1/LOD.  The
validation should include detailed comparisons with geophysical excitation
series.  As we know, the current IERS combined polar motion series are less
precise than the IGS series alone (Ponte & Ali, GRL, 29(15), 2002) and suffer
from significant biases.  Any new IERS combined polar motion series should be
at least as accurate as the IGS-only series, which will depend sensitively on
the handling of the co-location ties and their errors.

As Zuheir and I have noted, we see no practical way to combine satellite-based
biased-LOD information directly with VLBI-derived UT1/LOD in a rigorous
SINEX combination.  Consequently, a secondary combination step is needed to
take the VLBI-derived UT1/LOD results from the rigorous ITRF2004 combination
(which benefit from accurate GPS-based polar motion, as we have shown in the
paper by Ray, Kouba, and Altamimi, "Is there utility in rigorous combinations
of VLBI and GPS EOPs?", submitted to J. of Geodesy) and combine with
satellite-based LOD results non-rigorously (taking great care for time-varying
biases).  This is primarily a densification process.  We believe that a
standalone Kalman filter procedure (such as JPL's KEOF) would be ideal for
this step.  (In a completely analogous way, VLBI-derived nutation can be
combined with satellite-based nutation-rates, if these are available.)

2) Develop an operational weekly update procedure

As we outlined at Nice, two approaches (rigorous and approximate) can be
considered to maintain the ITRF2004 long-term solution on an ongoing basis.
This is needed especially to update EOP(t) values.  One or both of these
approaches must be implemented and tested, presumably mainly by the Product
Centers that will be responsible for future EOP updates.  The approximate
method is probably adequate, using technique-specific reference frame
solutions derived from the ITRF2004 combination.


At the Vienna meeting, I propose that the CPP objectives and activities be
redirected in these two ways.  A detailed plan should be developed and agreed
upon (which requires substantial advanced preparation!).  While I appreciate
his past efforts, Markus simply has not been sufficiently engaged in the CPP
and his new job at GFZ would seem to make that problem much worse.  Therefore,
as first priority, I propose that a new IERS Analysis Coordinator (or at least
CPP chair) be chosen to lead this activity.  My ideal scenario would be for
one of the core IERS Product Centers to invite Jan Kouba to visit and lead
this activity for a year or two.

With best regards,
--Jim