Dear Jim , dear colleagues Being now a somewhat external observer of the IERS adventures, I tend to think that Jim's initiative is not untimely. Since the inception of IERS#2, different combination currents have been followed by different groups, and little profound discussion on the objectives and the methods took place. I believe that the various meetings planned in Wien should be used to correct that lack of listening to each other. Jim's statement of the problems encountered in global combinations seems to me fairly constructive, and the solutions he argues for, classical in their approach and realistic in their proposed implementation. The fact that variance-covariance matrices are used all along in the combination process does not change the need for: 1) a master solution that realizes the IERS Reference System, with as many global parameters as reasonably feasible; 2) any desired densification of parameters in time, space, and technique: time series of station coordinates, EOP, etc, with new specific problems to be solved. I hope that the CPP group and the DB will catch this opportunity to help IERS make a real step forward. Missing it would be a real pity. With my best wishes. Martine -- Martine Feissel-Vernier Observatoire de Paris and Institut Geographique National Telephone: 33 (0) 1 64 15 32 83 DT/LAREG Fax: 33 (0) 1 64 15 32 53 8 Av. Blaise Pascal, Champs sur Marne 77455 Marne la Vallee Cedex 2, France -- On Mon, 14 Mar 2005, Jim Ray (NGS 301-713-2850 x112) wrote: > 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 > >