Dear Markus, FYI, earlier this week I returned to NGS from the BIPM. (Apologies to anybody who got bounced e-mail messages from me while my e-mail forwards were tangled recently.) Thanks for your message. Some comments are embedded: > Date: Wed, 29 Sep 2004 10:13:31 +0200 > From: Markus Rothacher > To: cpp@iers.org > Subject: Status of the IERS Combination Pilot Project > > Dear Colleagues, > > first of all I would like to thank the IVS (Axel Nothnagel > and his group, GIUB) and the ILRS (Cinzia Luceri and Roberto > Devoti, ASI) for their efforts to generate intra-technique > combined weekly solutions for the IERS CPP. These solutions Please allow me to add my thanks too. I am especially impressed with the apparent comprehensiveness of the ILRS combined solutions. On the other hand, it seems to me that the IVS combined solutions are limited in their usefulness. The files I checked seem to be a simple average of the two or three VLBI solutions (all using the same software). It appears that only the R1 network involved, consisting of just half a dozen or so VLBI stations. This is really not very helpful. An even more fundamental problem is that the EOPs are included as offsets only, no rates, at the mid-point epoch. Without the EOP rates, it makes combination with other SINEX files rather difficult. (As I understand it, the ILRS solutions do not include EOP rates either, but their reference epoch is always noon, which does allow them to be combined with other noon solutions.) The IVS combinations are a promising first step, but I do not see how they can be used by the CPP at this point. > are now available at the IERS CB. Together with the IGS combined > solution from Remi Ferland (NRCan) and the DORIS solutions from > Pascal Willis (JPL) we now have "official" solutions for each of the > space geodetic techniques. The solutions are arriving in time, too, > i.e., within the two months after the observations. > > It is due time now -- especially in view of the upcoming IERS > Workshop in San Francisco -- to go one step further and to > seriously start generating and delivering the weekly > inter-technique solutions. I would like to encourage the > combination centers to start this process ... > If you find any problems concerning the weekly SINEX files > available from the individual techniques, please send an e-mail > to "cpp@iers.org". At the Nice meeting, Zuheir and I gave a two-point presentation. One of the points we stressed was that we could not see any way to make a sensible multi-technique combination at the weekly level in isolation. In order to link the independent techniques, some tie information is required. The needed ties are not measured weekly and therefore must be introduced into the multi-technique combination as global information, otherwise significant time-varying distortions will be introduced and the ties will be strongly over-weighted. In our opinion, a meaningful multi-technique combination must start with a long-term combination, preferably at least 5 years but even longer would be better. Therefore, we asked that the IERS explicitly request the longest possible span of weekly solutions from all the techniques. This was already envisioned in the original CfP: "Submissions for all earlier weeks are also sought, where feasible." However, as far as I know, this request was never fulfilled. In our Nice presentation, we also described two different approaches that could be used to update the long-term solution once it has been generated by making weekly incremental combined solutions. There was a limited amount of discussion of our presentation in Nice but I do not recall that anybody questioned the validity of our points. Since that time, there has been no further discussion that I know about. Therefore as far as I am concerned, unless somebody can explain our failure in understanding the problem, it is not possible to satisfy your request currently. Long-term solutions are available outside the CPP domain for DORIS (thanks to Pascal) and for the IGS (back to 1999, thanks to Remi), but we do not have homogeneous combined solutions for SLR or VLBI for sufficiently long periods. So, we cannot "start generating and delivering the weekly inter-technique solutions", as requested. I do not believe anybody can either. Using long-term solutions from outside the CPP database, Zuheir and I did make a study of the VLBI-GPS local ties and we distributed a report to the CPP and the survey WG. We have received no comments on that report. Since that time we have made an update of that study and will give papers on the results at the AGU meeting. Much as I like San Francisco, I will not plan to spend an extra 4 days there unless I see some definite evidence that the time would not be wasted. > I would also like to encourage the other groups that were > answering the CfP of the IERS CPP and did not yet deliver > to start with their contributions (special products, > validation, ...). We need all the available expertise to get > this challenging combination tasks done in a thorough way ... > For instance, it would also be preferable to get a > combined solution from the IDS at some point in time. This > would help to validate the quality of the DORIS solutions. > > Very soon you will get more information on the sessions > planned for the IERS Combination Pilot Project Workshop in > San Francisco this December (Dec. 9-10, 2004). Please make > sure, that you can come to San Francisco and contribute to > the meeting and the discussions. Under the circumstances, it is not clear to me that there is any justification for such a workshop. Personally, I am not interested in spending two days to see a lot of ppt presentations of the nice things that could be done ... someday. Can anybody say that they will be prepared to report anything new or useful? I do not think that Zuheir and I will have anything more than the papers will we anyway present for the AGU, which are tests using solutions from outside the CPP database. Zuheir, please correct me if I am incorrect about this. Best regards, --Jim > > With best regards, > Markus Rothacher > IERS Analysis Coordinator > > -------------------------------------------------------------------------------- > Prof. Dr. Markus Rothacher Phone : ++49-89-289-23191 > Forschungseinrichtung Satellitengeodaesie Fax : ++49-89-289-23178 > Technische Universitaet Muenchen E-mail: markus.rothacher@bv.tum.de > Arcisstrasse 21 > D-80290 Muenchen / Germany