Minutes ILRS/AWG Workshop #12
December 13, 2004, San Francisco, CA

Monday, December 13

G. Appleby opened the meeting at 9:00 AM (see agenda in Table 1). An attendance list is listed in Table 2. Many of the items for discussion were introduced by the chairman using a summary presentation (see Appendix).

The present EOP + POS project was summarized.  This project concerns the computation of weekly station positions at the mid-point of each 7-day interval and EOP’s at 1-day intervals, together with the combination of these results. Input solutions are provided by 5 analysis centers: ASI, DGFI, JCET, GFZ, NSGF.  ASI, the primary combination center, and DGFI, the back-up combination center, perform the combination.  There was some concern expressed by the way that Tom Johnson at USNO uses the ILRS products for the NEOS product (see Action Item 1).

Representatives from ASI could not be in attendance.  A summary report was issued and reviewed (see Appendix).

A status report by DGFI was given (see Appendix).

A discussion followed about the difference between providing normal equations with minimal constraints in the combination solution or providing loose constraints solutions (see Action Item 2).  The minimal constraint solutions of course offer the non-expert user a product for coordinate time series, etc., whereas the loosely constrained solutions are primarily of use in further inter-technique combinations. The DGFI report mentioned a problem seen with the Beijing SLR data while performing a particular weekly solution.  This opened up the question as to how problems such as these should be handled in a routine way (see Action Item 3).  There was also a question raised about how the outside world should have access to, and use, the primary and back-up combination ILRS solutions (see Action Item 4).

D. Gambis presented an external evaluation of the ILRS EOP products.  This opened a further discussion of what constitutes the official ILRS product and how it should be used by the outside world. (see Action Items 5 and 6).

M. Rothacher reviewed plans for the new IERS Pilot Project and the formation of a successor to the weekly solutions now being computed, and to the current ITRF2000.  Could the ILRS respond to the schedule proposed and exactly what is desired from the ILRS on the part of the IERS (see Action Items 7 and 8)? It was pointed out that the five ILRS analysis centres currently involved in the weekly pilot project were also well advanced in their ‘back’ solutions for this IERS PP (see Appendix).

The provision of further ILRS ‘products’ was discussed. It was clear that there is a ‘market’ for at least LAGEOS orbits, and since the ACs are routinely each week producing LAGEOS 1&2 and Etalon 1&2 orbits, it was thought that it would be relatively simple to place these four ephemeredes in SP3 format files each week, as per the IGS orbits. No decision was made about how to produce an ‘official’ ILRS product, however, since that would imply the need for a combined product. It was agreed to keep this topic under review.

E. Pavlis reviewed the current benchmarking project and the approval of new Analysis Centers, like BKG.  His results suggest that BKG should be provisionally approved, having passed the orbit test and the residual test.  BKG still must pass the “blind” test, but Pavlis expects this to be completed soon. See Action Item 9.

A discussion followed about the present global coverage of the ILRS.  Especially troubling is the “Pacific hole”, caused by the Arequipa and Maui stations having had their operation curtailed, and the weather problems being experienced at TIGO in South America.   Concern was also expressed about the operations of the Tahiti station.  Of course, the fall-back on data acquisition caused by the NASA cut-backs is also a serious problem. The AWG strongly urges early resolution of these serious situations.

The question of the definition of what constitutes an ILRS Analysis Center and Associated Analysis Center arose again, now that the ILRS is virtually fully operational for EOP’s and station position products for the IERS (see Action Item 12).

We discussed the procedures to be used for the approval of new ILRS stations coming on-line (see Action Item 10).

M. Rothacher brought up the question of applying atmospheric loading to ILRS solutions (see Action Item 11).

B. Richter questioned the status of the “Arc-Length Project” that had been proposed by Maria at an earlier AWG meeting.  It was agreed that Maria, herself, is responsible for the project and should be asked to chair a small study group to look into the situation.

Next Meeting. We agreed that the next AWG meeting should be held during the EGU in Vienna, April 2005, a half-day meeting probably being sufficient.

The meeting was closed at noon.

Action Items following IERS CPP and ILRS AWG meetings during Fall AGU, 2004.

1. The ILRS should clarify with NEOS (i.e., Tom Johnson) what the official ILRS EOP products are and how they should be used.  It became clear both at the Combinations meeting in Napa on Saturday and at the AWG meeting in San Francisco on Monday, that IERS did not know about ILRS’ EOP product, aligned to ITRF2000.  In particular, Johnson may not have a full understanding of exactly what the assumptions are for the ILRS EOP generated products.
2. There should be a discussion within the ILRS/AWG, and a consensus made, on whether the official ILRS ‘pos+eop’ product should be a “loose constraints” solution (as is produced now by ASI) or a normal equations with minimal constraints solution (as advocated by DGFI), or even whether both products should routinely be generated.
3. Procedures should be worked out at the ILRS Combinations center to deal with any “problems” encountered with the weekly solutions.  This concerns interactions between ASI and DGFI, what feedback there should be to the individual Analysis Centers, and when feedback should be made to individual stations.
4. Ensure that the IERS has a clear understanding of the status of the “official” ILRS ‘pos+eop’ submittal and what should be done when the “official” solution is not in place (i.e., going to the official backup solution).  Right now, it appears as if both solutions (i.e., ASI and DGFI) are “captured” by the IERS.  The ILRS should be assured as to which of its products are being used by the IERS and why.
5. Daniel Gambis’ presentation caused a great deal of misunderstanding (check Action Item # 4, above).  It appears as if IERS is not using only the official ILRS product.  Gambis is also using other input from several of the individual ILRS Analysis Center solutions, in generating the IERS product.  Thus, the ILRS must check NEOS and IERS requirements.  Are they the same?
6. Are they different (see Action Item 6, above)?
7. Are ASI and DGFI ready to participate in the IERS ‘back’ pilot project (as outlined by M. Rothacher)? IERS are now certainly aware of the status of the ‘back’ processing work underway by the five ILRS ACs.
8. Confirm Rothacher’s requirements for the ILRS for the IERS Pilot Project and when ILRS input is required.  It appears as if Rothacher’s “need” is for weekly solutions, going back for at least 10 years.  Altimimi’s requirements/recommendations seem to be different.  It is likely Rothacher will publish very soon a call for participation, with a tight deadline (February 2005?) for submissions. It is probable that solutions other than the official ILRS combination product will be forthcoming from the analysis community, but essential that the official solution is made available in time.
9. Clarify path for routine application of “benchmarking” for potential new ACs. Welcome work by JCET on BKG’s submission and encourage completion of the process. 
10. Re-define the ILRS AWG’s role in and the procedures necessary for quality assessment of data from new stations.
11. Determine and agree a date by which Analysis Centers must apply atmospheric loading in their solutions. It was agreed that this requirement need not be in place for the current ‘back’ solutions, but should be enforced thereafter.
12. Re-define or confirm the definitions of ILRS Analysis Centers and Associate Analysis Centers.  Are the present definitions within the ILRS Terms of Reference sufficient? Further, consider recommending revision of definition of ‘AC’ (see http://ilrs.gsfc.nasa.gov/science_analysis/analysis_centers.html )

Peter Shelus
15th December 2004.

Table 1: Agenda

Table 2: Attendance

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Last modified date: Wednesday, November 01, 2006
Author: Ron Noomen
Author: Peter Shelus
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