Minutes ILRS/AWG Workshop #8
April 3-4, 2003, Nice, France

Agenda

1. Opening

Welcome by Noomen. Thanks to SAO/ILRS/Pearlman for arranging location for meeting and services. Approval of agenda (Appendix 1). Brief introduction of participants; their names and e-mail addresses are listed in Appendix 2.

2. Minutes from previous AWG meeting

Not discussed explicitly. Most of the issues of the meeting in Lanham 2002 will pass the floor again in the current workshop in Nice.

Appleby briefly reported on the station/satellite specific center-of-mass corrections, previously presented at the International Workshop on Laser Ranging in October 2002 (Appendix 3). The results will be made more unambiguous for analysts, at least for the cannonball satellites initially, and then be made available on the ILRS web pages (action item Appleby/Otsubo/Torrence).

3. Actions since AWG Lanham

The action items of the previous meeting in Lanham were reviewed. About half has been fulfilled; the remainder will appear on the action item list coming out of this Nice meeting.

4. Announcements

4.1. ILRS related presentations

Two presentations that relate to the general activities and/or science results of the ILRS are mentioned here. The first one is a publication in Eos (Vol. 84, No. 6, February 11, 2003, p.51), entitled "Laser ranging workshop draws international research community", by Noomen and Klosko. It is a summary of the International Workshop on Laser Ranging that was held in Washington DC in October 2002. The other one is a presentation "ILRS contribution to current and future IERS products" given by Gurtner (co-authors Appleby, Noomen and Shelus) during the IERS Retreat in Paris, on March 31 and April 1 2003.

In addition, Exertier reported on a paper written by Barlier and Lefebvre, entitled "A new look at planet Earth: satellite geodesy and geosciences" (in: "The Century of Space Science", p. 1623-1651, Kluwer, 2001).

4.2. IERS Retreat

Noomen reported on the IERS Retreat, which was held in Paris on March 31 and April 1, 2003, to discuss about the future of the IERS. On the first day, Gurtner gave a presentation on various issues of the relation between ILRS and IERS: products, requirements, etcetera (the text can be found on the ILRS web pages). Noomen attended the discussions on the second day, where the presentations of the previous day were summarized, discussion groups were formed and general conclusions were drawn. A summary of the issues that are relevant to the ILRS is given in Appendix 4.

An important element to be mentioned here explicitly is the announcement of an IERS Combination Pilot Project, which will develop a time-series of weekly global station coordinates solutions, based on input from the various technique services. The ILRS is expected to participate with an official combination product here. Eventually, the results coming out of this time-series will replace more traditional IERS products like ITRF2000-type solutions.

Another specific issue raised in the IERS Retreat was the concern expressed by Capitaine on the situation of LLR tracking of the Moon: at the moment, only 2 stations appear to do LLR operationally (the French 7845 system and McDonald). Luceri remarked that MLRO has proven lunar tracking capability since about March 1, 2003. Shelus reported on the promising development of the new observatory at Apache Point. Manning mentioned that LLR capability could be an issue in the reconstruction of the observatory in Mt. Stromlo, which would be a valuable observation point for its unique coverage of the southern portion of the Moon's orbit around the Earth. An official letter of support on this issue would contribute to such a discussion (action item Noomen).

5. SINEX issues

Husson gave an overview of various SINEX issues (cf. Appendix 5). He is in the process of constructing a SINEX file that includes the corrections (in terms of range, epoch, or meteorological) that need to be applied to the observations. Currently, the year 1999 has been processed; Husson is working on later years and will also include the corrections to be applied to older data. These corrections apply to LAGEOS and Etalon data (action item Husson).

Mareyen (through email correspondence) brought up the issue of a possible incomplete or ambiguous documentation of the reference point in SINEX files. It holds in particular for fixed stations that have shifted their reference point definition at a certain moment (Zimmerwald and San Fernando). The problem can be resolved by including the Site Id Block and the Eccentricity Block in the SINEX solutions; for ILRS purposes, both blocks have to be specified as mandatory (action item CB). In solutions that are not "under full control" of the ILRS, like ITRF2000, this problem might persist.

Another element, related to the SINEX format, is the use of DOMES numbers. Mareyen run into the problem that it is not always possible to find the correct DOMES number (incl. reference point indicator) for "old" stations and/or occupations. This will have to be inventoried and brought to the attention of the people maintaining the DOMES ids (action item Noll).

An issue, although not directly related to SINEX, is the availability or preservation of historic SLR data. Doubts have arisen on the completeness of the data files going back to the years before the start of the MERIT campaign (September 1983), and the years prior to 1980 in particular. Noomen has experienced, as an example, that for certain months in the late 1970s LAGEOS-1 data from only 1 station is available. It is generally considered that one of the strengths of the SLR solutions lies in the long time-series that can be provided, a capability that should be preserved for the future (action item Noll).

6. Pilot project "harmonization"

Husson gave an update on the harmonization activities, intended to give a unique and unambiguous message to the SLR stations on the quality of their observations (preferably on a pass-by-pass level, but may not be achievable to better than a few cm) (Appendix 6). Currently, 6 analysis institutes perform an operational QC analysis on at least a weekly basis, and 4 of them have shifted to ITRF2000 station coordinates (site positions is a known source of discrepancies) (action item CSR, MCC). The disagreement on pass-by-pass range biases is large (a few to several cm).

If one aggregates the QC results over a month, there can still be cm level offsets, but a time series of these offsets will generally track each other to the sub-cm level. There are seasonal trends in the long term averages from the better performing sites, which is probably due to unmodeled geocenter movement and/or atmospheric pressure loading. A change to a site's coordinates (especially height) will induce a discontinuity in the site's apparent range bias. Therefore, it is imperative than analysis centers document coordinate updates (action item QC groups). In addition, an error in a site's height rate will induce a long term drift in the apparent range bias. Husson also pointed out that a "real" range bias change at one station also affects the residuals of neighboring stations, but to a lesser extent by virtue of the orbital link between stations in close proximity. In summary when interpreting QC reports, the following items need to be taken into consideration:

• Aggregate the pass-by-pass results over a month.
• The station coordinates/velocities used by the analysis centers.
• A bias change in a neighboring site will influence your "apparent" site bias.
• Geocenter movement and atmospheric pressure loading, if not modeled, will induce seasonal signals in the "apparent" bias estimates

Husson also explored two new complimentary analysis techniques. Recognizing the correlation between range bias trends for neighboring stations and the typically "slow" change in orbital perturbations or errors, Husson has tested a technique where the time-series of range biases for a reliable station is taken as a reference, and is subtracted from the results available for neighboring stations (Appendix 6). This has been done on a monthly basis, and has led to impressive results: as an example, steps in biases and data corrections are now fully observable. In particular, the CSR solutions are capable of following physical on-site engineering changes. Possible problems in the height rates of stations (e.g. in ITRF2000, or another station coordinates representation) are also reflected in the outcome of this so-called "short-arc collocation". In this respect, remarks were made on the ITRF2000 vertical rates being "suspect" for a number of site including Riyadh, San Fernando, and Riga. The results from the 1999 "pos+eop" test 28-day coordinate solutions were also used as another bias estimation technique. This technique is in principle the best approach for determining absolute biases, since station positions are estimated simultaneously with range bias. The weakness of this technique is that there may not be adequate LAGEOS data from most sites in 28-days, in order to separate a range bias from station height change. When 28-day site bias estimates were averaged for one year, there was excellent consistency (to a few mm) between the range bias estimates obtained from ASI, CRL and CSR.

Finally, Husson also reported on a site tie analysis, which may be used to identify errors in the SLR coordinates solution, the GPS solution, and/or the observed site tie. Details can be found in Appendix 6.

In conclusion of this agenda item, Noomen briefly reported on the transition of station coordinates in use for the QC analysis performed each week in Delft: a shift from SSC(DUT)93L05 (extrapolated over more than 10 years by now) to ITRF2000 resulted in a reduction of the rms-of-fit from about 30-35 mm to about 20 mm on average (with LAGEOS-2 being at the level of about 16 mm); suggestions for further improvements were also given (Appendix 7).

7. Pilot project "benchmarking and orbits"

7.1. Status reports

Husson gave an introduction on this pilot project (Appendix 8). Its overall goal is to identify and (help) eliminate blunders in the software and/or processing by individual analysis groups who want to contribute to official ILRS products: specifically, a test procedure is in the make which will give a pass/fail judgement on an analyst's treatment of a particular test dataset of observations and the quality of the results.

To this aim, four different solution types have been defined (A-D), with various degrees of freedom for the analyst. To judge the characteristics and quality of each solution that is handed in for evaluation, 5 different types of criteria are proposed: range corrections, orbit solutions, EOP solutions, station coordinates solutions, and residuals. So far, 7 analysis groups have provided solutions.

Husson gave a flavor of the current status of the contributions by comparing these, taking the JCET solution as (arbitrary) reference (cf. Appendix 8). The orbit solutions are rather diverse: the products coming out of the "A" computations (direct integration) show along-track differences that may build up to several mm, but also to several meters (similarity of software may not necessarily play a decisive role here). In the "B" solutions, where the orbit is fitted, along-track orbit differences typically may reach the level of various dm. A similar degree of inconsistency is observed for the "C" orbits (solving for other parameters as well; computation model still prescribed), with the exception of the IAAK results that tend to reach several meters. The "D" results (free computation model) show diverse results, with differences building up to decimeters (GFZ, GEOS) or meters (ASI, DGFI).

The range corrections are typically very consistent: differences are about 0.1 mm at most. The differences in the residuals reflect orbit differences to a large extent, and show similar patterns as has been reported for "orbits". As for station heights, this comparison addressed the vertical component only. The "C" results may show differences of up to 5 cm, whereas the "D" results are consistent to 1 cm (with the exception of the DGFI results, which has a different range bias treatment with corresponding effects on the station heights). The comparison of the EOP results identified a misinterpretation of the a priori values in the SINEX files generated by JCET (action item analysts).

Müller reported on the status of activities for the DGFI contributions to this benchmarking project (Appendix 9). At the moment DGFI is unable to contribute with "A", "B" or "C" solutions since the prescribed computation model is not fully implemented in the DGFI software yet. This holds for ocean tides and loading, accelerations modeling, the C2,1 and S2,1 terms of the gravity field, the model for solar radiation pressure and geocenter motion. Also, the LOD representation is still an issue. DGFI is in the process of including proper representations of these model elements, and expects to "deliver" within a few weeks.

Pavlis reported on his comparisons of contributions for the benchmark project (Appendix 10). When comparing the x/y/z components of orbit solutions by ASI and JCET, good agreements were observed for the A/B/C solutions, but differences of up to 50 cm were found for the D type. The comparison of the JCET solutions with GEOS solutions yielded a discrepancy of 200 cm for the "A" solutions, and values up to about 10 cm for the B/C/D solutions. The comparison with GFZ results showed differences of up to 100 cm for the "A" orbits, and about 10 cm for the B/C/D results.

As an alternative, Pavlis also looked at the differences in radial, cross-track and along-track directions (both for position components and for velocity components). This basically confirmed the problems identified with the x/y/z comparison (ASI "D" and GEOS "A"). A third option for comparison representations is in Keplerian elements. This brought to light a consistent 28 mm offset for the semi-major axis of solutions provided by NERC.

7.2. Future

A discussion ensued on how to proceed with this benchmarking project. Since a number of analysis groups have a problem with the implementation of the along-track, piecewise continuous acceleration model, it was decided to reformulate the "A" element and have the orbit integration done without those accelerations. This needs to be included in the description available on the ILRS web pages, as well as brought to the attention of the analysts. Other descriptions that need to be made more explicit refer to the data weighting relative to the a priori standard deviations of parameters, the epoch of station coordinates, and the UT vs. LOD issue (action item Husson, analysts).

Since the project is to result in a pass/fail grade for individual solutions, specific test criteria were discussed. First of all, the range corrections (center-of-mass, troposphere and relativity) will be inspected; the rms of the difference w.r.t. a reference standard may be no more than 0.1 mm. This reference will be generated from the average of the "D" solutions, but these already are known to be consistent at a very high level.

As for station coordinates and EOP solutions, a reference solution will be developed from the "D" solutions, first mapped onto ITRF2000 (using the ILRS AWG Core Stations only). In this process, editing and weighting will have to be applied to a certain extent, depending on the actual consistency of the solutions. The final pass/fail verdict will also be based on "D" solutions, and the provisional criterion is for the rms difference w.r.t. the average to be within 2 times the rms of the position/EOP residuals coming out of the reference determination.
Station coordinates and EOPs follow basically the same procedure, but quality assessments can be made independently.

The orbits element of the pass/fail grade will consider both "A" and "D" results. The reference for the "A" comparison will be a direct average (leaving room for editing at this moment), whereas the standard for the "D" solutions will be developed from the solutions propagated into the ITRF2000 frame (the orbit solutions are typically given in an earth-fixed reference frame); again, the editing issue is left open for now. The pass/fail result of the final contributions is again a provisional factor 2 times the residuals of the computation of the average. Importantly, the radial, cross-track and along-track elements of the orbit (differences) will be judged individually.
The residuals will not be used in the pass/fail assessment, since it is recognized that they do not provide new, independent information.

The analysts will still have to provide "B" and "C" solutions, but they may be used to help identify possible problems with rejected "A" and/or "D" solutions by comparing them with similar results computed by other analysis groups. No reference solutions will be made for these "B" and "C" solutions. Analysts are requested to generate new solutions before June 1 (action item analysts), and subsequently reference solutions will be generated before June 15 (action item Husson/Torrence) and pass/fail assessments on individual solutions will be given on June 30 at latest (action item Husson).

8. Pilot project "positioning and earth orientation"

Noomen gave an introduction on this pilot project. He reminded the participants on the client, the specific goals and the developments that have taken place during the past 3 years. During the AWG meeting in Lanham, a time schedule was agreed upon, according to which the AWG would be prepared to present an official product on EOPs to the international community by the time of the current meeting in Nice. However, this has not materialized, although the reactions to the Call for Participation have been very good: 7 proposals for analysis contributions and 4 proposals for combination centers were received. The situation will be reviewed after hearing the status reports of the various groups.

8.1. Individual contributions

ASI

Luceri reported on ASI's developments (Appendix 11). In particular, ASI is working on the automation, and expects to have an operational system running by mid-May at latest. In addition, ASI has provided weekly input to the IERS SINEX Campaign, basically adhering to the AWG guidelines. EOPrates were also delivered, but accompanied with heavy constraints. ASI has become an official IERS Combination Research Center.

Geosciences Australia

On behalf on Govind, Manning reported that Geosciences Australia has worked on data covering the period October 1999 until December 2000, thereby solving various SINEX problems. Manning has no information on a possible target date when Geosciences Australia will have an automated analysis system fully operational.

DGFI

Müller reported on the DGFI activities, which concentrate on the modifications that are necessary for LOD estimation and for satisfying the benchmark tests. DGFI will continue the contributions when the software problems have been solved.

GFZ

This institute is relatively new to these ILRS activities, so Koenig gave a somewhat broader overview of GFZ developments (Appendix 12). The core of their activities consists currently of developments to pass the benchmark tests. This necessitated various modifications to the EPOS software (LOD estimation, units conversions, etcetera). As for the benchmark project, a "C" and a "D" solution has been generated. The "C" solution shows differences w.r.t. IERS C04 of up to 2 marcsec in x/y-pole (which is comparable to the solutions generated by JCET), and a clear slope in UT solutions (the situation is a bit different for JCET, probably because of constraint handling). Station coordinates differences w.r.t the "C" JCET solution are about 5 cm rms (without elimination of systematic effects), with very large error bars.

As for the "D" solutions, EOP results were obtained that are very close to the a priori values. Station coordinates solution differences w.r.t. the "D" solution derived by JCET were still at the 5 cm level, but with much more realistic error bars.

Koenig also brought up a possible problem with the description of the a priori standard deviations of estimated parameters and observation weights, in particular their relative values (action item Husson).

IAAK

Shuygina reported on developments in the ERA software, in particular on the various elements in the computation model (Appendix 13).

JCET

Pavlis reported that his automatic procedure for generating solutions is running, but further tests have withheld him from delivering so far. He took the opportunity to discuss his contributions to the IERS Combination Research Center activities (Appendix 14). These cover a period of 4 years, processing is done in weekly batches, station coordinates and EOPs are estimated only (in addition to satellite-specific parameters). The epoch of the station coordinates appeared to be an issue, which could not be fully corrected for by doing covariance propagations. Mapping the weekly solutions into ITRF2000 yielded transformations of 10-12 mm rms for each component, and a trend which is caused by differences between the two (series of) solutions (errors in ITRF2000?). The results are available in SINEX format on the JCET web pages. The reference to the mid-point of each data interval, rather than an arbitrary fixed epoch, is one of the most urgent development issues.

NERC

Appleby is also working on developments for the automated generation of EOP/network solutions. Another activity is the development of the benchmark solutions, where an intriguing problem appears to be a very significant 28 mm bias in the semi-major axis of the "C" solutions.

8.2. Comparisons and combinations

ASI

Devoti gave an overview of the proposed service (Appendix 15). The fundamental elements will be (i) preprocessing, (ii) combination and (iii) QC and delivery. At the moment, about 70% of the software is ready. Testing is being performed with the 1999 "A" series of solutions. These monthly solutions show an rms difference w.r.t. ITRF2000 of 5-9 mm. The combined EOPs fit better to the IERS C04 series than the individual ones, although the CSR solution performs slightly better. Weighting factors are applied to each of the 5 input solutions such that the χ-square for each is more-or-less identical (resulting in scaling factors applied to the covariance matrices of between 1 and 100; for linear uncertainties the square root is to be taken).

DGFI

Kelm (Appendix 16) discussed the status of the DGFI combination procedures: the level of automation (fully), its status (100% ready), test results (based on the 1999 "5A" solutions, plus IERS SINEX campaign results). The following steps are taken: (i) deconstrain the normal equations and compute eigenvalues, (ii) reduce bias parameters and obtain a reduced normal equation, and (iii) apply the minimal constraint and solve for the parameters. Weight factors range from 0.12 to 1.5 (on a linear scale).

JCET

Pavlis is in the process of automating the procedures, and expects to be "up and running" in the course of May (Appendix 17).

NCL

Nurutdinov sketched the procedures in his TANYA software, which has been updated to also include EOPs: (i) validation, (ii) unconstraining, (iii) estimation of the GNET combined solution, (iv) Helmert transformation, (v) outlier removal, (vi) variance component estimation, (vii) product generation, and (viii) reporting. Tests have been performed with the "BB4" solutions covering the period January-June 2002 (Appendix 18).

8.3. Future

The future of the "positioning and earth orientation" project was discussed next, including requirements of the client (IERS) and practical restrictions for the ILRS analysts. After a lengthy debate, it was decided to modify the current Call for Participation into one with a weekly processing schedule, which better matches the frequency of the IERS Bulletin A, its first customer (action item Noomen/Appleby/Shelus). The exact deadline for such analyses needs to be assessed, and depends on the date of release of the Bulletins A (action item Noomen). Although different ideas were brought to light, it was agreed to leave the current elements of the proposal intact (28 days, 1-day EOPs i.e. x/y-pole and LOD, station coordinates at mid-point, provide full covariance information) since this gives the widest range of opportunities to optimize the final solution. New deadlines for contributions are: handing in of analysis solutions on May 31 at latest, test period running until October 1, evaluation of results in first 3 weeks of October and selection of analysis contributors (i.e. the outcome of the benchmark project) and prime/backup combination center during the next workshop in Wettzell (last week of October).

Rothacher stressed the importance and uniqueness of SLR when it comes to determining geocenter and scale. He repeated the invitation that was already expressed during the IERS Retreat (cf. agenda item 4.2), for an official ILRS contribution to the new IERS Combination Pilot Project. Here, ILRS is asked to contribute with weekly solutions covering station coordinates and daily EOPs (the latter including epoch values plus time derivatives), each individual solution being based on 7 days of tracking data. Although the exact details need to be worked out, and it was strongly emphasized that for the SLR technique a 28-day period would be preferable for high-quality station coordinates and geocenter determination, the solution requirements posed by this new IERS project were honored by the AWG. Noomen volunteered to participate in the new IERS Combination Working Group, and will initiate an official response to the formal invitation that ILRS can expect to come from IERS (action item Noomen/Appleby/Shelus).

9. Miscellaneous

9.1. Atmospheric refraction

Riepl gave an update of the status of studies on tropospheric refraction, in particular the developments after the meeting in October 2002 (Appendix 19). There, a proposal was made for specific test case to test new dispersion models (zenith delay formulations and mapping functions). So far, two analysis groups (ASI and JCET) have participated in these investigations.

Luceri reported on the developments at ASI (Appendix 20). In particular, 3 different options were tested: (i) the standard Marini-Murray formulation, (ii) the combination of the Saastemoinen zenith delay and the Mendes mapping function, and (iii) the Saastemoinen model extended with the Ciddor dispersion formula, again completed with the Mendes mapping function. ASI has investigated the effects on LAGEOS-2 observations, taken in the period 1999-2001 and analysed in monthly data intervals. Coordinates were kept fixed at ITRF2000, which might affect the outcome of the residuals of course (i.e. favoring the model that was used for the input of ITRF2000, notably Marini-Murray).

One direct observation was that the far majority of the low-elevation SLR data was taken by 3 stations only: 7835, 7839 and 7845 (action item CB: encourage the global network to improve, wherever possible, this situation). Other direct observations were made on the effect of the various modeling options: for "green" SLR systems (wavelength 532 nm), nominal differences between any of the various options of up to 1 mm were observed, whereas at a wavelength of 423 nm (Zimmerwald) the nominal differences might increase to up to 6 mm at low elevations. Next, orbital computations were done and the residuals inspected to see which model is most favorable. Here, a mixed conclusion had to be drawn: 1) the model Mendes+Ciddor generally gives residuals less biased than the Mendes model, 2) at 532 nm the Marini-Murray gives better results (up to 2 mm) for observations below 30 degrees, 3) at 423 nm the Mendes+Ciddor is better (up to 2 mm) at all elevations. Clearly, the influence of the model for station coordinates (and its background) is playing an important role here. Finally, the situation for the MLRO system was investigated as a special case: this system obtains high-frequency range observations at green and UV wavelengths, allowing a completely independent assessment of the tropospheric delay. Based on actually observed delay differences and their model equivalents, it was concluded that the Saastemoinen/Ciddor/Mendes combination best approximated reality at the full range of elevations.

Pavlis reported on his investigations (Appendix 21). He processed full-rate SLR data on LAGEOS-1 and LAGEOS-2 in weekly data arcs, covering the years 1999-2002. As modeling options, Pavlis used (i) the standard Marini-Murray formulation or (ii) a modified Saastemoinen model for zenith delay in combination with the Mendes mapping function. The data on LAGEOS-1 appears to be slightly more affected by the choice between the two options than the LAGEOS-2 data is. The data distribution as a function of elevation may play a role here.

9.2. Station qualification

Pearlman addressed this issue (Appendix 22). For a number of years, the ILRS has tried to initiate a station qualification system with 3 options: Core, Operational or Associate Station. For several reasons, this has not come true. Pearlman proposes a new qualification system, with two options only: Operational or Associate Station. Details of the proposal: (i) all currently active stations will become Operational Station, (ii) new stations will initially become Associate Station, (iii) depending on the satisfaction of various criteria (site log, number of passes, format and integrity, evaluation by AWG), a new station can be "upgraded" to Operational status, (iv) an Operational Station may be downgraded to Associate status if it does not satisfy Operational requirements anymore, (v) the tracking network will be evaluated on a quarterly basis, with provisions for long (up to 9 months at most) downtimes. The proposal was agreed upon by the ILRS AWG, with the exception that the AWG did not honor the necessity for minimum tracking criteria on Stella and/or Starlette.

In addition to this qualification, the AWG decided that for internal analyses purposes it will continue the previous system of 3 possibilities, the results of which will be made available on the ILRS AWG web pages. The AWG will use the so-called Shanghai criteria for this purpose, although exceptional circumstances (like geographic distribution) might be used to modify the outcome. Action item Husson: develop automatic system for assessing AWG Core Stations.

9.3. Analysis center qualification

Considering the many developments that are taking place at this moment, this agenda item is postponed until a next AWG meeting.

9.4. Dynamic tracking priorities

Pearlman initiated a discussion on ways to make better use of the geometric strength and weaknesses of the network, in particular because of the large number of satellites that require SLR tracking, the varieties in terms of required passes, and the concentration of SLR stations in Western Europe (Appendix 23). Two options are (i) to reduce or increase the priority of a particular satellite (on a station-dependent basis?), and (ii) to avoid tracking overlap between stations in close proximity. A difficult question is how to define, let alone optimize, science output. A long discussion ensued, with no specific or clear outcome. It was also recognized that for some specific applications, such as altimeter calibrations, the existence of simultaneous SLR tracking would be advantageous.

10. Next meeting

The next AWG workshop will be held on October 26 and 27 (i.e. a Sunday plus Monday) in Wettzell, Germany. During the remainder of that week, other ILRS meetings will be organized at the same location.

11. Action items

Noomen summarized the various action items (old, and new ones coming out of this meeting) (Appendix 24).

12. Closure

Noomen thanked the participants for their contributions and their open and frank participation in the discussions.

May 6, 2003
R. Noomen, G. Appleby, P.J. Shelus

Appendices:

1. Agenda
2. List of participants
3. Overview of satellite/receiver/wavelength specific center-of-mass offsets (Appleby)
4. IERS Retreat summary (Noomen)
5. SINEX format issues (Husson)
6. Harmonization (Husson)
7. DEOS quick-look analysis developments (Noomen)
8. Introduction "benchmarking" (Husson)
9. Benchmark solutions DGFI (Müller)
10. Benchmark solutions JCET (Pavlis)
11. EOP+network solution ASI (Luceri)
12. EOP+network solution GFZ (Koenig)
13. EOP+network solution IAAK (Shuygina)
14. EOP+network solution JCET (Pavlis)
15. EOP+network comparison/combination ASI (Devoti)
16. EOP+network comparison/combination DGFI (Kelm)
17. EOP+network comparison/combination JCET (Pavlis)
18. EOP+network comparison/combination NCL (Nurutdinov)
19. Refraction modeling overview (Riepl)
20. Refraction studies ASI (Luceri)
21. Refraction studies JCET (Pavlis)
22. ILRS station qualification (Pearlman)
23. Dynamic tracking priorities (Pearlman)
24. ILRS AWG action items

ILRS Analysis Working Group workshop #8
Nice, France, April 3-4, 2003

1. opening
2. minutes AWG Lanham
3. actions since AWG Lanham
   
   1. reports, presentations
4. annoucements
   
   1. ILRS related presentations, publications
   2. IERS Retreat
5. SINEX issues
6. pilot project "harmonization"
   
   1. status report
   2. future
7. pilot project "benchmarking and orbits"
   
   1. status report
   2. future
8. pilot project "positioning + earth orientation"
   
   1. individual contributions
      · ASI
      · Geosciences Australia
      · DGFI
      · GFZ
      · IAAK
      · JCET
      · NERC
   2. comparisons and combinations
      · ASI
      · DGFI
      · JCET
      · NCL
   3. future of "positioning + earth orientation"
      · time line?
9. miscellaneous
   
   1. atmospheric refraction
   2. station qualification
   3. analysis center qualification
   4. dynamic tracking priorities
10. next meeting
11. action items
12. closure

Appendix 2: Attendance

Appendix 3
Overview of satellite/receiver/wavelength specific center-off-mass offsets
G. Appleby

Appendix 4

IERS Retreat summary (Noomen)

IERS:

• Represent space geodesy and individual space geodetic techniques
  
• Combine best elements of individual techniques
  
• Continue, expand an d improve current products
  
• Possible "new" fields/products: geocenter (explicitly), planetary "geodesy", relativistic products, orbits

ILRS and IERS

ILRS -> IERS:

• Absolute station coordinates, velocities
  
• Geocenter, scale
  
• Earth Orientation Parameters
  
• Validation and calibration GPS/GLONASS spacecraft parameters (phase centers)
  
• Validation and calibration gravity field solutions
  
• Validation of parameters/models (troposphere, relativity, tides, …)

IERS -> ILRS:

• Analysis standards
  
• Station (inter-technique) site ties (incl. validation)
  
• Rapid provision of coordinates of new stations
  
• Political support

IERS Retreat: IERS Combination Pilot Project

(evolves from IERS SINEX combination campaign)

rigorous combination of networks + EOPs + quasars:

• rigorous
  
• weekly process
  
• input: weekly network solutions, daily EOPs
  
• contributions expected/solicited from:
  - IERS technique centers (combination product, or 1 individual solution)
  - combination solutions at measurement level
  
• 4-6 weeks delay
  
• timeline:
  - May 1, 2003: installation of Combination WG
  - end of June: release of CFP
  - Sep 15: deadline for proposals
  - beginning Oct: evaluation of proposals
  - Jan 1, 2004: start of pilot project
  
• eventually, combination product will replace (that of) IERS PCs
  
• eventually reprocess old SLR, VLBI, GPS, … data
  
• time-series of weekly solutions, evolve into multi-year solution

IERS Retreat: discussion group on "techniques"

Bruyninx, Noomen, Nothnagel, Weber

Analysis topics:

• ocean tidal loading: IERS to recommend conventional model (users: or better) => action item "SBL" and "Conventions"
  
• atmospheric pressure loading:
  - values at station and/or grid (interpolation), 6h resolution, max 7 days latency => action item "SBL"
  - regression coefficients
  - IERS to define reference pressure
  
• update conventions (per chapter) when necessary and timely

Products:

• geocenter: definition and requirements => action item Noomen: initiation and testbed various techniques

Network:

• GPS at all sites => action item Services
  
• GPS Core stations => action item IGS
• barometer readings and dissemination => action item IGS

IERS Retreat: miscellaneous (1)

Altamimi:

• multi-technique EOP solutions better than single-technique EOP solutions
  
• estimation of EOP rates badly influences SLR EOP results

Gambis: format change C04 / Bulletin B

Trend towards rigorous combinations

Relation between IERS and IGGOS ?

Ray: International Earth Reference and Rotation Service (IERS)

Schutz, Schwintzer: gravity missions and altimeter missions need good and consistent EOP + reference frame

Kouba: single-technique combination first, then inter-technique

IERS Retreat: miscellaneous (2)

Capitaine (discussion group on "astronomy"):

• "deficiencies": continuity of LLR observations
  
• dynamics Earth-Moon system
  
• now down to 2 stations
  
• continue present level of tracking intensity
  
• Rothacher: Wettzell => LLR
  
• IVS also?

Schwintzer (discussion group on "gravity and geophysical fluids"):

• SLR needed for temporal variation in low-degree zonals and stability of gravity field solutions
  
• collocation in space (GPS, SLR, DORIS, …)

Schwintzer: time-series of gravity field terms
<= 1, <= 2 ?
(geocenter, orienation)
(currently: GFZ only)

IERS WGs on

• Combination
  
• Datum
  
• Collocation

Appendix 5

SINEX format issues
V. Husson

Appendix 6

Harmonization
V. Husson

Appendix 7

DEOS quick-look analysis developments (Noomen)

            RESIDUAL RMS (CM)

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station coordinates modeling:

• ITRF2000 since January 1, 2002
  
• fixed, unless s of epoch value in ITRF2000 larger than 20 mm
  
• positions of new station estimated (at least initially)

future:

• multi-wavelength capability
  
• tropospheric refraction updates
  
• atmospheric pressure loading
  
• station/satellite specific signatures

Appendix 8

Introduction "benchmarking"
V. Husson

Appendix 9

Benchmark solutions DGFI
H. Müller

Appendix 10

Benchmark solutions JCET
E.C. Pavlis

Appendix 11

EOP + network solution ASI
V. Luceri

Appendix 12

EOP + network solution GFZ
R. Koenig

Appendix 13

EOP + network solution IAAK
N. Shuygina

Appendix 14

EOP + network solution JCET
E.C. Pavlis

Appendix 15

EOP + network comparison/combination ASI
R. Devoti

Appendix 16

EOP + network comparison/combination DGFI
R. Kelm

Appendix 17

EOP + network comparison/combination JCET
E.C. Pavlis

Appendix 18

EOP + network comparison/combination NCL
K. Nurutdinov

Appendix 19

Refraction modeling overview
S. Riepl

Appendix 20

Refraction studies ASI
V. Luceri

Appendix 21

Refraction studies JCET
E.C. Pavlis

Appendix 22

ILRS station qualification
M.R. Pearlman

Appendix 23

Dynamic tracking priorities
M.R. Pearlman

Appendix 24

ILRS AWG action items

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