Minutes from CSTG SLR/LLR Subcommission General Assembly

Shanghai, China
November 12, 1996

Michael Pearlman/CFA

The Plenary Session of the SLR Subcommission of the CSTG was held in Shanghai on November 12, 1996. The agenda for the meeting is included in Attachment 1. The list of attendees is included in Attachment 2. Contact Michael Pearlman if you require hardcopies of these attachments.

Reports on Relevant Recent Paris Meetings (Peter Shelus)

IAU Working Group on Reference Frame Workshop (October 7 and 8, 1996)

The ICRF (International Celestial Reference Frame) is now defined by the VLBI Reference Frame, consisting of a subset of some 600 radio sources. A description of the catalog that defines the frame, together with the methods that were used to derive it, will appear as an IERS Technical Note as well as a journal article. Further, the Hipparcos catalog has been linked to the VLBI catalog to form the optical counterpart of the ICRF. Effort is underway to link the ICRF to other frames of reference, i.e., the Tycho catalog, the Solar System, the FK5, and others. Final discussions during the workshop centered on the maintenance of the VLBI Catalog (mainly source structure monitoring), extension and densification of all relevant catalogs, and what role the IERS might play in the maintenance of the ICRF.

ITRF Workshop (October 9 and 10, 1996)

Deliberations within the ITRF (International Terrestrial Reference Frame) Workshop included the possibility and advisability of providing, via the IERS, geocenter and scale series that were comparable to the EOP (Earth Orientation Parameter) series now being computed by the IERS, as well as what and when to provide an improvement to ITF94. Also discussed was the possibility of comparison campaigns to investigate the sensitivity of the various techniques, (GPS, DORIS, and SLR) to the geocenter and stressing the importance of global and multitechnique solutions for all derived ITRF parameters. There was also some discussion about the formation of a consultative ITRF subgroup, similar to that so successfully implemented for the ICRF.

EOP Technical Workshop (October 11, 1996)

Changes were announced in the IERS EOP series beginning in 1997. The effects of several nutation terms, at the 1 mas level, must be added. Analysis centers should see changes in bias and rate. Details are in the IERS Annual Report. Discussions were also held concerning the handling of daily and subdaily variations in EOP series. It was decided to continue the publication of series without the effects of daily and subdaily tides. Rigorous procedures will be established for users to obtain proper subdaily values. Concerning regular IERS EOP publications, it is desired to reduce paper mailings, with more use of electronic distribution.

1996 IERS Workshop (October 14 and 16, 1996)

The thrust of the workshop was a critical examination of the present and future role of the IERS, its customer base, and its relationship with other entities. Six working groups (Assessment of Current Missions, Astronomical Reference Frames, Vertical References, Topography and Crustal Deformations, Unification of Regional Reference Systems, Earth Rotation Dynamics and Geophysical Fluids) submitted specific recommendations for reorganization to the IERS Directing Board. Position papers were presented, discussions were held, and final recommendations were made. The IERS Directing Board is now to review the workshop results and formulate a revised course for the IERS.

CSTG Meeting (October 17 and 18, 1996)

Coordination and consolidation of space geodetic analyses, including the consolidation of formats and the combination of solutions were discussed. Strong recommendations were made for using a SINEXtype format for ease in the interchange of analytical results and the combination of solutions. The SLR community must maintain a strong presence in this decision making process. There was a brief subcommission meeting of the precise satellite microwave systems (GPS, GLONASS, DORIS, PRARE) and a discussion of an IUGG Fundamental Reference and Calibration Net (essentially an update of the concepts embodied in FLINN). The new CSTG Working Group on Coordination of Techniques, chaired by Tom Herring, had its first meeting. John Degnan and Richard Eanes, members of the SLR Subcommission, are on the Working Group.

Technique representation on the CSTG Directing Board is by institution not individual. At the moment, the SLR Representative on the CSTG Directing Board is the University of Texas, in the person of Bob Schutz. The LLR Representative to the CSTG Directing Board is CERGA, which has delegated Peter Shelus/CSR to replace Christian Veillet, who took a position elsewhere. A replacement has to be selected for Bob Schutz who wants to withdraw.

International GPS Campaign (Alan Murdoch)

The SLR Subcommission initiated a tracking campaign on GPS this fall in response to the IERS and IGS requests. Predictions and time offsets are being issued regularly, and data centers are on line to analyze the data. Between October 19 and November 11, ninety-two passes (681 normal points) were taken on GPS-35 and forty-nine passes (335 normal points) were taken on GPS-36. A number of simultaneous events between stations have been recorded which will provide opportunity to study system performance. Joint solutions with GPS data have also been encouraged.

A second tether mission (ATEX) is being planned for launch by the U.S. Navy. The plan includes GPS receivers in addition to cornercubes at the end masses.

EUROLAS Cluster Concept (Andy Sinclair)

EUROLAS had a small meeting in April at RGO to discuss their assessment of the cluster concept presented by Gerhard Beutler. They agreed to pursue and support the concept based on

1. Improving data quality,
2. Sharing technical expertise,
3. A means of diagnosing station biases, and
4. Overcoming weather and scheduling conflicts.

A note by Andrew Sinclair describing the concept of the Cluster of European SLR Stations is included in the report of the Porto CSTG Subcommission meeting. At the Shanghai meeting Sinclair reported progress on some of the action items:

1. Graz held a small workshop in June, at which a thorough examination was made of various problems that can bias SLR data, and various courses of action were decided. These were reported on in the main session of the Shanghai Workshop.
2. Zimmerwald have set up a realtime system for exchange of status information within the Cluster, and so far this has been implemented by Wettzell, Herstmonceux and Zimmerwald.
3. Graham Appleby (RGO) has set up a system for quality assessment of SLR data within the cluster, using shortarc analysis of simultaneously tracked passes, and at this stage the results are being reported monthly via WWW.

Analysis Issues (Andy Sinclair)

At the Porto meeting, the geophysicists complained that they could not understand the station coordinates from SLR. Graham Appleby has compared SLR stations coordinate solutions from ITRF, CSR, and GSFC to try to understand the problem (see Attachment 4). An initial discrepancy of several meters was resolved when it was realized that the GSFC solution was referred to the intersection of axes rather than the geodetic marker for the station. Stations can submit data with either reference, but the reference and required eccentricity must be specified. The NASA stations customarily refer measurements to the geodetic marker (as do GPS stations), while most of the European stations use the intersection of axes as the reference. Mobile stations, which are often in operation prior to a final determination of eccentricity, tend to use the intersection of axes. (Short period mobile stations operations have become less of an issue with SLR ). The IERS has a numbering system that makes the reference unambiguous; each marker is assigned a unique "DOMES" number which should be furnished with any solution. DOMES numbers are assigned by the ITRF sub-bureau of the IERS (boucher@schubert.ign.fr or altamimi@schubert.ign.fr). Recommendation for actions:

• The reference point for fixed SLR stations should be standardized to the geodetic markers.
• The eccentricity file should be standardized.
• SLR solution submitted for general should use DOMES numbers as globally unique identifiers for reference points.

Once the correct references and eccentricities were used, and restricting comparisons to only the best quality stations, solutions agreed to 3 -5 cm. Small reference frame adjustments brought this down to 1-2 cm. Adding all of the stations to the solutions degraded the comparison significantly. A number of these stations have significant biases.

Coordination of Analysis Centers (Richard Eanes)

In Porto in June, it was agreed that we need to strengthen the coordination of the SLR Analysis Centers and to standardize the output products. Richard Eanes was asked to chair an Analysis Working Group under the SLR Subcommission to assess the need and the feasibility, and if appropriate, to develop an action plan. Eanes' is in the process of contacting: Peter Dunn, Ron Noomen, Ramesh Govind, and representatives from DGFI and other analysis groups for participation. He plans to have the group in action shortly and to have a proposal to present to the IERS meeting in March 1998.

As both the SLR representative to the IERS and the head of this new working group, Eanes is trying a pilot program approach using one combined series to describe the full set of geodetic parameters (station position, Earth center-of-mass, etc.). He plans to have this ready for IERS submission in March. This timeframe fits in well with Tom Herring's CSTG Working Group on Technique Intercomparison and Jim Ray's activity on the ITRF. The first step in developing a solution integrating all techniques, is to develop a SLR only solution.

CSTG Working Group on Technique Intercomparison (John Degnan)

The first meeting of the CSTG Working Group on Technique Intercomparison was held in Paris in conjunction with the IERS meetings. The Working Group was formed to examine how the different measurement techniques should be compared and integrated. Collocation of different instrument types (SLR, GPS, Doris, etc.) would provide opportunity to resolve present differences of coordinate systems.

The Working Group stressed the need to adopt a version of the SINEX format adopted by IERS so that data centers of all techniques can exchange and combine data for integrated analyses.

Richard Eanes Analysis Working Group should review the SINEX format to see if it is appropriate for SLR or if changes are required.

Full Rate Data (Vincenza Luceri)

Vincenza Luceri asked the Subcommission to reconsider the decision to cease the routine shipment and archiving of SLR full-rate data. The request was turned down, but the need to enhance the normal point data format with statistical information on data distribution was stressed.

Revised Normal Point Description (Andy Sinclair)

Andy Sinclair reviewed a study conducted at RGO on the effects of data distribution and processing on range bias (see Attachment 4). There are many modes of station operations (single vs. multiple photon operation, various detectors, various data screening levels. etc.), but the results are:

• On Lageos the range bias errors are in the few mm region.
• For the lower satellites, some system biases may be as large as a few cm.

He feels that a comprehensive resolution of this bias issue will require a more thorough understanding of the engineering characteristics of the systems. Andy is suggesting that there are more urgent factors affecting data quality at present, and that as the Keystone and SLR 2000 single photon systems approach operation, we should form a working group including EUROLAS, NASA, WPLTN, Lunar, etc. to work the problem.

Data Formats

At the Berne Meeting we agreed to some minor changes in the normal point format:

• To use the term "RMS" throughout, instead of a mixture of "RMS" and "standard deviation",
• A scheme to allow for wavelengths greater than 1 micron,
• To include a flag to indicate replacement data.

These changes have been reviewed and finalized. Andy Sinclair and Van Husson are preparing a brief note for the next Subcommision Newsletter. They will also rewrite the SLR Normal Point Description (see CSTG SLR Subcommission Newsletter dated April 1990).

Van Husson and Andy Sinclair from the Subcommission Data Working Group made a proposal to improve the traceability of SLR system configuration (see attachment 6). Some systems configurations are now being changed for different satellites and keeping track of these configurations at the data analyst level has been difficult if not impossible. The proposal would redefine the three flags currently used in the header record (bytes 45, 46, and 47) in order to be able to flag changes to the system and to denote use of alternative operational configurations, and would add an online station configuration file to be kept current by the individual stations. The file would include all of the information necessary for the analysts. There was general agreement that this capability was needed.

The Subcommission Data Working Group will work through the details on the Proposal for Improving Traceability of SLR System Configurations including the station configuration file design and update scheme. They would try to meet an implementation date in early 1997.

Predictions

Time Bias Corrections

Predictions are routinely updated with time bias corrections to improve acquisition and pointing. Several groups generate and distribute time bias corrections, but they are not standardized in format or distribution. The SLR Subcommission formed a working group to develop standardized time bias functions and distribution mechanism.

The Working Group on Time Bias Corrections will formulate its recommendations and report back at the Steering Committee Meeting on Friday afternoon.

IRV's

The Prediction Working Group chaired by Andy Sinclair has issued a recommendation for stations to do some consolidating in the IRV header line to allow space for additional information such as notice of multiple IRV sets per day and a means of relating IRV's to time bias functions (see Attachment 7). The Subcommission agreed with the recommendation.

Proposal for Retracking Old Satellites with SLR

Biancale/CNES and Schwintzer/GFZ have proposed that we reinitiate tracking on some early SLR satellites (BE-C, D1-C, and D1-D) for refinement of the gravity field (see Attachment 8).

The satellite inclinations fall in the 40 degree range where the gravity field has some deficiencies.

The distributed arrays on these older targets might only support decimeter accuracies and the magnetic stabilization will limit southern hemisphere (orbital) coverage. Concern was expressed that these satellites could not improve the field beyond our present knowledge. We also need to understand their real data needs.

The Analysis Working Group should evaluate this proposal and come back with a recommendation.

Subcommission Organization and Structure

Steering Committee Membership

The Subcommission agreed that both the analysts and the data centers should have representation on the Steering Committee. There was also agreement among the WPLTN members to consolidate their positions on the Committee. It was agreed that the Steering Committee membership would be comprised as follows:

It was agreed that:

• Richard Eanes and Ron Noomen would continue their membership, but as the Analysis representatives.
• EDC would be asked to furnish the Data Center representative, probably Wolfgang Seemueller.
• Peter Shelus would continue as the Lunar representative.
• The three networks would select their own representatives
• The Steering Committee will select its Chairperson and Recorder at a meeting to be held in concert with the Laser Workshops; the Chairperson and the Recorder will also be Chairperson and Recorder of the full Subcommission. The next selection of officers will take place at the next Workshop.

Subcommission Role

A small group headed by John Luck was asked to review and update the draft Subcommission Charter included in the notes from the Porto Meeting. The revised Charter will be used as part of a Subcommission reorganization discussion to take place at the Steering Committee meeting on Friday afternoon.

The revised draft charter will be prepared for the Subcommission Steering Committee meeting on Friday afternoon.

CSTG Endorsement of the Laser Workshop

The Subcommission members endorsed a recommendation that the Laser Workshop should have recognition from the CSTG.

A small group headed by John Luck agreed to draft a resolution of invitation to the CSTG to be presented to the Workshop participants at the final Business Session.

John Degnan will send a letter to Gerhard Beutler with the Subcommission's recommendation.

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