Minutes from CSTG SLR/LLR Subcommission Meeting

Plenary Session

Maratea, Italy
June 13, 1997

Michael Pearlman/CFA

The Plenary Meeting of the SLR/LLR Subcommission of the CSTG was held in Maratea, Italy on July 13, 1997. The agenda for the meeting and the attendance list are included at Attachments 1 and 2.

The SLR Subcommission maintains an up-to-date Web address at http://cddisa.gsfc.nasa.gov/cstg/slr_cstg.

Summary of the NASA SLR Review Committee Report

A summary of the SLR Review Committee Report prepared at the request of NASA's Office of Mission to Planet Earth was given by Mike Pearlman at the WEGENER General Assembly meeting on June 11. Copies of the report were made available to the attendees and an article has been prepared for the 13th Bulletin of the CSTG to be issued in July. Copies of the full report are available from Mike Pearlman.

Lunar Laser Ranging

Summary of the JPL LLR Meeting in March

(words provided by Peter Shelus but not presented)

A meeting of LLR interested parties was held at JPL on March 19-20, 1997. Observations continue at Texas, Grasse, and Wettzell. Texas MLRS data volume is now better than that obtained with the original 2.7-m system, but at accuracy and precision levels an order of magnitude better. The Grasse OCA station is now operating at the mm level with continued excellent volume. Observations at Wettzell and Orroral, although possible, have their problems, both physical and political. The death of LLR at Maui was again the source of great wringing of hands and gnashing of teeth. MLRO is in the wings, probably to start within the year.

Analysis is being performed at many centers including JPL, CfA, Montana and Texas in the US, France, Germany, and Italy. Principal effort continue on gravitational and relativistic parameters, the lunar interior based upon lunar librations, Earth orientation (rotation, precession, and nutation), and solar system dynamics. Dave Smith presented a summary of efforts being started to range not only the Moon but nearby planets of the Solar System using micro-lasers and optical transponder technology. Development work toward the SLR2000 will play a major role in that effort.

There has been a push to put LLR data into a format identical to SLR data making it easier to co-mingle data types. An effort is also underway to produce LLR predicts in IRV-type format so that more stations could make use of them. Randy Ricklefs, Van Husson, and Andy Sinclair are the major movers and shakers. Encouragement to (appropriate) SLR stations to try the moon under excellent instrumental and environmental conditions is underway.

The meeting was quite positive with respect to the symbiosis of SLR and LLR. Many of the participants are now quite comfortable with the concept of the moon being another target in the general satellite ranging community. Copies of the viewgraphs with participants, agenda, and a summary of the more important aspects of that meeting are included in Attachment 3.

LLR Applications and Requirements for Relativity

Ken Nordtvedt also reported on the JPL meeting, but he focussed more on the relativity applications and data requirements. He discussed the need for: more precise tests of Einstein's strong principle of equivalence (does everything fall at the same rate?), tests of the relativistic precession of inertial reference frames caused by the curvature of space induced by the mass of the sun, and tests for discontinuous time spans. Nordtvedt stressed the need for accurate lunar laser data (<2 cm), a network of stations, more robust data sets, and better scheduling of observations. He also discussed the importance of interplanetary ranging with transponders.

He mentioned that the integration of the SLR and the LLR within the Subcommission and the close relationship with the IERS were considered important issues at the JPL meeting.

International Laser Ranging Service

John Degnan presented the latest version of the International Laser Ranging Network (ILTN) Terms of Reference (see Attachments 4 and 5). Under the ILTN, the international SLR/LLR infrastructure will be organized into an integrated service to provide more effective support to the scientific user community through satellite and lunar laser ranging data and related data products for geodetic and geophysical research. The ILTN is modelled after the IGS and takes advantage of present infrastructure where appropriate. It will include the current stations, Operations Centers, Data Centers (regional and global) and Analysis Centers, and will provide the coordination with the IERS and other international bodies. It will be organized into (1) standing working groups to conduct the daily business of the ILTN, (2) ad-hoc working groups to address special tasks of finite duration, and (3) a Central Bureau to oversee the integrity of the ILTN, to facilitate the interfaces, and to assure that the final data products meet user requirements. A Governing Board of 18 members selected to represent all aspects of the activity will provide the overall direction. The present standing working groups include: (1) Missions, (2) Data Standards and Processing, (3) Networks and Engineering, and (4) Analysis.

Werner Gurtner discussed the role of the Missions Working Group which will review requests (including Support Plans) for laser tracking by new prospective missions and make recommendations to the ILTN for acceptance and level of support, or rejection. The Working Group will generate procedures for special requirements and will provide updated recommendations for network tracking priorities (see Attachment 6).

Tom Varghese described the Networks and Engineering Working Group (see Attachment 7). The role of the Working Group will be to (1) expedite resolution of data anomaly problems; (2) maintain an up-to-date system configuration data base for the network sites, (3) create a "knowledge base" for technology, algorithm processes, problems and resolutions; (4) perform engineering analysis in support of current operations and new missions; and (5) facilitate engineering improvements. The Working Group will strive to optimize data product output and to foster stronger interaction and greater feedback with the field stations.

Andrew Sinclair described the Data Formats and Procedures Working Group that will be responsible for all data procedures up to and including the generation of full-rate and normal point data. (see Attachment 8) This will include predictions, data preprocessing procedures, data formats, formats for documenting station pertinent information, and procedures for on-site quality control (jointly with the Networks and Engineering Working Group).

In Richard Eanes absence, John Degnan presented the concept for the Analysis Working Group which will be responsible for overseeing the work of the Analysis Centers to assure that properly evaluated data products, in standardized format, reach the users in the timeframe specified by the ILTN Governing Board. The Analysis Working Group will work to standardize the SLR data products and to establish a uniform means of comparing and combining results from different centers. Tom Herring's CSTG Working Group is examining the SINEX format to see if it can be adapted to fully satisfy the SLR/LLR needs.

Gerhard Beutler requested that a draft of the ILTN document be ready for inclusion in the CSTG Bulletin #13 to be issued in July. He also suggested that we might want to consider a transition period, or an ILTN pilot service, beginning in 1998, while we work through and develop some of the procedures.

Pippo Bianco suggested that we should expand our view and include a wider range of applications including planetary ranging.

John Bosworth asked if the users can develop their own Support Plans or if it was more reasonable for the Missions Working Group to work with them. The need for more discipline and pre-planning by the missions was discussed and it was pointed out that the requirements for the Support Plan were less than those usually provided by ATSC. John Degnan gave some additional clarification on the requirements for the ILTN Support Plan (see Attachment 9). It was also suggested that the Missions Working Group be responsible for closure with users, to make sure their accepted requirements are being fulfilled.

None of the Analysis Centers are currently analyzing all of the SLR data, and some concern was expressed about our ability to provide quality control. It was noted, however, that all of the satellites have users and someone is doing their analysis.

Both the Networks and Engineering Working Group (NEWG) and the Data Formats and Procedures Working Group (DFPWG) included system configuration in their charters. The difference, it was pointed out, is the NEWG will be responsible for collecting and maintaining the data base while the DFPWG will be responsible for the data base formats.

At the moment, SLR and LLR will have separate Analysis Coordinators. Eventually they should be combined.

The Central Bureau should provide the daily overview and "guiding hand" for the network. The Central Bureau can be distributed among several organization, using available infrastructure and resources to best advantage.

Other questions that were discussed and need further thought are:

1. How do we enforce the policies of the ILTN?
2. How do we resolve conflict?
3. What are the relative roles of the Central Bureau verses the Working Groups? Structure needs to be more formalized.

Global Station Distribution and Disposition of Transportables

EUROLAS Summary

Andrew Sinclair reported on the EUROLAS meeting in Grasse in April (see Attachment 10). Data quality control using short arc methods is progressing well as is shared tracking responsibility on GPS from Wettzell, Graz, and Grasse (LLR). INTAS is making progress with Maidanak, but not Katzively. The prospects for continued operation at Riga is poor.

Global Station Performance

Van Husson reported on the global SLR network performance in 1996 and rated stations on performance criteria discussed at the Laser Workshop in Shanghai (see Attachment 11). Most SLR stations are ranging to LAGEOS at night. Many stations are also ranging in daylight; several have also had success in daylight with the high satellites. The big issue is still range bias stability; only a dozen stations have consistently demonstrated range stability to 1-2 cm. We need to come to grips with the marginal (and sub-marginal) stations.

NASA Relocation Plans

David Carter reported on the SLR relocation plan at NASA. MOBLAS-8 is in the process of being moved from Quincy to Tahiti (see Attachment 12). Training of personnel is underway at Quincy. The facility at the new site is scheduled for completion in August, and setup should be completed by year's end. Discussions are underway with agencies in both South Africa and India regarding relocation of MOBLAS-6 and TLRS-4 respectively. MOBLAS-6 has been upgraded for relocation. A decision on the improvements to TLRS-4 is pending.

Upgrade of the Chinese and Russians Stations

Chinese Network

Mike Pearlman presented an update on the US efforts to provide assistance to the Chinese and Russians SLR stations. NASA has provided the third installment for funds to purchase key hardware items for the Chinese stations. So far, about $135K has been provided with some components going to all of the operating stations. Support is now being focussed primarily on bringing the Kunming station into operation and improving the quality and quantity of the Shanghai data.

John Luck reported that an engineer from the Changchun Station is at Orroral for training. The EOS "portable standard" is scheduled to go to Changchun this summer for collocation.

Wolfgang Schlueter reported the MLRS-1 is scheduled to go to China next May for a one-year occupation at Lhasa and subsequent occupations at Xian and several other sites. The program is being conducted with the Chinese National Bureau of Surveying and Mapping.

Russia and Former States of the Soviet Union

An SAO proposal with the Crimean Astronomical Observatory to the Civilian Research and Development Foundation for the Independent States of the Former Soviet Union (CRDF) to upgrade the Simiez SLR stations has been approved. Funding is anticipated this fall for a two-year program to improve hardware, software, and operations.

Keystone Status/HTLRS Plans

Hiroo Kunimori presented the status of the Keystone project (see Attachment 13). Four systems have been placed around Kashima and one at Koganei. Tests are underway on the first baseline and data should be available for the analysis community within several months. Once operational, Keystone will support global requirements as well as the localized crustal motion studies. Of great interest here is the combination of eye safe operation at synchronous high pulse repetition rates (up to 100 Hz.).

The HTLRS will reoccupy the island sites at Isigaki, Wakkanai, Chichi, and Marcus over the next several years in conjunction with Simosato.

TIGO

Wolfgang Schlueter reported on the status of TIGO. The laser system is currently at Delft with delivery scheduled for later this year. Announcements of opportunity will be going out for responses by potential host countries starting in the 1999 timeframe. Highest priority will be for Southern Hemisphere sites.

India

Tom Varghese reported on his meetings with the Indian Space Research Organization (ISRO) regarding the relocation of TLRS-4 (see Attachment 14). The most likely sites are in the Bangalore region (Bangalore, Kavalur, etc), but the possibility of a site in Iammu and Kashmere is also being discussed. Upgrades required to make TLRS-4 a viable long term ranging instrument are under discussion at NASA.

FTLRS

Richard Biancale told us that data from five months of FTLRS occupation at Corsica are now being analyzed and should be available to the community soon. They have made no decisions regarding FTLRS support for satellite tracking campaigns, but it will be available to support calibration activities.

Recent and Upcoming Missions

Overview

Alan Murdoch gave an overview of the recent and upcoming missions (see Attachment 15). Data volume continues to increase as the number of satellites being tracked increases and as systems are upgraded. Data yield on TiPS, one of the most challenging targets, has increased dramatically as experience has increased and technique has improved. The network has also been actively supporting ADEOS/RIS since last fall, but it appears that the satellite may have some orientation problems (?). Five satellites: Westpac (WPLTN-1), Sunsat, GFO-1, ERS-VII, and ATEX are all scheduled for launch over the next 12 months. Over the next several years, polar orbits will become much more common.

GPS Campaign: Network Performance Summary

Win Decker gave a rundown on the CSTG GPS Laser Tracking Campaign conducted in late 1996 (see Attachment 16). The intent was to (1) develop all SLR orbit trajectories for GPS 35 and 36 for direct comparison with GPS data, (2) acquire continuous multistation tracking during GPS eclipse events to better understand GPS attitude control, and (3) examine GPS-retroreflector offsets with simultaneous tracking by several SLR stations. A combined total of 116 passes were taken on the two satellites by 15 stations over a period of a month for the primary campaign. The data was sparse, but comparisons with GPS could be made. Particularly noteworthy were the performances at Maui, Texas, and NRL/SOR. (It should be noted that Yarragadee was down for upgrading during this period.) The eclipse and antenna offset events were scheduled, but none received tracking support from more than two stations at a time due mainly to poor weather and low return signal strength.

GPS Campaign: Analysis Results Summary

Erricos Pavlis presented a summary of the GPS SLR results on data taken over the previous eight months, starting with the official CSTG campaign (see Attachment 17). Over this period, 18 stations provided data; the major contributors were: Maui, Monument Peak, Yarragadee, and Wettzell. The SLR data was biased by 6 -7 cm from GPS orbits and the introduction of even a small amount of SLR data into the GPS orbits had a large effect, changing the orbit for the entire configuration. The data set was disappointing; only a few periods of global SLR tracking were adequate to support independent SLR-based orbit computations. The data set was not adequate to independently investigate SLR biases, but even a small set of the SLR data was found to have a strong influence on the GPS orbits. We need to work on the data yield.

Diadem Campaign Status

Richard Biancale reported on the status of the Diadem Campaign (see Attachment 18). Tracking was approved by the Subcommission on D1C and D1D for six months starting in late April for gravity field refinement. During the month May, 22 stations took data, averaging about 16 passes per day. Tracking from the Northern Hemisphere was good, but data from Riyadh would be very helpful. Magnetic stabilization of the satellite precludes most of the Southern Hemisphere stations, but a magnetic anomaly in the region of South America permits critical participation by Arequipa. Biancale is hopeful that French Polynesia will got some data and requests that the Australia stations continue data acquisition attempts (even a small amount would be very helpful).

Current orbital modeling is in the vicinity of 25 cm using EGM96. It is anticipated that the new data should provide a "reasonable" improvement.

New Business

LAGEOS Spin Rate and Photon Thrust

John Degnan presented some material on LAGEOS spin rate and photon thrust prepared by Doug Currie from the University of Maryland (see Attachment 19). Thermal effects (photon thrust) are the largest unmodeled error in the LAGEOS orbit analysis. It manifests itself as a once per revolution force that exerts a periodic along-track error that can be significant. Currie's program to study this effect is based on monitoring the evolution of the satellite spin vector by observing the solar reflections (glint) from the retroreflector surfaces. Data is collected by video recording with proper time reference. Currie is asking stations with video capability to support his experiment, which in turn should help us, improve the orbit modeling. Some data has already been acquired by Zimmerwald, and the MOBLAS systems are planning to participate.

A "Current Status Report of the Russian ZEYA Mission" provided by Vladimir Glotov is included as Attachment 20 for your reference.

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