INTERNATIONAL LASER RANGING SERVICE (ILRS)

J. J. Degnan¹ and M. R. Pearlman²

¹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

²Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA USA 02138, USA

CONTRIBUTIONS OF THE ILRS

The ILRS collects, merges, analyzes, archives and distributes Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR) observation data sets of sufficient accuracy to satisfy the objectives of a wide range of scientific, engineering, and operational applications and experimentation. The basic observable is the precise time-of-flight of an ultrashort laser pulse to and from a satellite, corrected for atmospheric delays. These data sets are used by the ILRS to generate a number of fundamental data products, including: centimeter accuracy satellite ephemerides, Earth orientation parameters, three-dimensional coordinates and velocities of the ILRS tracking stations; time-varying geocenter coordinates, static and time-varying coefficients of the Earth's gravity field, fundamental physical constants, lunar ephemerides and librations, and lunar orientation parameters

ORGANIZATION AND ROLE OF THE ILRS

The ILRS Tracking Stations range to a constellation of artificial satellites and the Moon with state-of-the-art laser ranging systems and transmit their data on a rapid basis (at least daily) to an Operations or Data Center. Stations are expected to meet ILRS data accuracy, quantity, and timeliness requirements, and their data must be regularly and continuously analyzed by at least one Analysis or mission-specific Associate Analysis Center. Each Tracking Station is typically associated with one of the three regional subnetworks (National Aeronautics and Space Administration (NASA), EUROpean LASer Network (EUROLAS), or the Western Pacific Laser Tracking Network (WPLTN).

Operations Centers collect and merge the data from the tracking sites, provide initial quality checks, reformat and compress the data if necessary, maintain a local archive of the tracking data, and relay the data to a Data Center. Operational Centers may also provide the Tracking Stations with sustaining engineering, communications links, and other technical support. Tracking Stations may perform part or all of the tasks of an Operational Center themselves.

Global Data Centers are the primary interfaces between the Analysis Centers and the outside users. They receive and archive ranging data and supporting information from the Operations and Regional Data Centers, and provide this data on-line to the Analysis Centers. They also receive and archive ILRS scientific data products from the Analysis Centers and provide these products on-line to users. Regional Data Centers reduce traffic on electronic networks and provide a local data archive.

Analysis Centers receive and process tracking data to produce ILRS data products. They are committed to produce the products on a routine basis for delivery to the Global Data Centers and the IERS using designated standards. Full Analysis Centers routinely process the global LAGEOS-1 and LAGEOS-2 data and provide Earth orientation parameters on a weekly or sub-weekly basis. They also produce other products such as station coordinates and velocities and geocenter coordinates on a schedule consistent with IERS requirements and provide a second level of data quality assurance in the network. Associate Analysis Centers produce specialized products, such as time-varying gravity field measurements, fundamental constants, satellite predictions, precision orbits for special-purpose satellites, regional geodetic measurements, and data products of a mission-specific nature. Associate Analysis Centers are also encouraged to perform quality control functions through the direct comparison of Analysis Center products and the creation of "combined" solutions using data from other space geodetic techniques. Lunar Analysis Centers produce LLR products such as lunar ephemeris, lunar libration, and Earth rotation (UT0 - UT1). In the field of relativity, LLR is used for the verification of the equivalence principle, estimation of geodetic precession, and examination of the relative change in G.

CENTRAL BUREAU

The ILRS Central Bureau (CB) is responsible for the daily coordination and management of ILRS activities. It facilitates communications and information transfer and promotes compliance with ILRS network standards. The CB monitors network operations and quality assurance of the data, maintains all ILRS documentation and databases, and organizes meetings and workshops. In order to strengthen the ILRS interface with the scientific community, a Science Coordinator and an Analysis Specialist within the CB take a proactive role to enhance dialogue, to promote SLR goals and capabilities, and to educate and advise the ILRS entities on current and future science requirements related to SLR. The Science Coordinator leads efforts to ensure that ILRS data products meet the needs of the scientific community and there is easy online access to all published material (via Abstracts) relevant to SLR science and technology objectives.

The CB has been actively providing new conveniences (such as targeted email exploders) and adding to the technical and scientific database. The information available via the ILRS Web Site has grown enormously since its inception, and many new links to related organizations and sites have been established. The site provides details and photographic material on the ILRS, the satellites and campaigns, individual SLR station characteristics, a scientific and technical bibliography on SLR and its applications, current activities of the Governing Board Working Groups and Central Bureau, meeting minutes and reports (including annual reports), tracking plans, etc. In the future, much more material will be made available online along with an enhanced search capability to quickly isolate specific material of interest. Due to the impending retirement of the CB's first Director, John Bosworth of NASA , Dr. Michael Pearlman of SAO has assumed the role of CB Director and Ms. Carey Noll of NASA will assume Dr. Pearlman's previous role of ILRS Secretary.

The Central Bureau maintains a comprehensive web site as the primary vehicle for the distribution of information within the ILRS community. The site, which can be accessed at:

http://ilrs.gsfc.nasa.gov/

  includes the following major topic titles: About the ILRS, Current Events, Working Groups, Satellite Missions, Network Stations, Data Products, Science/Analysis, Engineering/Technology, Reports, Frequently Asked Questions (FAQs), and Links. Mirrored sites are also available at the Communications Research Laboratory (CRL) in Tokyo and the European Data Center (EDC) in Munich. The site also includes SLR related bibliographies, Earth science links, historical information, collocation histories, and mail exploders. An on-line brochure provides charts for SLR presentations. A hard copy library of early documentation has been assembled and is listed in the on-line bibliography.A new ILRS Reference Card was recently developed to provide easy online access to much of this material and to targeted email exploders.

GOVERNING BOARD AND WORKING GROUPS

The Governing Board (GB) is responsible for the general direction of the service. It defines official ILRS policy and products, determines satellite tracking priorities, develops standards and procedures, and interacts with other services and organizations. There are 16 members of the Governing Board (GB) - three are ex-officio, seven are appointed, and six are elected by their peer groups (see Table 2). The first GB completed its two year term in Fall 2000. A new Board was elected over the summer and installed in November 2000 at the 12^th International Workshop in Matera, Italy. John Degnan was elected by the current GB to serve a second two year term as Chairperson.

 Table 1. ILRS Governing Board (as of May 2001)

Within the GB, permanent (Standing) or temporary (Ad-Hoc) Working Groups (WG's) carry out policy formulation for the ILRS. At its creation, the ILRS established four Standing WG's: (1) Missions, (2) Data Formats and Procedures, (3) Networks and Engineering, and (4) Analysis. In 1999, an Ad-Hoc Signal Processing WG was organized to provide improved satellite range correction models to the analysts. The Working Groups are intended to provide the expertise necessary to make technical decisions, to plan programmatic courses of action, and are responsible for reviewing and approving the content of technical and scientific databases maintained by the Central Bureau. All GB members serve on at least one of the four Standing Working Groups, led by a Coordinator and Deputy Coordinator. Table 1 lists the current GB membership, their nationality, and special function (if any) on the GB.

Fortunately, the WG's have attracted talented people from the general ILRS membership who have contributed greatly to the success of these efforts. The Missions WG has formalized and standardized the mission documentation required to obtain ILRS approval for new missions and campaigns. They continue to work with new missions and campaign sponsors to develop and finalize tracking plans and to establish recommended tracking priorities. The Data Formats and Procedures WG has been tightening up existing formats and procedures, rectifying anomalies, providing standardized documentation through the web site, and setting up study subgroups and teams to deal with more complicated or interdisciplinary issues. The Networks and Engineering WG has (1) developed the new ILRS Site and System Information Form which is being distributed to the stations to keep the engineering database current, (2) provided a new online satellite-link analysis capability for system design and performance evaluation, and (3) initiated the development of the ILRS technology database. The Analysis WG has been working with the ILRS Analysis Centers to develop a unified set of analysis products presented in the internationally accepted SINEX format. Three associated pilot programs are underway to assess differences among analysis products from the different centers. The Signal Processing Ad-Hoc WG is working on improved center-of-mass corrections and signal processing techniques for SLR satellites.

ILRS NETWORK

The ILRS Network as of May 2001 is shown in Figure 1. Traditionally the network has been strong in the US, Europe, and Australia. Through international partnerships, the global distribution of SLR stations is now improving, especially in the Southern Hemisphere. NASA, working in cooperation with CNES and the University of French Polynesia has established SLR operations on the island of Tahiti with MOBLAS-8. In cooperation with the South African Foundation for Research Development (FDR), NASA has relocated MOBLAS-6 to Hartebeesthoek (which already has VLBI, GPS, and DORIS facilities) to create the first permanent Fundamental Station on the African continent. Both systems are operational. . Operations at the new Australian station on Mt. Stromlo, which replaced the older Orroral site near Canberra, are going extremely well in terms of both data quantity and quality.

The NASA TLRS-3 system at Universidad de San Agustin in Arequipa, Peru, has carried the total SLR tracking load for South America in recent years. However, BKG (Germany) has selected Concepcion, Chile, for the site of its newly developed multi-technique Totally Integrated Geodetic Observatory (TIGO). The TIGO- with SLR, VLBI, GPS and absolute gravimetry techniques - will provide a Fundamental Station in South America when it becomes operational in late 2001. In Argentina, NASA has been negotiating a possible transfer of TLRS-4 to the University of La Plata. A possible joint Chinese-Argentine SLR station at the San Juan Observatory in western Argentina, with SLR equipment furnished by the Beijing Astronomical Observatory, is also being discussed .

The Peoples' Republic of China has made substantial investment in SLR stations and technology over the past two years. The SLR station in Kunming was recently re-established, bringing the total number of Chinese permanent sites to five (Shanghai, Changchun, Wuhan, Beijing, and Kunming. The data quality and quantity from the permanent Chinese stations continue to improve, most notably at Changchun. The Wuhan SLR station has been recently moved to a site outside the city where there is significantly better atmospheric seeing, and construction is nearing completion on two mobile Chinese SLR stations which will occupy additional sites within China, as part of a national geodetic program. The new Russian SLR station started operations near Moscow in 1999, and permission is being requested from the Russian government to integrate it into international SLR operations. A Russian SLR station in Novosobirsk has recently applied for ILRS membership. In Japan, The Communications Research Laboratory (CRL) in Tokyo continues to operate two of its four Keystone sites at Kashima and Tateyama in the Tokyo area. The future of the Koganei and Miura sites is unclear. The Simosato site, operated by the Japanese Hydrographic Institute, will continue to provide data in this technically highly interesting region. The Japanese Space Agency, NASDA, is also negotiating for the construction of a new state-of-the-art SLR station.

Sites in the United States and Europe have been relatively stable over the past several years, with efforts continuing to improve overall performance or reducing the cost of SLR operations. The new state-of-the-art Matera Laser Ranging Observatory (MLRO) with both SLR and lunar ranging capability has now been installed at Matera and is presently undergoing final acceptance testing. The new French Transportable Laser Ranging System (FTLRS) is undergoing checkout in preparation for tracking support of JASON and other active satellite missions from a site in the Mediterranean region. The unmanned SLR2000 prototype is nearing completion at NASA and field tests are scheduled to begin in Fall 2001.

Figure 1. ILRS Network (as of May 2001)

ILRS TRACKING PRIORITIES AND CAMPAIGNS

The ILRS is currently tracking about two dozen targets, including passive geodetic (geodynamics) satellites, Earth remote sensing satellites, navigation satellites, engineering missions, and lunar reflectors (see Table 2). The newest missions include the German CHAMP mission (GFZ), which was added in July 2000, and the oceanography mission, GFO-1 (US Navy), which was recently elevated from campaign status to permanent tracking. In addition, three new GLONASS (72, 79, and 80) satellites are being tracked in support of the IGLOS campaign. Recently, Etalon 1 and 2 were elevated in priority at the request of the ILRS Analysis Working Group in order to ascertain whether or nor SLR's determination of Earth Orientation Parameters (EOP) could be improved.

The ILRS assigns satellite priorities in an attempt to maximize data yield on the full satellite complex while at the same time placing greatest emphasis on the most immediate data needs. Priorities provide guidelines for the network stations, but stations may occasionally deviate from the priorities to support regional activities or national initiatives and to expand tracking coverage in regions with multiple stations. Tracking priorities are set by the Governing Board, based on application to the Central Bureau and recommendation of the Missions Working Group.

During the past year, tracking campaigns have included: (1) ERS-1 to support tandem Synthetic Aperture Radar (SAR) experiments with ERS-2; (2) the GEOSAT Follow-on (GFO-1) altimetric mission, (3) the South African SUNSAT remote sensing satellite, and (4) revived GEOS-3 and Beacon-C tracking for gravity field improvement .

Since several remote sensing missions have suffered failures in their active tracking systems or have required in-flight recalibration, the ILRS has encouraged new missions with high precision orbit requirements to include retroreflectors as a fail-safe backup tracking system, to improve or strengthen overall orbit precision, and to provide important intercomparison and calibration data with onboard microwave navigation systems.

UPCOMING MISSIONS

At one time, the main task of the international SLR Network was the tracking of dedicated geodetic satellites (LAGEOS, Starlette, etc.). Although we have had requests to revive tracking on older satellites already in orbit (e.g. Beacon-C) to further refine the gravity field with improved accuracy laser data, new requests for tracking are now coming mainly for active satellites. The tracking approval process begins with the submission of a Missions Support Request Form, which is accessible through the ILRS web site. The form provides the ILRS with the following information: a description of the mission objectives; mission requirements; responsible individuals, organizations, and contact information; timeline; satellite subsystems; and details of the retroreflector array and its placement on the satellite. This form also outlines the early stages of intensive support that may be required during the initial orbital acquisition and stabilization and spacecraft checkout phases. A list of upcoming space missions that have requested ILRS tracking support is summarized in Table 3 along with their sponsors, intended application, and projected launch dates.

 Table 2. ILRS Tracking Priorities (as of April 2001)

Once tracking support is approved by the Governing Board, the Central Bureau works with the new missions to develop a Mission Support Plan detailing the level of tracking, the schedule, the points of contact, and the channels of communication. New missions normally receive very high priority during the acquisition and checkout phases and are then placed at a routine priority based on the satellite category and orbital parameters. After launch, New Mission Reports with network tracking statistics and operational comments are issued weekly. The Central Bureau monitors progress to determine if adequate support is being provided. New mission sponsors (users) are requested to report at the ILRS Plenary meetings on the status of ongoing campaigns, including the responsiveness of the ILRS to their needs and on progress towards achieving the desired science or engineering results.

MEETINGS AND REPORTS

The ILRS organizes semiannual meetings of the Governing Board and General Assembly, which is open to all ILRS Associates and Correspondents. The 5^th ILRS General Assembly was held in November 2000 in Matera, Italy, in conjunction with the 12^th International Workshop on Laser Ranging. The 6^th ILRS General Assembly was held in March 2001 in conjunction with the EGS Symposium in Nice, France. Detailed reports from past meetings can be found at the ILRS web site.

 Table 3. Upcoming Missions (as of April 2001)

The 7^th ILRS General Assembly will be held at the Centre de Congres Pierre Baudis in Toulouse, France, on Friday morning, 21 September 2001. The meeting is held in conjunction with the SPIE/Europto Symposium on Remote Sensing (September 17-21, 2001), which includes a session on Laser Radar Techniques (Sept. 17-18) as well as open ILRS -sponsored Working Group sessions and calibration workshops A special Joint ILRS/WPLTN symposium will be held in Riyadh, Saudi Arabia, on the following Sunday and Monday (September 23-24).

The first ILRS Annual Report (1999) was published last year It is available as hard copy from the CB or online at the ILRS Web Site. The 2000 ILRS Annual Report is in preparation.

ILRS Analysis Center reports and inputs are used by the Central Bureau for weekly review of station performance and to provide feedback to the stations when necessary. Special weekly reports on on-going campaigns are issued by email. The CB also generates Quarterly Performance Report Cards and posts them on the ILRS web site. The Report Cards evaluate data quantity, data quality, and operational compliance for each tracking station relative to ILRS minimum performance standards. A catalogue of diagnostic methods, for use along the entire data chain starting with data collection at the stations, has emerged from this process and will be made available on the ILRS web site. The evaluation process has been helpful in comparing results from different Analysis and Associate Analysis Centers, a role soon to be assumed by the Analysis Working Group.

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