1997 CSTG Satellite and Lunar Laser Ranging Subcommission Report

John J. Degnan
Laboratory for Terrestrial Physics
NASA Goddard Space Flight Center
Greenbelt, MD 20771 USA

Table of Contents of Report
1. INTRODUCTION
2. SUBCOMMISSION AND RELATED MEETINGS
3. NASA SLR REVIEW
4. NEW STATIONS
5. RECENT CSTG-SPONSORED TRACKING CAMPAIGNS
6. NEW MISSIONS
7. TECHNOLOGY HIGHLIGHTS
8. SCIENCE HIGHLIGHTS
9. SUMMARY

Table of Contents of APPENDIX A: ILRS DRAFT TERMS OF REFERENCE
0 INTRODUCTION
1 TRACKING STATIONS
2 OPERATIONAL CENTERS
3 DATA CENTERS
4 ANALYSIS CENTERS
5 ANALYSIS AND LUNAR COORDINATORS
6 CENTRAL BUREAU
7 GOVERNING BOARD
8 WORKING GROUPS
9 ILRS ASSOCIATE MEMBERS
10 ILRS CORRESPONDENTS

Missions Working Group Charter
Data Formats and Procedures
Networks and Engineering Working Group Charter
Analysis Working Group Charter

Table 1: CSTG SLR/LLR Subcommission Steering Committee Membership

1. INTRODUCTION

The past year has been an important and exciting one for the SLR and LLR communities. As will be described in ensuing sections, the relevance and scientific value of SLR was reaffirmed by an international review panel, the SLR satellite constellation continues to grow at a rate of several satellites per year, exciting new scientific applications and methods of data analysis for SLR are developing, and technology continues to advance at a rapid pace.

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Since last year’s report, the CSTG Satellite and Lunar Laser Ranging Subcommission has held two General meetings - one in Porto, Portugal in June 1996 following the Seventh WEGENER Symposium and the second in November 1996 in Shanghai, China held in conjunction with the Tenth International Workshop on Laser Ranging Instrumentation and sponsored by the Academia Sinica/Shanghai Observatory. Sumary reports on the two CSTG General Meetings were compiled and mailed to SLR Associates of record as was a summary of the Shanghai Workshop. These reports can also be accessed online at our Subcommission Web Site:

    http://cddis.gsfc.nasa.gov/cstg/slr_cstg.html

The Shanghai Workshop was a great success and demonstrated that there is continued strong international interest and participation in the technical development of SLR. By vote of the participants in the Shanghai meeting, the next Workshop will be hosted by IfAG in Deggendorf, Germany, from September 21-25, 1998.

At the CSTG SLR/LLR Steering Committee Meeting following the General Meeting in Shanghai, three proposals were put forward and approved. John Luck of the Orroral Observatory in Canberra Australia proposed that the next Workshop be an official activity of the CSTG and its parent organization, the International Association of Geodesy (IAG). The Subcommission Chairman will carry this request forward to the CSTG Executive Board.

The second proposal was brought to the Steering Committee by Professor Gerhard Beutler, President of CSTG. The Steering Committee was requested to investigate the feasibility of an International Laser Ranging Service (ILRS), modelled in part after the highly successful International GPS Service (IGS). Various members of the Steering Committee contributed to a draft of an ILRS Terms of Reference and the Charters for four Working Groups - Missions, Data Formats and Procedures, Networks and Engineering, and Analysis. John Degnan, Werner Gurtner, Andrew Sinclair, Peter Shelus, and Thomas Varghese drafted various sections of the document and valuable inputs and critiques were provided by Van Husson, John Luck, and Peter Dunn. The draft is posted on the Subcommission Web Site, and we invite, indeed solicit, comments from the entire SLR community. These can be communicated to me via email at jjd@ltpmail.gsfc.nasa.gov. The proposed ILRS organization mimics certain features of the IGS but also continues, and augments, certain functions traditionally carried out by the SLR/LLR Subcommission Steering Committee and its Working Groups.

The current ILRS draft was presented to the CSTG Executive Board at the Baltimore AGU Meeting on May 29, 1997 and will be presented again at the next CSTG SLR/LLR Subcommission to be held at 9:00 am on June 13, 1997 in Maratea, Italy, in conjunction with the Eighth WEGENER Symposium. Comments will be accepted from the floor and discussed at an open meeting of the Subcommission Steering Committee in the afternoon. Upon completing the final version of the draft, the ILRS proposal will be presented to the CSTG Executive Board. It is anticipated that ultimately a Call for Proposals will be issued for the establishment of a Central Bureau as called for in the Terms of Reference.

In Shanghai, the Steering Committee heard and approved a proposal from the Western Pacific Laser Tracking Network (WPLTN) which disestablished the Chinese Network and Japan/Australia representatives on the Steering Committee and instead provided for three representatives from each of the major subnetworks of SLR stations, i.e. NASA, EUROLAS, and WPLTN. This resulted in an effective increase in representation (from two members to three) for both the NASA and WPLTN networks. NASA has since named David Carter, the new SLR Manager, to join John Degnan and Michael Pearlman on the Committee. Similarly, Hiroo Kunimori of Japan, has joined Yang FuMin of China and John Luck of Australia as the newest member of the WPLTN team. We also began the restructuring of the Steering Committee to meet the needs of the new organization. The current list of Steering Committee members - complete with affiliations, mail and email addresses, phone and fax numbers - are contained in the accompanying Table 1. The segment of the SLR community they represent is identified in the left-hand column, and the SLR/LLR community is invited to correspond with their Steering Committee representatives on topics of concern to them.

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During the period September 1996 through April 1997, NASA Headquarters commissioned an international panel of scientific and technical experts, largely from academia, to review the NASA SLR Program. The Committee was chaired by Professor Irwin Shapiro of the Harvard-Smithsonian Astrophysical Observatory (SAO). The final report of that review reaffirmed the importance of SLR and LLR to the scientific community, recommended some reduction in NASA SLR operations and/or the transfer of responsibilities to cooperating foreign partners in order to at least partially meet the funding reductions requested by NASA Headquarters, and urged NASA to look to the future and develop the new technologies needed for truly autonomous station operations. The Review Committee report and recommendations are discussed in much greater detail in the companion article in this volume written by Dr. Michael Pearlman, who served as the Executiive Secretary for the NASA Review Committee.

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The inadequacy of the global distribution of SLR tracking stations has been a matter of discussion and debate for several years. Most recently, at the EUROLAS Business Meeting in April 1997, the future disposition of the European transportables to help fill major gaps in geographic coverage was a major topic of discussion. Over the past year, several events have helped to ameliorate the situation, but more must be done.

In July 1996, international members of the Final Acceptance Team (FAT) for the Saudi Arabian Laser Ranging Observatory (SALRO) visited the site near Riyadh to perform final tests of the system. The FAT team declared the system operational and recommended that the system sponsors at King Abdulazziz City for Science and Technology (KACST) accept the system from EOS Inc. of Canberra, the system designers. This station, the first on the Arabian plate, helps fill a major geographical gap in SLR tracking coverage.

The first of five remotely controlled "Keystone" systems, also built by EOS Inc., is undergoing field tests at the Communications Research Laboratory (CRL) near Tokyo. The system is still in engineering status but has reported successful tracking of several satellites including LAGEOS.

NASA has recently announced that it will move its newly-upgrade MOBLAS-8 station from its longstanding site at Quincy, California, to Tahiti in French Polynesia. The move is expected to take place in the summer of 1997 and follows the completion of a cooperative agreement between NASA, CNES, and the University of the Pacific. Under the agreement, CNES will provide for all aspects of station operations while NASA will continue to provide sustaining engineering and logistics support. NASA is also actively pursuing the transfer of the MOBLAS-6 station to a site in South Africa as well as the transfer of the TLRS-4 system to a site near Bangalore, India.

After some initial technical problems and delays, the SLR portion of the German TIGO system is nearing completion and is expected to be operational this summer. Work continues on the Chinese mobile station as well. The Japanese HTLRS station is currently idle at Simosato.

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The Subcommission sponsored two tracking campaigns over the past year. A GPS tracking campaign was carried out in the October to December 1996 time frame. The goals of the first GPS tracking campaign were as follows:

(1) SLR-derived Ephemerides: To develop an all-SLR ephemerides, lasting initially for up to 4 weeks, for the GPS 35 and 36 satellites for the purpose of acquiring an intensive SLR data set which can be used for direct comparisons to GPS-derived ephemerides and other studies .

(2) Continuous Multistation Tracking during GPS Eclipse Events: To provide continuous multistation (preferably 3 or more SLR sites) SLR tracking of GPS 35 and/or 36 during any eclipse episode. SLR tracking should begin 15 minutes before GPS enters the Earth’s shadow and continue for 30 minutes after GPS exits the Earth’s shadow.

(3) GPS Phase Antenna Offset: To provide improved estimates of the GPS antenna phase center/laser retroreflector offset through "near-simultaneous" (defined as over the same time interval) tracking of GPS 35 and/or 36 by multiple (four or more) SLR stations for several continuous intervals of one to two hours each.

The results of the campaign were somewhat disappointing due to a lack of coverage in the Southern Hemisphere, where GPS-35 and 36 were largely in daylight, and some runs of bad weather at some of the better tracking sites in the Northern Hemisphere. Two short reviews of the GPS campaign will be presented at the upcoming Subcommission in Italy; Win Decker of ATSC will give a network perspective while Erricos Pavlis of GSFC/Univ. of Maryland will look at the campaign results from an analyst’s point-of-view. Summaries of the Maratea meeting will be posted on the Subcommission Web Site when available. A summary of GPS passes taken during the campaign, prepared by Scott Wetzel of ATSC, is displayed on our Subcommission Web Site.

More recently, the Subcommission approved a six month tracking campaign of the old Diadem-1C and Diadem-1D satellites, first launched by CNES in 1967. This campaign, requested by Richard Biancale of CNES/GRGS and Peter Schwintzer of GFZ/Potsdam, began on April 21 and strives to improve the gravity field by tracking these relatively low inclination (~40^o) satellites. Again, Southern Hemisphere coverage needs improvement, but initial reports from the analysts suggest that the campaign is having a beneficial effect on gravity field modelling.

Doug Currie of the University of Maryland has proposed a future campaign, to be presented at the Maratea meeting, in which SLR stations with the capability of detecting, recording, and timetagging sunlight glints off the LAGEOS 1 and 2 satellites do so. Many international stations were recently equipped with sensitive video cameras and recorders to monitor the relative motions of the tethered end masses on the NRL TiPS satellites. Recording the glints can provide a record of the satellite spin axis orientation and rotation rate which might explain some of the anomalous accelerations observed in the LAGEOS-1 orbit due to the "thrust" imparted by solar photons.

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The past year has seen the launch of ADEOS/RIS by Japan and ZEIA by Russia with several more expected in the coming months and the revival of Diadem-1C and Diadem-1D tracking. Several more satellites are imminent as described in Table 2.

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Technology continued to move forward during the past year. Principal thrusts were in the areas of increased automation, improved timing, and the removal of the atmospheric refraction error through two color techniques. In addition, many of the traditional stations (e.g. Zimmerwald, Shanghai, etc.) are upgrading their hardware to achieve new performance levels.

Through a major re-engineering of its two-decade old MOBLAS systems, NASA/GSFC now operates the systems approximately 24 hours per day, 7 days per week with a crew of four, i.e one person per shift. This was accomplished through the installation of aircraft safety radars, modernization to PC-based control computers and more user friendly software, the integration of all subsystems into a single trailer, the remoting of various controls, and the overall automation of a variety of functions. Major increases in data yields have resulted from the upgrades and increased temporal coverage.

In a parallel effort at GSFC, engineering activity on the totally autonomous and eyesafe SLR2000 system has picked up significant momentum following the endorsement in April by the NASA Headquarters SLR Review Committee. SLR2000 represents a radical departure from past SLR systems through its use of ultrashort pulse (100 psec), low energy (150 mJ), high repetition rate (2 KHz) passively Q-switched microlasers and correlation range receivers designed to extract mean signals as small as .0001 photoelectrons from the background noise. The system will use a quadrant ranging detector to simultaneously provide precision ranging and subarcsecond pointing corrections from the satellite returns. First field tests of SLR2000 are targeted for 1999.

The new state-of-the-art Matera Laser Ranging Observatory (MLRO), built by AlliedSignal Technical Services Corporation (ATSC) for the Italian Space Agency (ASI), is currently tracking satellites at GSFC. Besides incorporating a new high precision event timer to be used in both satellite and lunar laser ranging, ATSC will install two color streak camera hardware into the system as well.

The first of the Japanese "Keystone" systems is undergoing field tests near Tokyo. These largely autonomous systems, built by EOS, Inc., are designed to be operated with only remote monitoring from a central control room at the Tokyo CRL SLR site. Ultimately a single operator will monitor the operations of up to five SLR stations in the vicinity of Tokyo. This system also features a new generation of precision timing devices.

The German TIGO systems currently nearing completion will incorporate multiple geodetic techniques including SLR, GPS, VLBI, and gravity. The SLR system features the first ultrashort pulse two color Titanium Sapphire laser operating in wavelength bands near 400 and 800 nm for two color ranging.

Two color ranging received a major impetus with the launch last fall of the Japanese ADEOS satellite. ADEOS carries the Retroreflector in Space (RIS) experiment, a special hollow cube corner with a 50 cm aperture. Besides serving as Two groups at NASA/GSFC in the United States and the Wettzell Fundamentalstation operated by IfAG in Germany have proposed two color experiments to RIS. At the Second ADEOS Workshop in Yokohama last March, NASA/GSFC reported the first known simultaneous two color streak camera returns from ADEOS/RIS. Unlike other satellites, which often display complicated multicube target signatures which makes correlation and hence two color propagation delay estimation difficult, ADEOS returns were single-peaked at both wavelengths. As expected, ADEOS/RIS preserves the single-peaked temporal profile of the outgoing laser pulse and will provide an excellent testbed for determining the ultimate performance limitations of two color SLR systems.

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At the November 1996 workshop in Shanghai, the author proposed a compact laser transponder to be used in conjunction with a small automated Earth station such as NASA’s SLR2000 system. It was shown theoretically, that an asynchronous microlaser transponder on Mars exchanging pulses with an SLR2000 Earth station could transfer range and time at the decimeter and subnanosecond levels, day or night. This possibility was also presented by Dave Smith of NASA/GSFC to the recent LLR Working Group Meeting at JPL in March 1997 where it generated a great deal of interest among the participants. Such a capability would open up new research capabilities in the planetary and relativistic sciences.

At the recent AGU poster session in Baltimore in May 1997, Steve Nerem of the University of Texas Center for Space Research (UT/CSR) presented an intriguing new approach to the analysis of SLR data in extracting science information from temporal gravity field model. Instead of solving for the time rate of change individual zonal harmonics as is usually done, he solved for temporal coefficients associated with various geophysical models (e.g. ice models, global circulation models, postglacial uplift, etc.) expected to produce temporal changes in the gravity field. This allowed him to estimate parameters having a longitudinal, as well as latitudinal, dependence and which might be tied ultimately to physical parameters within the models themselves.

Etienne Samain and Patricia Friedlance of CNES have proposed the international T2L2 experiment (Time Transfer by Laser Link) in collaboration with a number of national space agencies. The T2L2 Payload will be placed on the Russian Space station MIR in 1999 and is expected to allow the synchronization and comparison of widely separated ground clocks with an accuracy of less than 50 psec, two orders of magnitude better than competing radio techniques (GPS, Two-Way). They propose to measure the frequency stability of a new generation of clocks (Atomic fountain, Trapped ion, H-Maser) at their ultimate performances level and open the way to new fundamental physics experiments and provide early validation of optical time transfer techniques to be used in future missions such as ACES (Atomic Clocks Ensemble in Space), SORT (Solar Orbit Relativity Test), etc.

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In conclusion, the past year has been an exciting one for SLR with the promise of new and exciting science applications (even extending beyond the Earth and Moon to the inner planets of the Solar System), major advances of the technology, and a reaffirmation of the scientific worth of the technique. Now that many of the potential crises appear to be behind us, it is hoped that the scientific and engineering communities can focus more effectively on the science and technology challenges that lie ahead.

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0 INTRODUCTION

The primary objective of the ILRS is to provide a service to support, through Satellite and Lunar Laser tracking data and related products, geodetic and geophysical research activities. The service also develops the necessary standards/specifications and encourages international adherence to its conventions.

The ILRS collects, merges, archives and distributes SLR/LLR observation data sets of sufficient accuracy to satisfy the objectives of a wide range of applications and experimentation. These data sets are used by the ILRS to generate, at a minimum, the following data products:

• earth rotation parameters
• coordinates and velocities of the ILRS tracking stations
• geocenter coordinates
• parameters of the Earth's gravity field
• high accuracy satellite ephemerides
• the determination of fundamental physical constants

The accuracies of these products are sufficient to support current scientific objectives including:

• realization of global accessibility to and the improvement of the International Terrestrial Reference Frame (ITRF)
• monitoring three dimensional deformations of the solid earth
• monitoring earth rotation and polar motion
• support the monitoring of variations in the topography and volume of the liquid earth (ocean circulation, mean sea level, ice sheet thickness, wave heights, etc.)
• scientific satellite orbit determinations
• climatological research
• calibration of microwave tracking techniques

Lunar ranging data has additional application in the following research areas:

Gravitational Physics

• Einstein’s Strong Principle of Equivalence
• Relativistic Precession of the Lunar Orbit
• Changes with Time of the Gravitational Constant

Lunar Science from Librational Studies

• Dissipation of Rotational Energy (is there a liquid core?)
• Shape of the Lunar Core-Mantle Boundary (Love number k2)
• Lunar Free Librations (stimulating mechanisms?)

Solar System Dynamics

• Intersection of Earth’s Equatorial Plane with Moon’s Orbit Plane
• Angle Between these Planes
• Dynamical Equinox
• Tying the Solar System to the ICRF

Earth sciences

• Variations in the Earth Rotation
• Nutations and precession
• Long period torque on the mantle due to the core

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The ILRS accomplishes its mission through the following components:

• Networks of Tracking Stations
• Operational Centers
• Data Centers and Archives
• Analysis Centers
• Associate Analysis Centers
• Analysis Coordinator
• Lunar Coordinator
• Central Bureau
• Governing Board
• Working Groups

ILRS Tracking Stations provide ranging data to a constellation of approved satellites (including the Moon), contained in a list of satellites compiled and approved by the ILRS Governing Board, through the use of state of the art laser tracking equipment and data transmission facilities allowing for a rapid (at least daily) data transmission to one or more Operational and/or Data Centers (see below).

The stations must meet requirements which are specified in separate documents. The tracking data produced by the ILRS stations are regularly and continuously analysed by at least one ILRS Analysis Center or one mission-specific Associate Analysis Center.

The Tracking Stations may be organized in regional or institutional subnetworks.

The Operational Centers are in direct contact with tracking sites organized in a subnetwork. Their tasks include the collection and merging of data from the subnetwork, data quality checks, data reformatting into a uniform format, compression of data files if requested, maintenance of a local archive of the tracking data, and the electronic transmission of data to a designated ILRS Data Center. Operational Centers also provide the tracking sites with sustaining engineering, communications links, and other technical support. In addition, Operational Centers can perform limited services for the entire network.

Individual tracking Stations can also perform part or all of the tasks of an Operational Center themselves.

The Regional Data Centers reduce traffic on electronic networks. They collect reformatted tracking data from Operational Data Centers and/or individual tracking stations, maintain a local archive of the data received and transmit these data to the Global Data Centers. Regional Data Centers may also meet the requirements for Operational Centers and Global Data Centers (as defined in the previous and following paragraphs) of strictly regional network operations and duplicate activities of Global Data Centers to facilitate easy access to the information and products.

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The Global Data Centers are the main interfaces to the Analysis Centers and the outside user community. Their primary tasks include the following:

• receive/retrieve, archive and provide on line access to tracking data received from the Operational/Regional Data Centers
• provide on-line access to ancillary information, such as site information, occupation histories, meteorological data, site specific engineering data, etc.,
• receive/retrieve, archive and provide on-line access to ILRS scientific data products received from the Analysis Centers
• backup and secure ILRS data and products.

The analysis centers fall into two categories: Analysis Centers and Associate Analysis Centers.

The Analysis Centers receive and process tracking data from one or more data centers for the purpose of producing ILRS products. The Analysis Centers are committed to produce the products, without interruption, at an interval and with a time lag specified by the Governing Board to meet ILRS requirements. The products are delivered to the Global Data Centers, to the IERS (as per bilateral agreements), and to other bodies, using designated standards.

The Analysis Centers provide, as a minimum, earth orientation parameters on a weekly or subweekly basis, as well as other products, such as station coordinates, on a monthly or quarterly basis. The Analysis Centers also provide a second level of quality assurance on the global data set by monitoring individual station range and time biases via the fitted orbits (primarily the LAGEOS 1 and 2 satellites) used in generating the quick-look science results.

Associate Analysis Centers are organizations that produce special products, such as satellite predictions, time bias information, precise orbits for special-purpose satellites, station coordinates and velocities within a certain geographic region, or scientific data products of a mission-specific nature. Organizations with the desire of eventually becoming Analysis Centers may also be designated as Associate Analysis Centers by the Governing Board until they are ready for full scale operation.

The laser ranging technique is a broad based one. As an observational technique, the division between lunar laser ranging and artificial satellite laser ranging has become largely a historical one. Due to many instrumental changes already taking place, and others soon to take place, ranging to the Moon is rapidly becoming no different from ranging to artificial satellites, and vice-versa. Indeed, the Moon itself is a satellite of the Earth. However, present differences in many areas related to observations (e.g., predictions and data formats) are still being reconciled. It must also be recognized that the major data analysis packages that are used for artificial satellite analysis (e.g., GEODYNE, UTOPIA, etc.) are not yet equipped to deal with lunar laser ranging observations and most of the LLR analysis packages are equally not yet compatible with SLR observations. Thus, it is prudent to maintain separate LLR and SLR coordinators for an, as yet, undefined time into the future.

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For the time being, both the SLR and LLR coordinators must work within their own disciplines to maintain observational and data integrities. However, they must also work together in an efort to unify both techniques, bringing together the best of both, and, when possible, learning from the other.

The Analysis and Lunar Coordinators serve as the respective technique Coordinators to the IERS. They are voting members of the ILRS Governing Board and elected by the Governing Board as the ILRS representatives on the IERS Directing Board.

The Analysis Coordinator chairs the Analysis Working Group which includes, at a minimum, one representative from each of the Global Analysis Centers and may contain representatives of Associate Analysis Centers as well.

The responsibility of the Analysis Coordinator is to monitor the Analysis Centers’ activities to ensure that the ILRS objectives are carried out. Specific expectations include global data quality control, station performance evaluation and reporting, and continued development of appropriate analysis standards and formats for the final science products. The Analysis Coordinator is also responsible for the appropriate combination of designated Analysis Centers products into a single and coherent set of products.

The Analysis Coordinator ensures that the ILRS products produced by the ILRS Analysis and Associate Analysis Centers conform with IERS requirements and standards.

The Central Bureau (CB) is responsible for the general management of the ILRS consistent with the directives and policies set by the Governing Board. The primary functions of the CB are to facilitate communications between the ILRS and the external scientific community, coordinate ILRS activities, maintain a list of satellites approved for tracking support, establish and promote compliance to ILRS network standards, monitor network operations and quality assurance of data, maintain ILRS documentation, produce reports as required, and organize meetings and workshops.

Although the Chairperson of the Governing Board is the official representative of the ILRS at external organizations, the CB, consonant with the directives established by the Governing Board, is responsible for the day-to-day liaison with such organizations.

The CB coordinates and publishes all documents required for the satisfactory planning and operation of the Service, including standards/specifications regarding the performance, functionality and configuration requirements of all elements of the Service including user interface functions.

The CB operates the communication center for the ILRS. It maintains a hierarchy of documents and reports, both hard copy and electronic, including network information, standards, newsletters, electronic bulletin board, directories, summaries of ILRS performance and products, and an Annual Report.

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The Central Bureau proposes to the Governing Board names of individuals to be elected as members at large to help ensure the proper representation of important contributing organizations.

The activities of the Central Bureau may be distributed between different groups and organizations according to written agreements and charters.

In summary, the Central Bureau performs primarily a long term coordination and communication role to ensure that ILRS participants contribute to the Service in a consistent and continuous manner and adheres to ILRS standards.

The Central Bureau is headed by a Central Bureau Director, who is an ex-officio member of the ILRS Governing Board. The Secretary of the GB is also provided by the Central Bureau.

The Governing Board is responsible for the general directions in which the ILRS is providing its services. It defines the official ILRS products, decides upon the satellites to be included into the ILRS tracking list, accepts standards and procedures prepared and proposed by the individual bodies of the ILRS and ensures, through its chairperson, the contact to other services and organizations.

The Governing Board consists of both appointed and elected members. The appointed members include:

The appointed members are considered ex officio and are not subject to institutional restrictions. The elected board positions are nominated by the ILRS components they represent or by the Central Bureau for a two year term. The At-Large members are intended to compensate for under-representation among the various components of the ILRS or to provide additional skills or knowledge of use to the Board in carrying out its duties. The total GB membership should be properly balanced in all respects with regard to supporting organizations, skill mix, geography, etc.

ILRS Associate Members, together with the GB, may nominate and vote for elected members of the GB at the General Election Meeting held approximately every two years in conjunction with the International Workshop on Laser Ranging. With the exception of At-Large members, nominees must be associated with the relevant ILRS component (e.g. Analysis, Data Centers, Lunar, etc.), and the GB will be final arbiter on an individual’s qualifications for a particular elected post on the Board. Nominations for the GB will be accepted by the Central Bureau at any time prior to the General Election Meeting. Election is by a simple majority of votes received. In the unlikely event of a tie vote, the GB will make the final selection in Executive Session.

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The GB Chairperson is elected by the Board from among their number for a term of two years, renewable for three terms. Nomination and selection of the Chairperson is carried out in GB Executive Session following the biannual General Election Meeting. The Chairperson does not vote, except in case of a tie. He/she is the official representative of ILRS to external organizations.

The GB exercises general control over the activities of the Service including modifications to the organization that would be appropriate to maintain efficiency and reliability, while taking full advantage of the advances in technology and theory.

Most GB decisions are to be made by consensus or by a simple majority vote of the members present, provided that there is a quorum consisting of at least ten members of the GB. In case of lack of a quorum the voting is by mail. Changes in Terms of References and the

Chairperson of the GB can be made by a 2/3 majority of the members of the GB, i.e., by twelve or more votes.

The Board shall endeavor to meet semiannually and at such other times as shall be considered appropriate or opportune by the Chairperson or at the request of at least eight members.

Members of the GB shall become IAG Fellows with the appropriate rights and privileges following two years of recognized service.

The Governing Board, at its discretion, can create or disband Working Groups. A Working Group (WG) may be either permanent (Standing) or temporary (Ad-Hoc) in nature. Standing Working Groups are created by the GB to carry out continuously evolving business of the ILRS. Ad-Hoc Working Groups are appointed to carry out special investigations or tasks of a temporary nature.

The Coordinator of each WG is selected by the GB from amongst its members to ensure close coupling of the WG with the GB and its goals. The WG Coordinator can independently appoint additional members to the WG from among the other GB members, ILRS Associate Members or ILRS Correspondents (see below). The WG Coordinator may also designate a Deputy to act on his/her behalf in his or her absence.

Currently, the Standing Working Groups are:

• Missions
• Data Standards and Processing
• Networks and Engineering
• Analysis

The charters for these working groups are attached as Appendices.

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Persons representing organizations which participate in any of the ILRS activities (missions, tracking, operations, data analysis, or archiving) and who are not members of the Governing Board are considered ILRS Associate Members. They may attend open (non-executive) ILRS meetings which are announced to the general community by the CB, place nominations for elected GB posts, and vote in ILRS elections. A directory, electronic and/or hard copy, of ILRS Associate Members is maintained by the CB.

ILRS Associate Members are considered IAG Affiliates with the corresponding rights and privileges.

ILRS Correspondents are persons on a mailing list maintained by the Central Bureau, who do not actively participate in the ILRS but express interest in receiving ILRS publications, wish to participate in workshops or scientific meetings organized by the ILRS, or generally are interested in ILRS activities. Ex officio ILRS Correspondents are the following persons:

• IAG General Secretary
• President of IAG Section V

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The primary task of the ILRS is the timely collection, merging, archiving, and distribution of laser ranging data to various approved spacecraft properly equipped with retroreflectors.

Currently, artificial satellites tracked by the global tracking network fall into four major categories:

• Geodetic (e.g., LAGEOS 1&2, Etalon 1&2, Starlette, Stella, Ajisai, GFZ-1)
• Altimetric (e.g. ERS 1&2, TOPEX/POSEIDON)
• Space Navigation and Positioning (e.g. GPS, GLONASS, METEOR-2/PRARE)
• Special (e.g. TiPS, METEOR/Fizeau, RESURS-3, ADEOS/RIS)

Geodetic satellites are dedicated passive satellites used in defining the Terrestrial Reference Frame and gravity field and in scientific studies of tectonic plate motion, regional crustal deformation, Earth rotation and polar motion, postglacial uplift, etc. Such satellites generally require frequent and long term tracking (decades) to achieve their scientific goals.

Altimetric satellites are usually limited life missions lasting approximately three to ten years. Typical scientific goals include dynamic sea surface topography, mean sea level and wave height determination, global ocean circulation, ice sheet thickness and topography, and, with future laser altimeters, land surface topography including biomass estimation. The frequency and duration of laser tracking required is usually dependent on the desired radial accuracy for the orbit, the presence or absence of complementary microwave tracking devices such as GPS, DORIS, or PRARE, and the periodic need for inflight calibrations of these techniques and/or the altimetric sensors themselves.

Space Navigation and Positioning satellites provide an alternate means (usually based on microwaves) of obtaining precise geodetic positioning on the Earth or precise navigation in space. Laser tracking provides an independent means of calibrating the performance of these systems, further defining satellite force models, and/or directly tieing their orbits into the SLR reference frame with its well-defined geocenter and vertical scale height. Tracking can be of long term interest (e.g. GPS) or relatively short term (e.g. METEOR 3/PRARE).

Special satellites usually have a unique, short-term scientific or engineering goal such as the study of tether dynamics in Earth orbit (TiPS), testing the performance of new retroreflector designs (METEOR 2/Fizeau, RESURS-3), the evaluation and optimization of two color SLR systems (ADEOS/RIS), or intercontinental time transfer experiments (METEOSAT P2/LASSO). In one case (ADEOS/RIS), lasers provide the precise orbit predictions which permit ground-based lidars to rapidly acquire the onboard reflector for atmospheric experiments.

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The Point of Contact for every organization requesting tracking support of new satellites or modification of the adopted tracking priorities is the ILRS Central Bureau.

It is the responsibility of the Spacecraft or Mission sponsor requesting laser tracking support to file a Laser Tracking Support Plan with the ILRS Central Bureau. In the past, NASA has generated the Laser Tracking Support Plans for most new missions, including many non-NASA experiments. These plans are required to obtain tracking approval at NASA and many non-NASA sites. In the future, it will be the responsibility of the spacecraft or mission sponsor to generate the plan and submit it in a timely manner to the Central Bureau. Pertinent information on the Mission will be extracted from the Plan and posted on the CSTG SLR/LLR Subcommission Web Page for easy access by individual stations.

The Laser Tracking Support Plan must provide, to the best of the sponsor's ability, the following information:

• Scientific and Technical Points of Contact for the Mission
• the scientific or engineering goals of the mission
• the proposed launch scenario and approximate dates
• expected duration of the mission
• special tracking requirements or restrictions (if any) during the pre-launch, launch, and post-launch phases of the mission
• the tracking sites requested to participate in the operational phase of the Mission and the frequency and duration of tracking required from those sites (required orbital accuracy may be substituted for this)
• the acquisition, tracking, and/or data services to be provided by the sponsoring organization for the Mission
• those acquisition and data services of a special or non-routine nature requested from the ILRS
• required timelines for data processing and delivery
• repository for scientific or engineering results resulting from the mission and a statement of availability of raw and/or analyzed data to the general SLR community

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A laser station can only track one satellite at a time. The rapidly growing constellation of satellites tracked by lasers, combined with a vastly different tracking requirement for each satellite and limited tracking resources, necessitates the formation of a Missions Working Group to review all requests for laser tracking and to make a recommendation to the Governing Board regarding tracking and its relative priority with respect to other approved satellites.

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The function of the ITLN is to generate accurate and precise laser tracking data, and to merge, archive and distribute these data. In doing so a variety of data handling and processing procedures are involved, and it is essential that standardised and efficient procedures should be utilized throughout the network. This Working Group is concerned with procedures affecting the data up to and including the generation of full-rate or normal point data. The subsequent procedures concerning the generation of higher level data products (such as Earth rotation parameters, station coordinates, etc.) are the concern of the Analysis Working Group.

There are two main aims of this Working Group:

i) to maximise the efficiency of the process of generating the laser data, by ensuring that accurate predictions are available and that standardised software procedures are available to produce a uniform quality data product,

ii) to ensure that the data product contains all the information needed by the analyst, and that the data and related information are available for the analyst in a convenient form.

2. Role of the Data Formats and Procedures Working Group

2.1 Predictions:

It will be the responsibility of the Working Group to document and maintain standards for:

Force model and reference frame of IRV integrator.
Format of IRV state vectors.
Standard methods to correct IRVs for unmodelled forces.
Standard format for time bias functions.
Standard software packages for generating predictions from IRVs.

The Working Group will endeavour to ensure that there are several groups within the network with the capability of generating IRVs and time bias corrections, and that there are efficient and rapid means of distribution.

The Working Group will endeavour to ensure that a standard software package is maintained for generating pass predictions from IRVs.

2.2 Data processing

The Working Group will document and maintain the standard algorithm for formation of normal points, and will endeavour to maintain standard software packages for fitting a trend function to pass residuals, for analysing the distribution of pass residuals, and calculating various reference points (mean, peak, etc)

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2.3 Station information

The Working Group will document and maintain formats for recording station information, such as:

eccentricity vectors
details of occupancy of sites
changes to systems
alternative operational configurations of stations.

2.4 Data format

The Working Group will maintain the documentation of the format for the final data products, full-rate data and site-formed normal points, and will coordinate the continuing activity to review and if necessary revise the format.

Many of the actions discussed above are already carried out by various organisations, groups and individuals within the laser ranging community. The function of the Working Group is not to change any of these arrangements, but to consolidate and build on them, and ensure that developments take place in a coordinated way.

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SLR systems constituting the ILRS provide ranging data to satellites (artificial or otherwise) to meet the requirements of the scientific community. SLR must consistently produce high quality data if it is to fulfill its potential role as a "geodetic calibrator". The diversity in hardware, software and technologies in the ground based SLR/LLR systems which make up the global network creates a unique set of monitoring problems for SLR as compared to other space based geodetic techniques. This diversity makes it necessary to establish an international entity dedicated to monitoring overall network performance and ensuring the quality and timeliness of the data it produces.

An increasing number of satellites are being launched with an expectation of SLR support. Adequate coordination between the analysis community and the network is essential to ensuring that mission needs and scientific/engineering goals are being met. Feedback from the data analysts on faulty or inconsistent station performance must be documented and corrective actions initiated as rapidly as possible. The need often exists to review network performance versus mission requirements on a periodic basis to ensure adequacy of the global SLR dataset for the full complement of approved missions.

Individual station capabilities often dictate data quality and quantity and the number of spacecraft that it can support. At present, data quality (i.e. precision, accuracy and stability) can vary by as much as an order of magnitude from one station to the next. Such a large disparity compromises the full potential of the SLR technique and requires a coordinated approach to achieve greater uniformity and versatility. A globally coordinated WG, which understands both the user needs and station constraints, can provide a broad perspective in arriving at an optimal approach.

The Networks and Engineering WG will seek to improve and optimize the input-output functions and products of the SLR network by working closely with the data analysis community and the individual SLR stations through appropriate interactions and feedback. The primary responsibility of the Networks and Engineering WG is to facilitate the generation, collection and distribution of data in a timely and efficient manner to the user community while meeting the data quality and quantity requirements of the approved missions and to serve as a catalyst in improving the overall performance of the global network. To this end, the WG:

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SLR Analysis Centers provide geodetic solutions of various types to the International Earth Rotation Service (IERS) where applicable and/or to the Global Data Centers for use by the scientific community. Solutions provided to the IERS are related to the Terrestrial Reference Frame (TRF) and include station positions and velocities in a geocentric reference frame as well as those components of the Earth Orientation Parameters observable via the SLR technique. Other data products of a more specialized nature include static and time-varying gravity field coefficients, precise satellite orbits, etc.

A major goal of the space geodetic community is to freely use space geodetic data of all types in establishing and maintaining the TRF and in performing other scientific analyses. This is most easily accomplished if the presentation format of the scientific data product is largely independent of the technique. In recent years, the GPS community has adopted the SINEX format, and, since the overwhelming volume of data will flow from the GPS network, it seems appropriate for the SLR and other space geodetic analysis communities to either adopt the SINEX format or to work together toward some other mutually agreeable format. This will ensure that SLR data, with its unique strengths and capabilities, will continue to be attractive to researchers anxious to use all the space geodetic tools at their disposal to achieve their demanding scientific goals.

SLR Analysis Centers provide independent solutions, but have sometimes  adopted reference frames and/or models for gravity, plate motion, tides,  nonconservative forces, etc. which were either non-standard or poorly defined. These force model differences can produce apparent disparities and inconsistencies between solutions from centers employing different analysis techniques. Inadequate description of the reference frame can also create confusion among outside scientists attempting to use the archived results in their own analyses. It is therefore important that certain standards be uniformly applied to all formal solutions submitted by the Analysis Centers to the IERS and that those force or other models for which no standards have been adopted be clearly stated. It is the function of the Analysis WG to ensure that all IERS standards be adopted in formal SLR submissions to that body, to lobby on behalf of the ILRS to change IERS standards when warranted, to establish and maintain a knowledge base of analysis and data presentation standards which are easily accessible to the SLR Analysis community, to assist the analysis community in adopting and adhering to ILRS standards and practices in a pro-active manner, and to advise the Missions WG when necessary in the evaluation and/or improvement of global SLR network support to existing and future missions.

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2.Roles and responsibilities of the Analysis Working Group

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Last modified date: Friday, November 05, 2004
Author: Carey Noll
Maintained by: Carey Noll