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IAG Travaux (2007)

INTERNATIONAL LASER RANGING SERVICE (ILRS)

M. R. Pearlman ¹, C. E. Noll ² and W. Gurtner ³

¹ Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA USA 02138, USA
² NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
³ Astronomical Institute, University of Berne, CH-3012 Berne, Switzerland

CONTRIBUTIONS OF THE ILRS

The ILRS was organized as one of the IAG measurement services in 1998. The service collects, merges, analyzes, archives and distributes Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR) observation data sets to satisfy the objectives of scientific, engineering, and operational applications and programs. The basic observable is the precise time-of-flight of an ultrashort laser pulse to and from a retroreflector array on a satellite, which are reduced for orbital and positioning products, once corrected for atmospheric delays and spacecraft center-of-mass. The Service also produces analogous lunar ranging observations. These data sets are used by the ILRS to generate fundamental data products, including: accurate 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. The ILRS generates a standard weekly product of station positions and Earth orientation for submission to the IERS, and produces LAGEOS combination solutions for maintenance of the International Terrestrial Reference Frame (ITRF). The ILRS participates in the Global Geodetic Observing System (GGOS) organized under the IAG.

ORGANIZATION AND ROLE OF THE ILRS

The ILRS accomplishes its mission through the following permanent components:

Tracking Stations and Subnetworks
Operations Centers
Global and Regional Data Centers
Analysis and Associate Analysis Centers
Central Bureau

The ILRS Tracking Stations range to a constellation of artificial satellites, the Moon, and eventually interplanetary spacecraft with state-of-the-art laser ranging systems and transmit their data on an hourly basis 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 Tracking Stations and the Analysis Centers and outside users. They receive and archive ranging data and supporting information from the Operations and Regional Data Centers, and provide these 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 retrieve data from the archives and process them to produce the official ILRS products. They are committed to follow designated standards and produce on a routine basis for delivery to the Global Data Centers and the IERS. Analysis Centers routinely process the global LAGEOS-1 and LAGEOS-2 data and compute weekly solutions of station positions and Earth orientation for combination and submission to the IERS. Analysis Centers also provide a second level of data quality assurance in the network. Analysis and Associate Analysis Centers produce station coordinates and velocities, geocenter coordinates, 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 Coordinator 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 that there is easy online access to published material relevant to SLR science and technology objectives.

The CB has been actively providing new facilities to expedite communication and performance review, 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 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, and much more.

The Central Bureau maintains the ILRS Web site, http://ilrs.gsfc.nasa.gov, as the primary vehicle for the distribution of information within the ILRS community. Enhancements to the ILRS Web site continue. The ILRS station information pages were expanded to include various reports and plots to monitor network performance. Station operators, analysts, and other ILRS groups can view these reports and plots to quickly ascertain how individual stations are performing as well as how the overall network is supporting the various missions. Detailed information on satellites supported by the ILRS is also available on the ILRS Web site, organized by mission.

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 sixteen members of the Governing Board (GB) - three are ex-officio, seven are appointed, and six are elected by their peer groups (see Table 1). A new Board was installed in October 2006 at the 15^th International Workshop on Laser Ranging in Canberra Australia.

Within the GB, permanent (Standing) or temporary (Ad-Hoc) Working Groups (WG) carry out policy formulation for the ILRS. At its creation, the ILRS established four standing WGs: (1) Missions, (2) Data Formats and Procedures, (3) Networks and Engineering, and (4) Analysis. A fifth WG on Transponders for lunar and planetary ranging was established in 2006. The Ad-Hoc Signal Processing WG, organized to provide improved satellite range correction models to the analysts, has now been subsumed into the Networks an Engineering WG. The WGs 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 WGs, led by a Coordinator and Deputy Coordinator (see Table 1). The WGs continue to attracted talented people from the general ILRS membership who contributed greatly to the success of these efforts.

The Missions WG, with a set of evolving formal and standardized documentation, has been working with new satellite missions to seek ILRS approval for SLR observing support. If such support is deemed necessary for the success of the mission, and is within the operational capabilities of the network, the WG works with the new mission personnel and campaign sponsors to develop and finalize tracking plans and to establish recommended tracking priorities.

Table 1. ILRS Governing Board (as of June 2007)

Hermann Drewes	Ex-Officio, President of IAG Commission	Germany
Michael Pearlman	Ex-Officio, Director, ILRS Central Bureau	USA
Carey Noll	Ex-Officio, Secretary, ILRS Central Bureau	USA
Bob Schutz	Appointed, IERS Representative to ILRS	USA
Werner Gurtner	Appointed, EUROLAS, Governing Board Chair	Switzerland
Giuseppe Bianco	Appointed, EUROLAS	Italy
David Carter	Appointed, NASA	USA
Jan McGarry	Appointed, NASA	USA
Yang Fumin	Appointed, WPLTN	China
Hiroo Kunimori	Appointed, WPLTN	Japan
Vincenzia Luceri	Elected, Analysis Representative, Analysis Working Group Deputy Coordinator	Italy
Erricos Pavlis	Elected, Analysis Representative, Analysis Working Group Coordinator	USA
Wolfgang Seemueller	Elected, Data Centers Rep., Data Formats and Procedures WG Coordinator	Germany
Juergen Mueller	Elected, Lunar Representative	Germany
Graham Appleby	Elected, At-Large, Missions Working Group Coordinator	UK
Georg Kirchner	Elected, At-Large, Networks and Engineering Working Group Coordinator	Austria

The Data Formats and Procedures WG has developed and implemented the new Consolidated Prediction Format (CPF) for a much wider variety of laser ranging targets including (1) Earth-orbiting retroreflector satellites, (2) Lunar reflectors, (3) asynchronous and synchronous transponders. The new expanded format capability, with more complete modeling representation, has improved tracking on lower satellites and has removed the need for drag and special maneuver files. Within the WG, the Refraction Study Group has made significant advances in updating the refraction correction model and in proposing novel techniques for further improvement. The new model is now accepted as part of the IERS Conventions. The WG has developed a new format for ranging data to accommodate extraterrestrial targets and transponders in addition to SLR satellite data. The Network and Engineering WG has organized a special kHz SLR workshop in Graz, to motivate and assist other stations to switch to this promising technique; slight adaptations in the existing normal point format were implemented to accommodate kHz SLR data. Programs to quality check normal points at the SLR station before transmission to operations centers were distributed to all stations. The SR620 Stanford counters, which have been used during the past year at several stations, are now measured for recently discovered non-linearities, to be able to apply possible range corrections on already measured SLR data. In addition, the EDF (Engineering Data File) system has been established in order to verify and intercompare SLR system functions at the hardware level. The Signal Processing Ad-Hoc WG is working on improved center-of-mass corrections and signal processing techniques for SLR satellites.

The Analysis WG completed its pilot projects to assess and resolve differences among analysis products from the Analysis and Associate Analysis Centers. Seven centers have qualified as Analysis Centers; two additional centers are in the qualifying process. A Combination Center and a Backup Combination Center have been in operation since 2004. The WG has developed and implemented the process to deliver LAGEOS derived site positions and EOP to the IERS as required on a weekly basis. A 1993-2005 reanalysis of the LAGEOS data was provided to the IERS in support of the development of ITRF2005. Work is underway to add additional official ILRS products including precision orbits and certified data corrections. A new reanalysis that includes the historical SLR data back to at least 1983 is in process.

The newly formed Transponder Working Group has begun to form a list of recommendations for the future development of interplanetary laser transponders for centimeter ranging and sub-nanosecond time transfer. Activities are underway at SLR sites in Europe and the US to conduct simulated interplanetary experiments using the current SLR satellite constellation. Such experiments would have a beneficial impact on interplanetary optical communications as well.

ILRS NETWORK

Satellite Laser Ranging(SLR) Network

The SLR technique is now over forty years old, having originated in 1964 with ranging to Beacon-B from GSFC. Systems have evolved from a manually operated mount with meter-level ranging systems to automated and semi-autonomous systems with sub-centimeter ranging accuracies.

The present ILRS network, as shown in Figure 1. The last four years have witnessed considerable activity within the ILRS. After some discouraging cutbacks in 2003-5, the ILRS network has had some resurrection. NASA and the University of San Agustin reopened the TLRS-3 system at Arequipa in late 2006. A rededication ceremony was held in early 2007. Fortunately the GPS receiver has been in operation since SLR closure in 2003, so some continuity has been provided during the intervening period. Several upgrades including the “restricted tracking capability” have been added to the system to enhance operations. The Mt. Haleakala station has also been reopened with the TLRS-4 at a new site about 100 meters from the old site. The system began producing data also in late 2006. A rededication of this site was held in late January 2007. Both stations have produced sufficient LAGEOS data to verify their performance. Staffing reductions persist at the MLRS (McDonald) and MOBLAS-7 (GSFC) and to a lesser extent at MOBLAS-4 (Monument Peak). The partner stations at Yarragadee, Hartebeesthoeck, and Tahiti are unaffected.

ILRS Network Map

Figure 1. ILRS network (as of June 2007)

The Mt. Stromlo station has been fully operational since its reconstruction after the forest fire in early 2003. The station is now the second largest data producer in the ILRS network after Yarragadee. The two Australian stations together produced about 14,000 passes in 2006.

The Chinese SLR network continues its very strong support for the ILRS network. The Changchun station maintained its exceptional performance with activities underway now to help strengthen daylight ranging. The new Shanghai station is now in operation after relocation; data yield is steadily improving. The new Chinese SLR station in San Juan, Argentina has performed impressively since beginning operations in March 2006 and has risen to one of the six largest producers of data in the network. This station has helped a great deal in the laser ranging coverage in Southern Hemisphere.

Improvements have also been realized in other stations in the ILRS network. The station in Riyadh, Saudi Arabia continues its impressive tracking operations. This is the only station that in the ILRS located on the Arabian Peninsula, so its importance cannot be understated. The TIGO system in Concepción, Chile has undergone substantial repairs and is now back in operation. Data yield has steadily increased over the last few years, but the station is fighting difficult weather conditions. The location of this station in South America should help greatly in the Southern Hemisphere coverage. The Graz system continues its impressive performance with 2kHz operation, a technology that will most likely become more prevalent in the network as time goes on. A 2kHz laser has also been purchased for implementation into the Herstmonceux station; several other stations are seriously considering this upgrade. The TIGO system in Concepción, Argentina and the upgraded Zimmerwald station continue with two-wavelength ranging using a titanium-sapphire laser operating at 423nm and 846nm to test this as a means for improving the atmospheric refraction correction. The station at Grasse, France has temporarily closed for major upgrading. The French Transportable Laser System (FTLRS) is now being readied for relocation to Burnie, Tasmania) to support altimeter calibration and validation. The storm damage has been repaired the GUTS facility in Tanegashima, Japan and operations have been underway, but data yield is still sparse.

Lunar Laser Ranging (LLR) Network

During the Apollo missions the astronauts deployed laser retro-reflectors near their landing sites, which are in continued use up to the present day. Today, the results from Lunar Laser Ranging (LLR) are considered among the most important science return of the Apollo era. The lunar laser ranging experiment has continuously provided range data for more than 37 years. The main benefit of this geodetic technique is the determination of a host of parameters describing lunar ephemeris, lunar physics, the Moon’s interior, various reference frames (the terrestrial and selenocentric frame, but also the dynamic realization of the celestial reference system), the Earth-Moon dynamics as well as the verification of metric theories of gravity and gravitational physics, such as the equivalence principle or any time variation of the gravitational constant.

The site operated by the Observatoire de la Côte d’Azur (OCA), France has collected the majority of the LLR data in recent years. The LLR station at the McDonald Observatory in Texas, USA is another major provider of the LLR data. Until 1990, the Haleakala laser ranging station on the island of Maui (Hawaii, USA) contributed to LLR activities with its 40cm telescope. Other stations with promising potential in lunar ranging are Wettzell, Matera, and Mount Stromlo. The new APOLLO lunar ranging station at the Apache Point Observatory (New Mexico, USA) took some data in 2006 with very impressive results. This station is designed for mm accuracy ranging. Today, MLRS (and OCA) are the only currently operational LLR sites achieving a typical range precision of 18-25mm, hopefully further sites may provide lunar data on a routine basis soon. Current LLR data is collected, archived and distributed under the auspices of the International Laser Ranging Service (ILRS).

ILRS TRACKING PRIORITIES AND MISSION SUPPORT

The ILRS is currently tracking 28 artificial satellites including passive geodetic (geodynamics) satellites, Earth remote sensing satellites, navigation satellites, and engineering missions (see Table 2). The stations with lunar capability are also tracking the lunar reflectors. In response to tandem missions (e.g., GRACE-A/-B) and general overlapping schedules, stations have begun tracking satellites with interleaving procedures.

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.

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.

Missions are added to the ILRS tracking roster as new satellites are launched and as new requirements were adopted. New missions included ANDE-RR, an atmospheric modeling satellite, and GIOVE-A the first engineering test satellite for the Galileo series. The network also supported a short calibration campaign on the ALOS satellite with optical sensors for terrestrial mapping. The ETS-8 synchronous satellite was also launched to a location over the western Pacific, but tracking was delayed until early 2007 while the satellite underwent engineering readiness tests. Missions for completed programs were deleted from the ILRS tracking list. The TOPEX/Poseidon project ended in 2006 after a remarkable 13 years of service, with SLR providing the sole source of POD during its last year of operations. SLR was the sole means of POD for the SAGE experiment on Meteor-3M which ended in 2006. The GP-B 18-months campaign also ended in 2006.

At one time, the main task of the international SLR Network was the tracking of dedicated geodetic satellites (LAGEOS, Starlette, etc.). Although the ILRS has 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 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 June 2007)

Satellite Priorities
Priority	Mission	Sponsor	Altitude (km)	Inclination (degrees)	Comments
1	TerraSAR-X	Infoterra/DLR/ GFZ/CSR	514	97.44	First priority for acquisition phase only
2	GRACE-A, -B	GFZ/JPL	485-500	89	Tandem mission
3	CHAMP	GFZ	429-474	87.3
4	GFO-1	US Navy	790	108.0	Altimetry/no other tracking technique
5	Envisat	ESA	796	98.6	Tandem with ERS-2
6	ERS-2	ESA	800	98.6	Tandem with Envisat
7	Jason-1	NASA/CNES	1,350	66.0
8	ANDE-RR Active	NRL	400	51.6
9	ANDE-RR Passive	NRL	400	51.6
10	Larets	IPIE	691	98.2
11	Starlette	CNES	815-1,100	49.8
12	Stella	CNES	815	98.6
13	Ajisai	NASDA	1,485	50
14	LAGEOS-2	ASI/NASA	5,625	52.6
15	LAGEOS-1	NASA	5,850	109.8
16	Beacon-C	NASA	950-1,300	41
17	Etalon-1	Russian Federation	19,105	65.3
18	Etalon-2	Russian Federation	19,135	65.2
19	GLONASS-99	Russian Federation	19,140	65	Replaced GLONASS-86 on 03/20/2003
20	GLONASS-95	Russian Federation	19,140	65	Replaced GLONASS-87 on 01/12/2007
21	GLONASS-102	Russian Federation	19,140	65	Replaced GLONASS-84 on 08/26/2005
22	GPS-35	US DoD	20,100	54.2
23	GPS-36	US DoD	20,100	55.0
24	GIOVE-A	ESA	29,601	56
Lunar Priorities
Priority	Retroreflector Array	Sponsor	Altitude (km)
1	Apollo 15	NASA	356,400
2	Apollo 11	NASA	356,400
3	Apollo 14	NASA	356,400
4	Luna 21	Russian Federation	356,400

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 (e.g., TerraSAR-X in 2007) 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, 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 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.

Table 3. Upcoming Missions (as of June 2007)

Mission	Sponsor	Planned Launch Date	Mission Duration	Altitude (km)	Inclination (degrees)	Application
GIOVE-B	ESA	2007		23,916	56	Radio navigation satellite system
GOCE	ESA	2007		250	96.5	Earth's gravity field and geoid modeling
Jason-2	NASA/CNES/ Eumetsat/NOAA	June 2008		1,336	66	Oceanography, T2L2
LRO-LR	NASA	October 2008		Lunar	N/A	POD for LRO
NPOESS	NOAA/NASA/DoD	2013		833	98.7	Sea surface height
PROBA-2	ESA	December 2007		721	98	Technology validation

OFFICIAL ANALYSIS PRODUCTS

The ILRS products consist of SINEX files of weekly station coordinates and daily Earth Orientation Parameters (x-pole, y-pole and excess length-of-day, LOD) estimated from 7-day arcs. Two types of products are distributed each week: a loosely constrained estimation of coordinates and EOP and an EOP solution, derived from the previous one and constrained to an ITRF, currently ITRF2000. Official ILRS Analysis Centers (AC) and Combination Centers (CC) generate these products with individual and combined solutions respectively. Both the individual and combined solutions follow strict standards agreed upon within the ILRS Analysis Working Group (AWG) to provide high quality products consistent with the IERS Conventions 2003. This description refers to the status as of January 2007. Each weekly solution is obtained through the combination of weekly solutions submitted by the official ILRS Analysis Centers:

ASI, Agenzia Spaziale Italiana
BKG, Bundesamt für Kartographie und Geodäsie
DGFI, Deutsches Geodätisches Forschungsinstitut
GA, Geosciences Australia GFZ, GeoForschungsZentrum Potsdam
JCET, Joint Center for Earth Systems Technology
NSGF, NERC Space Geodesy Facility

These ACs have been certified through benchmark tests developed by the AWG. The official Primary Combination Center (ASI) and the official Backup Combination Center (DGFI) follow strict timelines for these routinely provided products.

In addition to operational products, solutions have been provided covering the period back to 1993. A current effort is underway to extend the time series as far back as the mid 1970s. The ILRS products are available, via ftp from the official ILRS Data Centers CDDIS/NASA Goddard (ftp://cddis.gsfc.nasa.gov/) and EDC/DGFI (ftp://ftp.dgfi.badw-muenchen.de).

ILRS Contribution to ITRF2005

The time series of weekly solutions from 1993 to 2005, produced by the Primary Combination Center, was delivered to IERS/ITRS as an official ILRS contributed data set for ITRF2005. Several months of joint work within the ILRS AWG were devoted to the quality assessment of the contributing solutions from the ILRS ACs as well as the final combined solutions from the ILRS CCs. The final version of the combined ILRS time series was submitted in December 2005. Figures 2 and 3 show the origin and scale differences with respect to the old ITRF realization, ITRF2000.

These time series are essentially equivalent to the ones that have been generated operationally since after January 2007 and those to be soon reissued for the period of 1993-2006 and only differ from that in the applied tropospheric model (Marini-Murray for the old vs. Mendes-Pavlis for the new series) and the modelling of a range bias due to Stanford event timers’ non-linearities for a subset of stations. The description of the official contribution to ITRF2005 is available at http://itrf.ensg.ign.fr/ITRF_solutions/2005/doc/ILRS_ITRF2005_description.pdf.

Figure 2. Origin offsets from the official weekly ILRS product with respect to ITRF2000.

Scale factor from the official weekly ILRS product with respect to ITRF2000

Figure 3. Scale factor from the official weekly ILRS product with respect to ITRF2000.

Comparison of the ILRSA and ILRSB Combinations

The official ILRSA combination solution produced by ASI is routinely compared with the backup combined solution ILRSB produced by DGFI following a fundamentally different approach. The results show a good agreement between the two solutions and absence of any systematic differences. The tables and figure below briefly show this agreement in terms of:

mean 3D wrms of the site coordinates residuals with respect to ITRF2000 (Table 4 and Figure 4);
mean differences of the translation and scale parameters with respect to ITRF2000 computed using the two time series ILRSA and ILRSB (Table 5);
EOP residuals with respect to EOPC04 (Table 6) for the year 2006.

Table 4. 3D wrms of the site coordinates residuals w.r.t. ITRF2000

	ILRSA(mm)	ILRSB(mm)
All sites (mean)	21.5	26.0
Core sites (mean)	8.0	10.1

3D wrms of the core site coordinates residuals with respect to ITRF2000

Figure 4. 3D wrms of the core site coordinates residuals with respect to ITRF2000.

Table 5. Translation and scale (with respect to ITRF2000) differences between ILRSA and ILRSB

	TX (mm)	TY (mm)	TZ (mm)	Scale (mm)
Weighted Mean	1.14±0.18	-0.24±0.18	-0.10±0.41	0.05±0.26
WRMS	3.14	2.27	4.03	3.13

Table 6. EOP daily residuals with respect to EOPC04 for ILRSA and ILRSB

	ILRSA		ILRSB
	WMEAN	WRMS	WMEAN	WRMS
EOP-X (mas)	-0.055	0.153	0.020<	0.175
EOP-Y (mas)	0.193	0.153	0.244	0.197
LOD (ms)	0.003	0.047	-0.003	0.054

The individual as well as the combinations of the ILRS ACs and CCs are monitored on a weekly basis with a graphical and a statistical presentation of these time series through a dedicated web site hosted by the JCET AC at http://geodesy.jcet.umbc.edu/ILRS_QCQA/.

MEETINGS AND REPORTS

The ILRS organizes semiannual meetings of the Governing Board and General Assembly; General Assemblies are open to all ILRS Associates and Correspondents. These meetings are typically held in conjunction with ILRS workshops, such as the fall technical workshops (oriented toward SLR practitioners) or the biannual International Workshop on Laser Ranging. A summary of recent ILRS meetings is shown in Table 7. Detailed reports from past meetings can be found on the ILRS Web site.

Table 7. Recent ILRS Meetings (as of June 2007)

Timeframe	Location	Meeting
October 2003	Kötzing, Germany	ILRS Technical Workshop “Working Toward the Full Potential of the SLR Capability”
April 2004	Nice, France	Analysis Working Group Meeting
June 2004	San Fernando, Spain	14^th International Workshop on Laser Ranging 10^th ILRS General Assembly and WG Meetings Analysis Working Group Meeting
October 2004	Graz, Austria	ILRS Technical Workshop “kHz SLR Technology”
April 2005	Vienna, Austria	ILRS Working Group Meetings Analysis Working Group Meeting
October 2005	Eastbourne, UK	ILRS Technical Workshop “Observations Toward mm Accuracy” 11^th ILRS General Assembly and WG Meetings Analysis Working Group Meeting
April 2006	Vienna, Austria	ILRS Working Group Meetings Analysis Working Group Meeting
October 2006	Canberra, Australia	15^th International Workshop on Laser Ranging 12^th ILRS General Assembly and WG Meetings Analysis Working Group Meeting
April 2007	Vienna, Austria	ILRS Working Group Meetings Analysis Working Group Meeting
July 2007	Perugia, Italy	Analysis Working Group Meeting
September 2007	Grasse, France	ILRS Technical Workshop “Challenges for Laser Ranging in the 21^st Century” 13^th ILRS General Assembly and WG Meetings Analysis Working Group Meeting

ILRS Biannual Reports summarize activities within the service over the period since the previous release. They are available as hard copy from the CB or online at the ILRS Web site.

ILRS Analysis Center reports and inputs are used by the Central Bureau for 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. These results include independent assessments of station performance from several of the ILRS analysis/associate analysis centers. The statistics are presented in tabular form by station and sorted by total passes in descending order. Plots of data volume (passes, normal points, minutes of data) and RMS (LAGEOS, Starlette, calibration) are created from this information and available on the ILRS Web site. Plots, updated frequently, of multiple satellite normal point RMS and number of full-rate points per normal point as a function of local time and range have been added to the ILRS Web site station pages.

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INTERNATIONAL LASER RANGING SERVICE (ILRS)

M. R. Pearlman 1, C. E. Noll 2 and W. Gurtner 3

M. R. Pearlman ¹, C. E. Noll ² and W. Gurtner ³