International Laser Ranging Service

Sixth General Assembly of the International Laser Ranging Service (ILRS)

Nice, France
Wednesday, March 28, 2001

The Sixth General Assembly of the International Laser Ranging Service (ILRS) was held in Nice, France on March 28, 2001, in conjunction with the European Geophysical Society XXVI General Assembly. The agenda and the list of attendees for the Sixth General Assembly of the ILRS are included as Attachments 1 and 2.

Governing Board

The Governing Board (GB) Chair, John Degnan welcomed the participants and briefly reviewed some of the key developments over the past year (see Attachment 3). The ILRS celebrated its second anniversary this past September. A new Governing Board was elected and John Degnan was reelected as Chairperson. New Coordinators and Deputy Coordinators were chosen for the Working Groups. The Central Bureau, at GSFC, continued to oversee daily operations, communications, and implement policies set forth by the Governing Board. The ILRS Annual Report for 1999 was issued and the report for the year 2000 is in process.

The Working Groups have been very active. The Missions Working Group has formalized procedures for approving new missions. The Analysis Working Group has pilot projects underway to compare results from different analysis centers and to develop formal ILRS solutions for the IERS. The Data Formats and Procedures Working Group is streamlining and modernizing data formats and procedures to better handle new mission requirements and technologies. The Networks and Engineering Working Group has been upgrading procedures to expedite data flow and developing engineering data bases to improve network performance. The Signal Processing Ad Hoc Working Group and the Refraction Study Group are working to reduce the remaining systematic ranging errors from sources such as satellite signatures and refraction.

Network improvements continue in the Southern Hemisphere with the new stations in Canberra, Tahiti, and Hartebeesthoek. The new German TIGO system is planned for Concepcion, Chile later this year. Negotiations continue for the placement of a Chinese or US system in Argentina. Several new systems are under development in China and Russia. Fully automated systems are under development at both NASA and EOS (Australia).

Central Bureau

Satellite Requirements

Mike Pearlman reported on a Satellites Requirements Survey conducted by the Central Bureau to determine user needs and ILRS responsiveness to user needs (see Attachments 4 and 4a). We want to make sure that the data we acquire is being used and that we are responding to the requirements of our community.

Analysis Centers and Associate Analysis Centers were asked which satellites they use in their analyses, what they do with the data, and are we fulfilling their needs. Eighteen centers responded prior to the meeting (a couple more responded afterward). Satellite applications included EOP, reference frame, gravity field, station position and motion, relativity tests, POD for altimeter satellites, Q/C for stations, spacecraft modeling, and technique intercomparisons. Lunar applications included EOP, relativity, lunar gravity field, lunar ephemeris, tidal accelerations, and other lunar science. All of the satellites currently tracked by the ILRS are being used by at least two centers; most of the satellites are being used by four or more. General comments were not enough high satellite data, not enough Southern Hemisphere data, and too many weak stations in the network. The users of the lunar data wanted better stations and target coverage. It was noted that recent activities to build up Southern Hemisphere coverage and to categorize stations by performance (see below) are addressing some of the major comments.

Station Qualifications

The ILRS has the responsibility to provide the best possible data products to data users within the practical constraints of its operational entities. This service starts with the data production at the ILRS field stations. Presently there is a wide divergence in the performance of these stations; some are achieving very impressive performance levels, while others are not doing as well. Although station performance is improving over time, many stations still do not provide the consistent data quantity and quality necessary to support the studies of the data users. The shortcomings are due in part to weather and staffing levels, but there are also system, procedural, and financial issues that detract from performance. At the moment there is no station performance threshold for ILRS participation, nor is there any formal discrimination between the high and lower performing systems within the ILRS network. Data users, including those who are generating products for the IERS, ITRF, etc. and those who are doing technique intercomparisons receive all of the network data without any qualification. Each analyst makes his own evaluation of the data.

Mike Pearlman reported on the progress of a small study group, established at the Networks and Engineering Working Group Meeting in Matera in November 2000, to make a recommendation on the Qualification for ILRS Participation by SLR Stations (see Attachment 5). The group has agreed that the ILRS should remain inclusive (room for everybody), but the stations should be categorized on the basis of performance. Stations should be categorized as "Operational Stations" or "Associate Stations" based on whether or not they provide useful data to the analysis community. The criteria to distinguish useful data are now being defined by the Analysis Working Group and the Central Bureau. The threshold is not intended to be strenuously high, but the data must be of sufficient quantity and quality to have a positive impact on the solutions being generated. The procedure for new stations to join the Operational Network is also being defined. Associate Stations would include stations under development and under long protracted programs of upgrading. It would also include stations that are not performing well. Associate Stations would be encouraged to submit their data as it is taken so that analysis groups could provide scrutiny and advice.

Within the network of Operational Stations, we also plan to designate a core network of high performing stations that meet the ILRS Performance Criteria (see Attachment 5). A process of annual review to add and delete core stations and to reconfirm operational status for all stations would be implemented. The various categories of stations would be highlighted on the ILRS maps and charts.

Operational Issues

Van Husson reviewed ILRS operational issues (see Attachment 6). The new daily (and in some cases subdaily) prediction capability is now operational along with the new prediction exploder. It was agreed that we will place time bias, satellite maneuvers, and drag functions into separate exploders to better accommodate specific user needs. The latest Station Global Report Card has been issued; it was noted that the performance of the Chinese Network has improved significantly. Most stations are routinely submitting their Station Status Report. Nearly 18,000 ILRS web pages have been accessed by 1739 hosts so far in 2001. Site Logs have been submitted by all but the Russian systems. (The Site Logs for the Keystone systems were submitted just before the meeting.) A Site Log master file has been created and many format and data integrity issues have been resolved.

Data delivery continues to improve. Only a few stations have data latency beyond a day and most are reporting within a few hours. Network data volume has leveled off to a little more that 70,000 passes per year. We need to see how we can increase this. The "weekend effect" does not appear to be very significant, at least at the summary level view. It should also be noted that about 35% of the ranging data are in daylight.

Network Reports

EUROLAS

Werner Gurtner gave an overview of EUROLAS (see Attachment 7). The computer control system and the event timer was changed in the WLRS, yielding improvements in range stability. The telescope control system is also in the process of being replaced. Discussions on the revival of MTLRS-1 continue. Data quantity and quality continue to improve at the San Fernando station. Last year the station acquired over 400 passes on LAGEOS. The station is now getting some daylight passes on LEO satellites. The new calibration scheme inside the dome has been implemented and tests are underway with the new SPAD. The Cagliari station is down until mid-year for improvements in pointing and ranging accuracy. In Zimmerwald, the small telescope mirrors have been recoated with broadband dielectrics. Also, after the laser output was found to be significantly broadened, the pulsewidth was readjusted back to 100 ps, reducing the single shot rms. MLRO continues acceptance testing at Matera.

WPLTN

Hiroo Kunimori reviewed the WPLTN status (see Attachment 8). The two Australian SLR systems have been the highest performers in the network. Mt Stromlo is operating autonomously about half the time. The definitive report on the local ties between Mt. Stromlo, Orroral, Yarragadee, Hobart, and Tidbinbilla has been published (http://www.auslig.gov.au/geodesy/techrpts/localsvy/localtiesv4.pdf). The Chinese stations report several problems. Laser problems in Beijing, dome problems in Wuhan, and detector problems in Kunming are all being worked on. The Chinese SLR system for Argentina will be completed later this year. The site still needs to be developed. The operation of the Keystone stations in Japan is being suspended until the new budget year. The telescope from the Miura station has been dismantled and sent to the University of Kagoshima. JHD has made available the data from the HTLRS occupation of remote islands in the 1990's. The Simosato stations had mount problems, and the motors were replaced. LEO satellite performance has improved by lowering minimum elevation and optimizing shifts. The new NASDA SLR system (GUTS) in Japan is under development.

In Russia, the telescope pointing system at Komsomolsk is inoperative at the moment. A new telescope is planned for installation in early 2002. The Maidanak-1 system will be upgraded in late 2001 with new computers, pointing system, laser, and other components. The Maidanak-2 system has been upgraded and engineering measurements are underway. The station should be fully operational by mid-year. The SALRO is now operating and has provided a considerable amount of data for engineering scrutiny. A special symposium is scheduled for Riyadh on September 23-24 to commemorate the SALRO recommissioning.

NASA Network

John Bosworth reported on the NASA Network (see Attachment 9). The HOLLAS system is in the midst of a telescope control system replacement. Replacement should be completed by mid-year. In the meantime, operations continue on a one-shift basis. McDonald, Greenbelt, Monument Peak, and Arequipa are operational. In the partner stations, Yarragadee is operational. Hartebeesthoek was dedicated in November 2000 and is now operating seven days per week. The system in Tahiti is down with mount control problems. Engineers are due on-site in early April to repair and realign the system. TLRS-4 is in caretaker status while NASA seeks a Southern Hemisphere site, possibly in Argentina.

Work continues on the SLR2000. The prototype is expected to begin field testing in the Fall of 2001. Twelve production units are planned for fabrication.

Data Center Report

Wolfgang Seemueller gave an update on the ILRS Data Centers (see Attachment 10). The hourly data deliveries between EDC and CDDIS are working routinely. The new prediction exploders have been installed at CDDIS with backup at EDC tested. Erricos Pavlis and Van Husson have updated the eccentricity file, cleaning up many old problems. We need a single eccentricity file. We need to complete the file of station performance anomalies and update communications between station operators and Data Centers. Management and archiving of mail within ILRS should be improved.

Working Group Reports

Missions

Hiroo Kunimori gave the Missions Working Group report (see Attachment 11). The Working Group recommended (and the Governing Board approved) the extension of the Be-C Campaign through December 31, 2001 and the transition of GFO-1 from campaign to routine tracking status. GLONASS 84 has been added to the tracking roster for IGLOS. GLONASS 84 has a smaller, supposedly more efficient retroreflector, that uses the Fizeau effect. The new array, with about two thirds fewer cubes, is expected to give nearly the same return signal strength as the earlier satellites. Once again there was considerable confusion in the numbering of the satellite, with inconsistencies between NORAD, the World Data Center, and INTERCOSMOS. The problem was "resolved" through the orbital elements. A number of stations have already taken data on the satellite indicating that there is some cube enhancement.

The upcoming ENVISAT and ADEOS-II missions were approved. Tracking support was requested by NRL for the Starshine 2 and 3 satellites to be launched later this year for a space education program. The satellites would have polished mirrors for visual observations from the ground and a couple of dozen one-cm retroreflectors for ranging. The Working Group had some questions on the viability of the mission: Could the mirrors be seen from the ground with reasonable optics? Is the cube design adequate to overcome the velocity aberration? Is the signal link adequate for ranging? The Working Group is making an inquiry to NRL prior to any further action. Mike Pearlman will contact Elango to find out what's happening with the Indian IRS-P5 satellite.

The Working Group plans to implement a procedure to track user (customer) satisfaction. It was noted that the Central Bureau has already queried the analysis community on satellite data usage and ILRS responsiveness to their needs (see Attachments 4 and 4a). New Mission Request Forms are being put on the Website as the Central Bureau receives them. Mission Support Plans need to be developed for all of the new missions.

Kunimori introduced the H2A-LRE, a mission in a highly eccentric orbit for ranging calibration (over a wide range of distances), study of satellite spin evolution, and lifetime tests of inexpensive BK7 retroreflectors. The mission objective also included gravity field and atmospheric drag studies. H2A-LRE is a mission of opportunity on the new H2A rocket. The satellite is in final integration and launch could be as early as July 2001 if it is authorized. The Working Group had some reservation concerning the gravity field and atmospheric studies, but it agreed that once a Mission Support Request Form is submitted, it would endorse a one month campaign to see what kind of data would be forthcoming and how useful the data would be. Subsequent campaigns could then be planned. Chris Reigber suggested that military radar tracking, similar to that provided to CHAMP, could be beneficial at least in the early stages of the program.

The Missions Support Request Forms for several upcoming missions needs to be nurtured along. The need for a CCR array specification database for satellite signature studies was introduced. It was pointed out that we need a more comprehensive data set to address not only satellite signatures, but also to provide users on-line access to full satellite center-of-mass corrections. The Central Bureau will work with the Signal Processing and Missions Working Groups to develop a plan on how to proceed. Mike Pearlman, Graham Appleby, Van Husson, Scott Wetzel and a couple of the people from the analyst community will meet to address this issue.

Networks and Engineering

Werner Gurtner presented the current activities of the Networks and Engineering Working Group (see Attachment 12). As reported earlier, the Prediction Exploders requested by the working group have been implemented. The data submission backup was implemented and successfully tested.

A draft of a Station Qualification Plan has been circulated to the Governing Board and presented to the working group (see Attachment 5). Issues yet to be resolved are:

1. Reexamine whether the current LAGEOS tracking requirement should be relaxed for participation in the ILRS?
2. Develop a quantified threshold for qualification as an "Operational" station?

The site log implementation has progressed very well. Only a few stations, mainly in Russia, have yet to comply. The process, including a site log master file, has facilitated a major cleanup of station information discrepancies.

New systems just deployed or soon to be deployed are offering considerable improvements in performance and operations. The MLRO is on-site at Matera and undergoing acceptance testing. Aside from enhanced SLR operations with two-color ranging, this station should also provide a dramatic augmentation to international lunar ranging activities. The TIGO, the Transportable Integrated Geophysical Observatory, is being prepared by BKG in Germany for installation in Concepcion, Chile, later this year. This should help to provide better Southern Hemisphere coverage for the full suite of geodetic instruments included. Work continued on the development of the NASA SLR 2000 prototype. After field testing in 2002, the fully automated SLR system will be replicated and deployed. It is anticipated that a dozen production units will be built for NASA alone.

The report from the Calibration Workshop in Florence in September 1999 has been distributed by Ulli Schreiber. Planning continues for the next ILRS Workshop on September 19 - 21, 2001, in conjunction with the SPIE Remote Sensing Symposium in Toulouse, France. The theme of this workshop will be improving and verifying ranging accuracy. The meeting will include interdisciplinary workshop sessions where members of the different ILRS Working Groups work together to address some of the key issues currently limiting SLR performance. A full ILRS General Assembly will be held on the morning of September 21. Ulli Schreiber, John Degnan, Mike Pearlman, and several others will constitute the program committee.

At the request of the Working Group, Van Husson and Mike Pearlman are looking into the organization of an on-line station performance report integrating much of the information now available in the Global Report Card and the Weekly Station Status Reports.

Data Formats and Procedures

Wolfgang Seemueller presented reports provided by Stefan Riepl and Randy Ricklefs on the activities of the Refraction and Prediction Format Study Groups (see Attachments 13a, 13b). Neither Riepl not Ricklefs were able to attend the meeting.

Recent analysis of LAGEOS and LAGEOS 2 data by Richard Eanes' indicated that a bias as large as a centimeter might exist in the zenith delay term of the Marini-Murray model. Erricos Pavlis used the 1999-2000 data from these satellites to perform a validation of a new mapping function developed by Virgilio Mendes and colleagues. The new mapping functions are based on radiosonde data from 180 sites globally distributed. These mapping functions have also been validated with independent radiosonde data from a two-year period. Curiously, the results also indicate a ~1 cm bias for the zenith delay computation with the Marini and Murray and the Saastamoinen models. The analysis of the LAGEOS data that Erricos Pavlis presented, show that the mapping function embedded in the Marini and Murray model and currently used in SLR may be in error by a few mm (~5 mm) down in the elevation range of 10 - 15 degrees. The new mapping functions provide some improvement at these elevations and extend their validity down to 3 degrees above the horizon. An alternative model, which is an adaptation of a model by H. Yan, was also compared by Mendes' group to the radiosonde data. Their study indicates that this model is not nearly as significant an improvement over Marini and Murray as is the Mendes mapping functions. R. Hill from NOAA has also made available on line an improved refraction index calculation. Riepl is studying the effects of horizontal gradients. The new (Mendes) mapping function code will be made available on the Study Group web page very shortly for wider testing. Further details on the Study Group activities can be found at:

http://www.wettzell.ifag.de/publ/rsg/rsg.html.

Randy Ricklefs provided a report on the activities of the Prediction Formats Study Group. This group is working to develop a single laser ranging prediction format for Earth satellites, Lunar reflectors, and laser transponder targets on other solar system bodies and spacecraft in transit. The mix of requirements poses a challenge, but if a reasonable solution can be found, it would allow us to consolidate and simplify the general prediction system. The Study Group is considering tabular or polynomial files that would include time and geocentric state vectors at separations required to achieve optimal precision for specific targets. Many issues still need to be addressed.

Analysis

Ron Noomen gave the Analysis Working Group Report (see Attachment 14). The ILRS serves a variety of customers, both internal (such as stations and analysis centers) and external (such as IERS and the geophysical community). The importance of the fundamental capabilities and contributions of SLR has been recognized by IERS in its generation of ITRF2000, a state-of-the-art model for the terrestrial reference frame: SLR provides the definition of the origin of ITRF2000, and SLR and VLBI together define absolute scale. Historically, IERS has done the combination of solutions from each technique in its process of developing official solutions, like the ITRF terrestrial reference frames, or Earth Orientation Parameter (EOP) solutions. With the reorganization of IERS, this responsibility is migrating to the geodetic services. Each service is expected to provide a single, best solution for EOP and coordinates as its contributions to the official IERS products,. In addition, each technique is responsible for its more general quality control issues.

Within the Analysis Working Group, a number of pilot projects are underway to better understand the differences in the individual center solutions and to work towards the single best solution. At the moment, four different pilot projects are in various stages of development, addressing (1) EOP and site coordinates and velocities, (2) orbits, (3) software benchmarking and (4) QC results, respectively. The first project mentioned here currently uses a test data set covering thirteen 28-day periods on LAGEOS-1 and -2, which has now been expanded to add Etalon-1 and -2. The SINEX format for exchange of solutions has been a hurdle, but, with some special tailoring, it is now being used well. The participating groups are showing great enthusiasm and a bit of competition. There is still some unexplained variation in results among the groups, and testing continues. Modeling differences and fitting procedures are being examined more closely. To date, five Analysis Working Group Workshops have been held, the last one being in Nice just prior to EGS. At this workshop, ten solutions and seven combination/comparisons were presented.

At the request of the Analysis Working Group, the ILRS Governing Board has approved a six-month campaign on Etalon-1 and -2 to improve SLR derived observations of EOPs. In addition, the Etalon data will improve the determination of GM, range and frequency biases and other parameters.

The next Analysis Working Group Workshop is scheduled for Toulouse, France, on September 17 - 18, in conjunction with the ILRS Symposium.

Signal Processing (A/H)

Graham Appleby provided an overview of the activities of the Signal Processing Working Group (see Attachment 15). The study group is tasked with the production and verification of center-of-mass corrections for all of the retroreflector-equipped satellites that we are tracking. Their initial task was to provide corrections for the spherical satellites for the main observing systems. They are presently modeling the single photon response of the spherical geodetic satellites; using ERS-2 full-rate data from Herstmonceux to represent the tracking system response. Application to Ajisai, LAGEOS and Etalon is underway. Use of data from Graz is also being considered. The Working Group is looking for additional active members.

Spin motion of Lageos-2 is being studied via photometry at Herstmonceux and a satellite model (Otsubo, Wood et al.). They are also continuing to monitor GPS and GLONASS microwave orbits to study the characteristics of the different arrays. A request has been made to the Missions W/G for more array information on GLONASS-84.

The Working Group reports including those on Etalon and GLONASS array information can be found at:

http://nercslr.nmt.ac.uk/sig/signature.html

Current Missions and Priorities

Mike Pearlman discussed the current missions and their priorities (see Attachment 16). Several changes have been made to Mission Priorities since the last meeting. GFO-1 has now been approved as a routine tracking object. Sunsat has failed. The Be-C campaign has been extended through 31 December 2001 to support the analysis work at CSR. A third GLONASS satellite, GLONASS-84, has been added to support IGLOS. At the request of the Analysis Working Group, a six-month campaign on Etalon 1 and 2 has been approved starting April 1 to support improved EOP from SLR and better definition of fundamental constants.

The IGLOS Campaign continues to study the difference between SLR and microwave orbits and to isolate system dependent errors.

ERS-2 continues to function well after nearly six years. ESA plans to operate the satellite at least until the end of the ENVISAT Commissioning Phase in the spring of 2002. Although SLR is the secondary tracking system on ERS-2, the age of the PRARE ground equipment and the backlog on system repair has placed increasing importance on SLR.

Overview of Upcoming Missions

Scott Wetzel reviewed the status of upcoming missions requiring ILRS support (see Attachment 17a, b, c).

Envisat

Envisat should be on its way to the launch site in early April. Launch is scheduled for July, but there is some indication that this may be delayed slightly. The ILRS will provide intensive SLR tracking during the six-month Commissioning Phase (which includes global cross-calibration ERS-2), and then routine tracking thereafter. POD will rely on the combination of DORIS and SLR.

Jason

Jason is scheduled for shipment to Vandenberg in June and for launch in August. Intensive SLR tracking is planned for the first six month verification phase in which Jason and TOPEX/Poseidon will be orbiting in close formation (1 to 10 minutes apart). Routine SLR tracking will be required thereafter. POD during both the verification and follow-on routine tracking phase will use a combination of GPS, DORIS, and SLR. SLR will provide scale and centering of the orbit, vertical phase center offset verification for DORIS and GPS, and absolute calibration of the altimeter. Predictions will be provided by HTSI.

Starshine

Starshine 2 and 3 are student participatory missions sponsored by NRL. The surfaces of these satellites are nearly covered with small mirrors polished by students for ground-based visual observation in twilight. Small retroreflectors are also being placed on the surface to support laser ranging. NRL has organized student-observing activities around the world to participate. Since these are launches of opportunity, Starshine 2 is planned for launch in November 2001 and Starshine 3 (now ahead of Starshine 2) in August 2001. The ILRS Missions Working Group is very supportive of educational missions, but some technical questions on mission viability are being posed to NRL before the formal approval process is undertaken.

H2A-LRE

Hiroo Kunimori discussed the H2A-LRE mission earlier. The Missions Working Group had some reservation concerning the gravity field and atmospheric studies, but it agreed that once a Mission Support Request Form is submitted, it would endorse a one month campaign to see what kind of data would be forthcoming and how useful the data would be. Subsequent campaigns could then be planned.

Joint IVS/IGS/ILRS Working Group

Graham Appleby reported on the Joint IVS/IGS/ILRS Working Group (see Attachment 18). The IVS community is discussing the technical possibilities and limitations of using the VLBI network to map the microwave energy distribution from GPS satellites. Recent correspondence suggests that if the discussions conclude that VLBI observations are viable, the SLR technique may be proposed as a check/QC on the X-Y components of satellite positions that might be derived by VLBI along with the Z-component of the phase center. SLR of course is not very sensitive to X-Y position, but may help to determine satellite attitude.

Annual Report for 2000

Mike Pearlman outlined the plan for submissions of contributions for the 2000 ILRS Annual report (see Attachment 19). The rules are: be brief and concise; one version for both the paper and web reports; liberal use of web site links and references; fast turnaround, and focus on progress during the year and future plans. Contributions are due on April 30.

Next Meeting

The next ILRS Governing Board and General Assembly meetings will be held in Toulouse, France during the week of September 21, 2001 in conjunction with SPIE Remote Sensing Symposium.

Editor's Note: If you would like to receive the attachments, please send an email request to Mike Pearlman (mpearlman@cfa.harvard.edu). These minutes are also available in Microsoft-Word (minutes in MS Word).