International Laser Ranging Service

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

Florence, Italy
Wednesday, September 20, 1999

Editor's Note: Most Attachments are available on-line and all Attachments are available in hard-copy. Please contact Dr. Michael Pearlman (email Pearlman@cfa.harvard.edu), ILRS Central Bureau Secretary, if you would like a copy of an attachment.

The Third General Assembly of the International Laser Ranging Service (ILRS) was held in Florence, Italy on October 20, 1999, in conjunction with the EOS/SPIE Symposium on Remote Sensing and the Colloquium on SLR System Calibration Issues. The agenda for the meeting and the list of attendees are included in Attachments 1 and 2.

The Governing Board (GB) Chair, John Degnan, welcomed the participants and reviewed proposed changes to the ILRS Terms of Reference (see Attachment 3). Changes focused mainly on: clarification of the ILRS position within the IAG, provisions to amend the Terms of Reference, recognition of Lunar Analysis Centers, clarification of provisions to nominate and elect Governing Board members, leadership of Ad Hoc Working Groups, and clarification of ILRS membership. The revised document will be circulated via SLRmail prior to a final Governing Board vote. Members were requested to review the latest version by the end of October so that the ratification process can be completed.

Central Bureau

Van Husson reviewed the status and performance of the network (see Attachment 4). More than half the stations are still below the minimum performance standard of 1000 passes per year on low satellites (LEO) and only a dozen stations are at or near the LAGEOS minimum performance standard of 400 pass per year. A comparison of the 1999 data statistics with those of 1997 showed that performance at a number of stations has decreased over the last two years – what’s happening? Most stations are meeting the bias stability standards. A number of data compliance issues at network stations have been cleaned up over the past six months.

Mike Pearlman reviewed some of the key ILRS and Central Bureau activities over the past six months (see Attachment 5). Six tracking campaigns were conducted during the period. Campaigns continue on ERS-1, GFO-1, IGEX, SUNSAT, and Beacon-C. The IGEX Campaign has been reduced from its original eleven satellites to three, GLONASS 70, 72, and 79. GLONASS 70 recently failed and the selection of a replacement GLONASS satellite is in progress. Twelve new, low orbiting, missions requiring SLR support are scheduled for launch over the next two years. Since these are all active missions, we are going to have to focus on improving network efficiency and data throughput.

The Interactive Mission Support Request Form is now on line; several of the new missions and recent campaigns have already submitted their requests through this channel. Quarterly Performance Reports now include station compliance information as requested at the April Meeting. Analysis Reports, Historical Data on Geodetic Satellites, updated Working Groups Charters and Activities, and expanded links to other relevant sites have been added to the website. Weekly Campaign Updates are now distributed by email.

Mark Torrence reviewed the activities on the ILRS Science web page (see Attachment 6). ILRS participants were urged to access the expanded science bibliography and add any of their own contributions that may have been left out. New charts have also been added to the brochure on "SLR and its Contributions to Earth Science". Mark also discussed our current knowledge of GM; the most recently determined value has an uncertainty of 0.0002 km³/sec², or about 0.5 parts per billion. This level of uncertainty implies a knowledge of the Earth’s semimajor axis of about 3 mm. We need to reach another order of magnitude in accuracy (corresponding to 0.1 mm) to further our geoscientific understanding. (How’s that for a challenge?)

Key challenges for the ILRS over the next year were identified:

1. strengthen the science liaison activity;

2. maintain the momentum of the Working Groups in developing and addressing their action plans;

3. improve the tracking response to "very" low earth orbiting satellites through improved coordination and predictions in preparation for mission such as CHAMP and GRACE;

4. encourage and help tracking stations and analysis centers to meet their minimum performance criteria;

5. develop an ILRS "standard global solution" for submission to the IERS and for maintenance of the International Terrestrial Reference Frame (ITRF)

6. adopt a global data format (such as modified SINEX) to further encourage the use of different data types in combined solutions;

7. continue the development of the ILRS website and data bases, in the areas of technology, science and applications, and operations, and formalize the process by which updates are approved; and

8. continue the process of documenting configuration and standardizing processes/procedures

Subnetwork Reports

EUROLAS

Werner Gurtner reported on the activities of EUROLAS (see attachment 7). Network station status is now exchanged in real-time among Herstmonceux, Zimmerwald, Potsdam, and Grasse. NERC continues to provide quality control for EUROLAS stations using short arc and long arc analysis. Herstmonceux has implemented computer controlled beam alignment, a C-SPAD detector, and fully automated daily IRV generation. Graz has implemented multi-arc tracking, time walk elimination in the start pulse, and is planning to add three Dassault timing units to its time-of-flight measurement. Cagliari reported that they plan to add an MCP, GPS timing and new software over the next year. Wettzell has a new Nd:YAG laser and has both new control system software and two-color observation procedures under development. TIGO test observations were completed in September. A Call for Participation has been issued for TIGO operations overseas. Proposals are in process from India, South Africa, and Argentina. A selection will be made prior to mid-2000. It was noted that TIGO will play a very important role in helping to fill out the SLR global network. Plans are still in progress for a cooperative MTLRS-1 program with China, but the first step will be the replacement of the control system.

WPLTN

Hiroo Kunimori reported on the activities of WPLTN (see Attachment 8). Mt. Stromlo and Yarragadee SLR stations are performing very well. Mt Stromlo, operated by EOS under contract to AUSLIG, is operating full time. Work has recently been completed on an updated internal calibration design, on the installation of a new CCD camera, and on a barometer calibration. Engineering and development work is also underway on improved automation, updated software, and improved geodetic survey techniques. Yarragadee is now operated under contract to AUSLIG. Operations cover 12 hours a day, 7 days per week.

The Changchun station suffered encoder problems this summer, degrading system performance until replacement was completed in September. The station is now working fine. The Shanghai station installed a new C-SPAD in March and new meteorological equipment in September. The next upgrading activities there include a short distance calibration target and a narrower band filter. The Shanghai Observatory is now issuing LAGEOS analysis results every two weeks. The Beijing station has installed a new HP GPS receiver and a C-SPAD, and has started to use the RGO predictions. They are still having stability problems with their mount servo system and their laser. The Kunming Station started observing in late 1998. They have taken about 200 passes on LAGEOS, but CSR analysis indicates that there may be a range bias problem.

The Keystone SLR stations started operations in September 1998, but performance has been disappointing, due to the combination of weather, system instability, and operational inexperience (with the four station interconnected network). A delay in the delivery of the aircraft detection system has also inhibited operations. CRL is trying to increase productivity by increasing operations personnel and resolving system problems. They also plan to make an additional effort to demonstrate simultaneous tracking strategies. Simosato is addressing both laser and mount problems and damage caused by recent lightning strikes. HTLRS is presently engaged in its third occupation on the island of Ishigaki. Next year it will go to the most northern peninsula of Japan.

The Russian SLR network continues to operate. RISDE plans to upgrade the Maidanak station with a new 30 ps laser, an SR 620 counter, a TC 420 discriminator, a H5023 photomultiplier, and a new meteorological station. Schedule will be dictated by the availability of funding. Operations at the Komsomolsk stations have been severely inhibited due to extensive forest fires.

SALRO has not been operational for about two years due to systems problems. AUSLIG and EOS are undertaking an effort to repair the system.

The Russian MCC continues to generate TIRV predictions for WESTPAC and provides weekly reports on LAGEOS 1 and 2. CRL continues to generate bias analysis reports using AJISAI and LAGEOS.

WPLTN will undertake another regional Geodetic campaign in November (PCGIP99) in support of the Permanent Committee for GIS in Asia and the Pacific. The multi-technique campaign will use LAGEOS and Etalon as priority targets. The ILRS will be asked to provide global support.

NASA Network

Reporting for the NASA Network, David Carter brought us up to date on the relocation of NASA SLR systems to Hartebeesthoek, South Africa and La Plata, Argentina (see Attachment 9). Through a joint program with the South African National Research Foundation, MOBLAS-6 is being placed at the Hartebeesthoek Radio Astronomical Observatory and will therefore be collocated with VLBI, GPS, and DORIS. Crew training will begin at GSFC in November; transfer is scheduled for the February – March 2000 timeframe. Plans are underway to transfer TLRS-4 to the Argentine National University’s Radio Observatory outside of La Plata. A Memorandum of Understanding is in process, and crew training should be underway at GSFC by late spring. Transfer is tentatively planned for the fall.

Data Centers

EDC

Wolfgang Seemueller reviewed data holdings, facilities, procedures and current issues at the EDC (see Attachment 10). A disk was replaced in early 1999 following a crash. An apparent disk crash in August was a case of saturation when a commercial company tried to put video games on their ftp server. This has happened several times and actions that are being contemplated include: installation of a new ftp server, restricting access to users with passwords, and adding more storage on the computer system.

CDDIS

Standing in for Carey Noll, Van Husson reported on key issues at the CDDIS (see Attachment 11). A disk failure in August (in mysterious coincidence with the August failure at EDC) brought the CDDIS partially down for about a week. Tracking data could be put into the system but not retrieved. A workaround system to make tracking data available was set up by ATSC and CSR as in interim measure. Once the disk was replaced, backup files proved to be incomplete and a rather arduous process of file reconstruction from original sources was undertaken. The backup process is now being conducted more methodically, and the purchase of additional backup hardware has been recommended.

Working Group Reports

Missions

Hiroo Kunimori presented the activities of the Missions Working Group (see Attachment 12). The Missions Working Group met in Florence prior to the General Assembly. Issues discussed included: (1) provisions for handling special requests as part of mission support, (2) criteria for ILRS acceptance of new missions, (3) determination of future tracking priorities, and (4) development of a flow diagram of procedures for new missions. The approval procedures for campaigns conducted over the past 6 months were reviewed with an eye toward the large number of new missions on the horizon. Mission support requests for CHAMP and VCL are already in process. Discrepancies in GLONASS numbering continue to cause confusion. The Central Bureau will get together with Werner Gurtner and Graham Appleby to agree on a standard numbering system that the SLR community will use for its operational activities. Satellite identification acronyms and numbers will be added to the ILRS web site by the Central Bureau. Spacecraft center-of-mass changes during mission lifetime need to be investigated and worked with the Signal Processing (Ad Hoc) Working Group.

Networks and Engineering

Werner Gurtner discussed the activities of the Networks and Engineering Working Group (see Attachment 13). The latest draft of the new site log has been circulated for comments. The Site Log, based in part on the form developed by the IGS, must be easy to fill out and contain all of the necessary information on the systems and sites for the analysts and the engineers. Once the Site Log is in final form, the Central Bureau will work with the stations to complete the information. The Working Group is also developing application procedures for new stations, quality assessment and engineering data checks, SLR calibration procedures, and network documentation and knowledge bases. Members of the Working Group organized the Colloquium on SLR System Calibration Issues taking place this same week in Florence.

Data Formats and Procedures

John Luck reported on the Data Formats and Procedures Working Group (see Attachment 14). Several items requested at the April meeting at the Hague are in the process of being added the ILRS website, including maneuver notification format and procedures, drag function formats, prediction center lists, data transmission procedures, satellite standard names and designators, site occupancy designators and DOMES numbers, and Y2K information. The need for drag functions in predictions may disappear with the more extensive use of daily issued IRV’s. The Normal Point Study Group (Jaguar Team) has completed its work, concluding that there should be no prescription on the minimum number of accepted returns in a normal point for any of the satellites. The Prediction Study Group (Lion Team) is preparing a questionnaire (for circulation by the Central Bureau) to provide a better understanding of which IRV’s are being used by which stations. Through NERC, daily updated IRV’s are currently being issued to improve our tracking response to low-Earth orbiting satellites and to increase our general daylight tracking data yield. Compatibility of NP and FR data formats and schemes of data compression are also being examined.

Analysis

The activities of the Analysis Working Group were presented by Ron Noomen (see Attachment 15). Techniques for quality assurance of SLR observations at the stations, the data centers, and the analysis centers are being examined, including how best to (1) determine and quantify the QC information, and (2) feed this information expeditiously forward and backward along the data path. Of primary concern to the working group is the proper formulation of SLR data products and best way to prepare them for the data users, including a "standard ILRS solution", for submission to the IERS. As a first step, they have organized an activity to compare the standard solutions produced by the ILRS analysis and associate analysis centers in order to better understand how they differ and why. A one-month standard LAGEOS data set is available from the CDDIS for centers to generate their solutions in preparation for workshop in January.

Signal Processing (ad hoc)

Graham Appleby reported that the Signal Processing Ad Hoc Working Group is working to develop and test models for center-of-mass range corrections appropriate for various SLR systems for the different satellite arrays (see Attachment 16). At the moment, they are focusing on single-photon return levels on spherical satellite, return energy effects for single photon detectors, data clipping effects, statistics for monitoring system stability, and incidence angle effects on distributed arrays. To undertake this effort, they will need information and ranging practices on each system, raw and processed full-rate data for the major systems, models for single-photon detectors at high signal strengths, and models for multiphoton detectors. Better information on array characteristics and configuration will also be required on a number of satellites presently in orbit (GLONASS, Etalon, Starlette), and better prelaunch information on the composition and configuration of retroreflectors arrays is required on future missions. The new site logs should include sufficient information from each station to conduct these studies. (The Signal Processing Working Group should make sure that the Networks and Engineering Working Group that is developing the new site log format has all the benefit of its wisdom). Results to date by the Signal Processing Working Group are presented at:

http://mtfiles.nerc-monkswood.ac.uk/nercslr/sig/signature.html

A number of presentations related to the activities of the working group are being given at the Colloquium on SLR System Calibration Issues.

Campaigns

GFZ

Reinhart Neubert presented a report on the GFZ-1 Satellite Campaign (see Attachment 17). The satellite reentered on Jun 23, 1999 after 4 years in orbit. Data yield was unexpectedly high at the beginning of the campaign, and continued at a good level until the satellite orbit fell below 325-km altitude in late 1998. Only a few sparse points were acquired below 300 km. During the campaign, very productive stations included Herstmonceux, Graz, Potsdam, Grasse, Monument Peak, and Yarragadee. Experience from the campaign showed that we need to improve tracking on satellites below 350 km and on low orbiting satellites in daylight. GFZ-1 data has been used for gravity field model development, and surface force and atmospheric drag parameterization. GFZ, NASA, and CSR continue to use the data for these studies and for CHAMP and GRACE Mission planning.

ERS-1

Reinhart Neubert reported that ERS-1 continues to work well, relying on SLR for its tracking (see Attachment 18). SLR data yield is averaging about 360 passes per month from 24 to 29 stations. Based on the continued good health of the satellite and the success of the tandem mission with ERS-2, ESA will ask the ILRS to extend the tracking campaign beyond the end of 1999 as presently planned.

Topex/Poseidon

Mark Torrence presented the current status on Topex/Poseidon (see Attachment 19). The satellite systems continue to operate well. SLR tracking continues at high priority. The mission has been recognized as a great success, due in part to the high quality of the precision orbits required to support the ocean surface altimetry. The combination of SLR and DORIS data is routinely producing 2-3 cm radial orbits. Improvement in POD continues at GSFC.

GFO-1

Mark Torrence also reported on GFO-1 (see Attachment 20). This radar altimeter satellite is still relying totally on SLR since the GPS receivers are not yet operating. SLR tracking is averaging about 200 passes per month. Orbit radial accuracy is currently in the range of 10cm. GSFC continues to work on upgrading the POD, with hopes of achieving another factor of two in accuracy with improved gravity field and non-conservative force models Initial tests show the altimeter crossover fits to be 10 cm or better.

Beacon-C

The status on the Beacon-C Campaign was presented by Peter Shelus (see Attachment 21). The campaign, requested by M.K. Cheng from CSR, was undertaken to support studies of secular variations in the gravity field. The current six-month campaign, which started in mid-July, has provided 835 passes through mid-September. Three-day orbit fits are at the 10-cm level. Prior to this campaign, Beacon-C was supported by the SLR network from launch in 1965 through mid-1986. Of note here, the satellite is magnetically stabilized, and visibility is restricted to Northern Hemisphere and equatorial sites.

IGEX-98

Werner Gurtner reported on the IGEX-98 Campaign (see Attachment 22), which was undertaken as a part of a wider program to test the GLONASS system. The original IGEX-98 SLR campaign, involving eleven GLONASS satellites, ended in mid-April as scheduled. A follow-on six-month campaign (IGEX-II) was approved on GLONASS 70, 72, and 79. GLONASS 70 subsequently failed, and a replacement is presently being selected from plane 1 (GLONASS 80, 81, or 82). Global microwave tracking on the GLONASS complex will continue for several years; a decision is yet to be made whether a request for further SLR support after the current campaign will be made. Results from the tracking-to-date were reported at the IGEX-98 Workshop in Nashville, Tennessee just prior to this meeting. Analyses at CODE show the GLONASS microwave orbits to be of 10-20 cm quality. The range bias between the GLONASS orbits and the SLR data is about 4 cm. Several possible sources are being examined.

SUNSAT

Scott Wetzel presented material on SUNSAT (see Attachment 23), which was built by the University if Stellenbosch in South Africa and launched by NASA in February, 1999. The mission, one of imaging and communications, is supported by GPS and SLR tracking. The tracking data are also being used for comparison of inflight GPS systems with SLR. The retroreflector array is an annulus with eight inclined cubes mounted on the boom (facing downward). A six-month SLR campaign was authorized in mid-September. Many of the SLR stations have acquired some data on the satellite, but only 6-7 have been regular contributors. Several groups are analyzing the data. Inquiries are underway to secure rapid release of the GPS data to help support prediction activities.

Upcoming Missions

CHAMP

Reinhart Neubert reported that CHAMP is scheduled for launch in May 2000 (see Attachment 24). With its GPS receiver and onboard accelerometer, the satellite is being launched to provide fundamental improvement in gravity field modeling. SLR will provide orbital and reference system strength, calibration of the GPS measurements, and act as a backup in case of GPS system failure. GFZ will provide the orbit predictions. GFZ is in the process of completing the ILRS Mission Support Request Document.

ENVISAT-1

Scott Wetzel reported that Envisat-1 is scheduled for launch by the European Space Agency in November 2000 as a continuation on the ERS satellites for environmental monitoring (see Attachment 25). SLR will provide tracking support during the commissioning phase to calibrate the altimeter and the DORIS and to act as a backup tracking system. The retroreflector array is the 16 cm diameter, hemispherical array with nine cubes (eight in an inclined ring and one at the top) that is now becoming a ESA standard package (ERS-1/2). No formal submission has yet been made for ILRS support.

IRP-P5

Scott Wetzel presented some material on the Indian Remote Sensing Satellite-P5 (see Attachment 26). The satellite is planned for launch by the Indian Space Research Organization in the 2001-2002 timeframe for cartographic applications using a high-resolution panchromatic camera. The requirement for SLR tracking has not yet been specified, but the satellite will carry an ERS-1/2 type (ESA standard) retroreflector array. No formal submission has yet been made for ILRS support.

ICESAT

Scott Wetzel also reported that the NASA IceSat Mission, scheduled for launch in July 2001, will carry a Geoscience Laser Altimeter System (GLAS) to map the Earth’s surface topography (see Attachment 27). SLR will be used for validation and backup for the GPS receiver. The retroreflector array will be the nine cube hemispherical design used on GFO-1, ADEOS-II, and Jason. Details on the mission are given on the ILRS website. No formal submission has yet been made for ILRS support.

ADEOS-II

Hiroo Kunimori reported on ADEOS-II (see Attachment 28). ADEOS-II will carry an array of Earth observing sensors, an S/Ka band transponder, a GPS receiver, and retroreflectors. GPS and SLR will be used for both short arc and long arc orbit determination to support the Earth observing systems. The satellite will be launched in November 2000 from Tanegashima, Japan. Required orbital accuracy is 1 meter.

Jason-1

John Degnan presented material on Jason-1, a follow-on oceanographic mission to Topex/Poseidon scheduled for launch in mid-May 2000 (see Attachment 29). The satellite will carry a radar altimeter, a microwave radiometer, DORIS and GPS receivers, and an SLR retroreflector array. The program will provide a global view of the oceans over a period of 3-5 years. The retroreflector array, which was designed by GSFC, consists of a nadir cube and a ring of eight uniformly spaced cubes at 50 degrees from nadir, arranged on a hemisphere. This is becoming a standard design, combining reasonable angular aspect and minimum signal spread.

GP-B

Mark Torrence reported on Gravity Probe-B (see Attachment 30). Gravity Probe B, scheduled for launch in the fall 2000, is a gyroscope experiment being developed by NASA and Stanford University to test two extraordinary, unverified predictions of Einstein's general theory of relativity. The experiment will check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting in a 400-mile high, polar orbit. So free are the gyroscopes from disturbance that they will provide an almost perfect space-time reference system. They will measure how space and time are warped by the presence of the Earth, and, more profoundly, how the Earth's rotation drags space-time around with it. These effects, though small for the Earth, have far-reaching implications for the nature of matter and the structure of the Universe.

Satellite laser ranging will be used in conjunction GPS data for precision orbit determination. Details on the orbit and the retroreflector array are given in the attachment.

VCL

Mark Torrence reported on the Vegetation Canopy LIDAR (see Attachment 31). This NASA-University of Maryland mission will map the vegetation canopy and the land surface topography to an accuracy of 1-meter using a laser altimeter. POD will be provided with GPS as primary and SLR as secondary tracking mode. SLR will be used to calibrate the GPS, and support calibration and evaluation of the altimeter. Orbital accuracy of better than 30-cm radial and 1 meter along/across track is required. The launch date is currently scheduled for August 2000.

ILRS Tracking Priorities

Mike Pearlman reviewed the ILRS Tracking Priorities (see Attachment 32). Nineteen satellites and five lunar targets were on the priority list, including four satellites with defined campaign end dates. As mentioned earlier, the failed GLONASS 70 will be replaced by either GLONASS 80, 81, or 82, at the discretion of the IGEX Committee. The next additions planned at this time are CHAMP and Jason; both scheduled for launch in May 2000.

ILRS Annual Report

Mike Pearlman presented a draft table of contents with suggested page lengths for the ILRS Annual Report (see Attachment 33). Writers for each article and organizers for each section were identified. It was agreed that we would ask Gerhard Beutler to write a short introduction and that we should have the report ready for distribution at the ILRS meeting at Nice in April. To meet this date, the manuscripts should be provided in electronic form to the Central Bureau by 15 January. Mike Pearlman and Peter Shelus will work up lists of questions for each report section and suggested formats as a guideline for the writers.

Next Meeting

The next ILRS General Meeting and Governing Board Meeting will be held in conjunction with the EGS Meeting in Nice, France in April 2000. The possibility of allocating at least a full day for the General Meeting was discussed.