International Laser Ranging Service

ILRS NEWG Meeting (Nice 2001)

Nice, France
March 27, 2001

Attendees:

Members:

Graham Appleby
Wolfgang Schlueter
Ulli Schreiber
Werner Gurtner
John Degnan
Peter Shelus
Van Husson
Hiroo Kunimori
Mike Pearlman

Observers:

Richard Eanes
Scott Wetzel
Bruce Schupler
John Bosworth

Data Flow Backup Test

Data flow backup procedures have been instituted if EDC or CDDIS is off the air. The procedures were tested during a planned shutdown at EDC. Most stations responded properly.

Calibration Meeting Report

The report on the "Colloquium on SLR-system Calibration Issues" in Florence in September 1999 has been distributed. If you need additional copies, contact Ulli Schreiber or Mike Pearlman.

Prediction Exploder

The new Prediction Exploder has been implemented. The mailing list needs some cleaning up; a few addresses are not working. This will be discussed with Carey Noll. To better accommodate the individual stations, we agreed to implement separate exploders for time bias, air drag coefficients, and maneuvers.

Station Qualification

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. Their report is included as Attachment 1.

Basic assertions by the group were that the ILRS should remain inclusive (room for everybody), but that stations should be categorized as "Operational Stations" or "Associate Stations" based on whether or not they provide useful data to the analysis community. The threshold criteria for Operational Stations 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. Stations become operational by completing the site logs and by submitting their data to the Analysis community for validation. 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 group could provide scrutiny and advice.

Within the network of Operational Stations, a core network of high performing stations that meet the ILRS Performance Criteria would also be designated. A process of annual review to add and delete stations in this each category would be implemented. These station classes would be highlighted on the ILRS maps and charts.

This plan will be presented to the Governing Board for review.

Site Logs

Van Husson reported that the site log implementation has progressed very well. Only the stations in Russia have yet to comply. An Excel based site log master file has been developed with an auto filter for screening entries. A section by section review and cleanup is now underway. Many inconsistencies in eccentricity and other station parameters have already been resolved. Procedures need to be specified for adding additional information such as station biases to the Site Logs. A historical record of station biases, as least as far back as 1996, and preferably as far back as data and memory exists, should be carefully documented. A study group including Peter Dunn, Graham Appleby, and Van Husson were tasked to develop such a master database. The stations should be contacted to assemble the historical bias information. Hyperlinks to the IGS, IVS, and IDS site logs should be added to the ILRS web site. Do we want to develop Site Logs for deactivated stations?

SLR 2000

GSFC is working toward an October date to have the prototype SLR2000 system ready for testing. The telescope and the facilities are finished. An encoder problem is being addressed in software. Pointing is still 2-3 times larger that the specification, but it is well below 2 arcsec and getting better. The optical transceiver build is progressing. After a year of testing and documentation, production should start in early 2003. It is anticipated that 12 units will be built for NASA alone; the first units should be ready for deployment in early 2004.

TIGO

The new PC/Linux control system and Dessault event timer are installed. The system is working. The system could be on-site at Concepcion, Chile, this summer, but administrative issues are pacing the shipment. A government level exchange of letters is still required to cover customs and importation.

MLRO

Some hardware and software issues are still being resolved. The next stage of acceptance tests is scheduled for May. Preliminary data has been sent to CSR for scrutiny. Lunar data taken at GSFC prior to shipment looks good.

Zimmerwald

Small optics in the telescope have been recoated. A pulsewidth problem in the laser has been repaired by the vendor, and ranging rms has been improved significantly.

WLRS

In October, the control system was replaced with a PC/Linux system similar to that in TIGO. After some pointing problems, Zeiss replaced the telescope drives. Work continues to iron out system problems, but they expect to be back on the air soon.

Meeting in Toulouse in September

Planning continues for the next ILRS Workshop on September 19 - 21, 2001, in conjunction with the SPIE Remote Sensing Symposium in Toulouse, France. At the Calibration Symposium in Florence in September 1999, we formulated many questions and issues that need to be addressed on laser ranging calibration and performance. At the meeting in Toulouse, we will work the next step: how do we take this information and address the actual improvement in the systems. 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 and other interested ILRS Associates will work together to address some of the key issues currently limiting SLR performance.

The Working Groups will be given an opportunity to meet separately (in parallel) on Wednesday morning, September 19. That afternoon and the next day, combined sessions will meet to work on preformulated issues. Results and conclusions will be discussed on Friday at a full ILRS General Assembly. Ulli Schreiber, John Degnan, Mike Pearlman, and several others will constitute the program committee and will help formulate the program.

On-Line Station Performance Report

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.

Rev 7. 3/10/01
A RECOMMENDATION TO THE ILRS GOVERNING BOARD
on the
Qualification of SLR Stations in the ILRS

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 qualification of the data. The ILRS Governing Board, since its inception, has noted this shortcoming.

Our first attempt at qualifying the ILRS stations came late last year when the ILRS participated in an activity to designate the first set of global multi-technique sites that meet the International Space Geodetic and Gravimetric Network (ISGN) criteria. The criteria included (1) quantity and quality of data, (2) geological setting, environment, monumentation, adequacy of site tie surveying, and adequacy of operations, and (3) survey, calibration, and analysis issues. Our participation was the selection of SLR stations that were considered to be “well performing sites within the ILRS criteria”. The selection was made by the Central Bureau with input from some of the Analysis Centers. The sites selected were Herstmonceux, Grasse, Zimmerwald, Graz, Yarragadee, Mt. Stromlo, Haleakala, Monument Peak, McDonald, and Greenbelt. We will be asked to reevaluate and update this list each year. Other stations that may soon join this groups are Hartebeesthoek, Matera (MLRO), Wettzell, Arequipa, Changchun, TLRS-4 (La Plata), Potsdam, Metsahovi and Tahiti. Although we used an ad hoc procedure this past year, we recognized the need to formalize the process for the future.

A small study group was 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. This document was also to include a procedure for new stations to join the network. The study group consisting of Mike Pearlman, John Degnan, Van Husson, Graham Appleby, and David Carter was tasked to have a draft position ready by February 1.

Deliberations and Recommendations of the Study Group:

The Study Group tried to strike the right balance among giving confidence to our customers in our data, offering sufficient incentive to stations to strive for the highest level of performance; and at the same time not being so overly restrictive in our rules of membership that we stifle groups that are serious.

We considered two questions:

What are the qualifications for stations to be admitted to, or to remain a part of, the ILRS network?

Should we further classify stations based on performance?

The Study Group agreed on the following assertions:

• exclusion of any SLR station from ILRS participation might discourage that station from striving to improve its performance and might precipitate withdrawal of support from funding agencies;
• although the Terms of Reference (see Appendix below) does not say that LAGEOS ranging is a qualification for participation in the ILRS, we felt that this was an oversight in the initial wording, and that LAGEOS ranging is essential;
• the ILRS network stations should be designated as either "operational stations" or "associate stations" depending upon whether or not they provide any "value" to the user community; and
• a Core network of "high performance" stations should be established within the operational station network and highlighted for the user community.

Operational Stations

Operational stations must adhere to the data format, delivery and site log compliance requirements and must furnish data that is considered useful by the analysis community, as evidenced by the fact that the station's data is regularly and continuously analyzed by at least one ILRS Analysis Center or by one mission-specific Associate Analysis Center.

To be considered operational, a station must have submitted at least two months of tracking data (ideally LAGEOS 1 and 2) to the Data Centers for rapid evaluation and a determination of preliminary station coordinates by the Analysis Working Group. If the Analysis Working Group considers that the data is of sufficient quantity and quality, it would recommend that the Governing Board declare the station to be operational. Once the station is declared operational by the Governing Board, the Central Bureau will inform the community at large of the new station and provide preliminary station coordinates. Stations that cease to have meaningful performance for a period of a year or longer would have to requalify for ILRS status.

Associate ILRS Stations

Associate ILRS stations fall within two categories: (1) those that are in engineering status and have not yet produced useful data; or (2) previously operational stations that have not produced useful data for a year or more and must requalify for operational station status.

The Core Network

Core network stations must meet and in some cases exceed the ILRS guidelines for station performance. (Also see the Appendix below). The Study Group selected the following tentative requirements for the Core network stations:

1. Annual Data Yield:
1.1 1000 Low Earth Satellite (LEO) passes
1.2 400 LAGEOS 1/2 passes

2. Data Quality:
2.1 0.5 cm LAGEOS NP precision
2.2 2 cm short term bias stability
2.3 1 cm long term bias stability

3.Operational Compliance:
3.1 Data delivery within 12 hours
3.2 Specified data format
3.3 Current site and system information form (site log)

4. Operational GPS Receiver

While these requirements will be the primary determining factor in acceptance as a Core station, some flexibility may be allowed based on other factors such as geographical importance, etc. Core stations are further encouraged to seek collocation with other space geodetic techniques such as VLBI and DORIS. Core network stations will be highlighted on the ILRS web site and on network maps.

Membership in the Core network will be reevaluated once per year. The Central Bureau will gather station statistics and present them annually to the Governing Board. New stations that qualify for Core status will be proposed by the Central Bureau, reviewed for concurrence by the Analysis and Network and Engineering Working Groups, and voted on by the full Governing Board. Prior Core stations that have substantially fallen below qualifying performance during the previous year will be dropped from the Core network.

Qualification of New ILRS Stations

From time to time new SLR stations are built and their owners will want to join the ILRS. We should offer them every encouragement and assist them wherever possible. New stations should be able to join the ILRS, even while they are in the process of system construction and testing. During this phase, the ILRS could be extremely helpful.

To join the ILRS, a station must:

• submit the ILRS Station Response Form; and
• keep the ILRS Central Bureau abreast of progress toward an operational state
• Once a station is ready to become operational, it must:
• submit its ILRS Site Log Form;
• submit two months of LAGEOS data and request a review by the Analysis Working Group; and
• receive approval from the Governing Board

APPENDIX

ILRS Terms of Reference:

ILRS Tracking Stations range 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 which allow for a rapid (at least daily) data transmission to one or more Operations and/or Data Centers. The stations must meet data accuracy, quantity, and timeliness requirements that are specified in separate documents. The tracking data produced by the ILRS stations are regularly and continuously analyzed by at least one ILRS Analysis Center or one mission-specific Associate Analysis Center. Tracking Stations may be organized into regional or institutional subnetworks.

Posted Guidelines for the Performance of ILRS Stations

Posted Guidelines for the performance goals of ILRS stations are specified at:

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

These guidelines include:

1. Annual Data Yield:
1.1 1000 Low Earth Satellite (LEO) Passes
1.2 400 LAGEOS 1,2 Passes
1.3 100 High Satellite Passes

2. Data Quality:
2.1 2 cm LAGEOS precision (i.e. single shot RMS)
2.2 1 cm LAGEOS NP precision
2.3 2 cm short term bias stability *
2.4 1 cm long term bias stability**

3. Operational Compliance:
3.1 Data delivery within 12 hours
3.2 Specified data format
3.3 Current site and system information form (site log)

*the standard deviation about the mean of the pass-by-pass range biases from the CSR Weekly LAGEOS analysis.
** the standard deviation of the monthly range bias estimates from the CSR Weekly LAGEOS analysis for at least 8 of the last 12 months.