13^th International Workshop on Laser Ranging
"Toward Millimeter Accuracy"

Workshop Summary

The 13th International Workshop on Laser Ranging was held at the Hyatt Regency Capitol Hill in Washington, D.C., on October 7 through 11, 2002. The International Laser Ranging Service (ILRS) and the laser ranging community organize this event every two years to discuss progress in satellite and lunar laser ranging, and their application to scientific programs. The event this year was sponsored by the National Aeronautics and Space Administration (NASA), the Smithsonian Institution, and the ILRS. Over 150 people from 22 countries participated in the meeting, which included presentations, posters, and discussions on hardware, software, operations, analysis, and science topics.

The theme of the 2002 workshop, “Towards Millimeter Accuracy”, was the focus of the thirteen sessions that were held during the week. On the hardware side, the technologists reported on developments in timing devices, calibration techniques, lasers, detectors, control systems, and other advancements which has resulted in a 5 mm one sigma single-shot range precision at the best stations. The single-shot observations (typically generated at rates of 5 to 10 Hz) are compressed into so-called normal points. The latter are obtained, for example, by averaging the data over three-minute intervals for the LAGEOS satellites at 5900 km altitude, and 15-second intervals for LEO satellites, which effectively reduce random errors to the level of about 1 mm. The development of new kHz rate systems reported by researchers in the US, Austria, and Japan are expected to further reduce random errors.

More important than the random errors are the systematic errors in the observations since these are fully retained in the normal points, and high absolute ranging accuracy is the ultimate goal. The primary contributor to systematic error is the uncertainty in the atmospheric path delay which, for optical wavelengths, is primarily influenced by the so-called “dry”, or pressure, component of the troposphere. There is an increased effort among SLR analysts to more accurately model the tropospheric delay for any station at any wavelength and at elevation angles lower than 20o. These improved models use the most recent mapping functions (Mendes, Herring) and appropriate zenith delays. Tropospheric path delay uncertainties can be reduced to 3-5 mm at 10° elevation with sufficiently accurate onsite measurements of pressure, temperature, and relative humidity as inputs to the models.

In recent years, a number of multi-wavelength systems have been developed to directly measure the atmospheric delay. Since the delays induced by the atmosphere are wavelength-dependent and predictable, measuring the pulse time of flight at two or more wavelengths permits a direct measurement of the atmospheric delay (comparable to what can be done with the ionospheric delay in GPS measurements). Nominally, the differential times of flight must be measured with an accuracy of 2 picoseconds or better to substantially improve upon the results obtained using atmospheric models and in-situ ground measurements. As discovered in early two-color experiments at GSFC and Wetzell, the task is often made more difficult because of additional operational complexities introduced by streak camera receivers and the fact that the return waveforms from multicube satellites often have totally different temporal characteristics, thereby making the determination of differential time of flight somewhat ambiguous. Nevertheless, additional stations have taken up the two-color challenge. Representatives of the SLR station in Zimmerwald (Switzerland) reported on the technical status of their system, which operationally produces dual-wavelength observations starting in August of 2002. The laser stations in Matera (Italy) and Concepción (Chile) are undertaking a similar development and are expected to be online very soon. Finally, the worldwide tracking station distribution has also improved since the previous workshop with the establishment of new SLR systems in Concepción (Chile), Hartebeesthoek (South Africa), Riyadh (Saudi Arabia) and various locations in Russia and China.

A second source of systematic error is the “satellite signature effect”, i.e., the location of the effective instantaneous point of reflection within the satellite (the so-called optical phase center) relative to the satellite center-of-mass used for orbital calculations. The effect is most pronounced for target arrays deviating substantially from a spherical or hemi-spherical surface, but even nominally spherical satellites are characterized by slight variations in this distance as the laser beam changes its orientation relative to the array. Pulse timing is further affected by the characteristics of the station (laser pulse width, laser wavelength, detector bandwidth, threshold vs constant fraction detection, multi-photon vs. single photon detection, etc.). Nevertheless, good progress has been made here as well. Researchers have reported on the offset values for a variety of geodetic satellites, and uncertainties on the order of about 2 mm are now achievable for many satellites. These corrections are expected to be adopted by the analysis community shortly.

Numerous positive developments in both data quality and quantity will undoubtedly stimulate improved analysis techniques and science products coming from SLR and LLR. During the first day of the meeting, an overview of the contributions of the laser range observations and their analysis to our (geo)physical knowledge was given in the Science session. Here, reports and reviews were given on:

• SLR/LLR contributions to the implementation of the terrestrial reference frame (origin and scale in particular; both of which are accurate at an absolute level to a few mm),
• the long-wavelength geogravity field,
• observed temporal variations in the gravity field useful as a form of remote sensing of mass flux in the environmental system (i.e., the long time-span of the observations allows for the most reliable direct observation of the gravitational effects of post-glacial uplift and changes in ice-cap mass balance),
• satellite altimetry (the observation of ocean currents, the absolute sea-level and variations therein and the absolute calibration of the altimeter instruments themselves),
• possible new satellite missions to test specific elements of Einstein’s General Law of Relativity,
• monitoring Earth rotation and orientation (in spite of recent inroads by GPS, the SLR technique remains a good contributor thanks to its long and consistent time-series), and
• improved understanding of the Earth-moon interaction, lunar dynamics (such as librations), and exploration of exotic topics like G-dot.

There were several exotic applications of the laser techniques which were discussed, some of which have reached fruition. These include:

• applying lidar techniques as altimeters to directly map the topography of extraterrestrial bodies. This has been accomplished for Mars and the asteroid 433 Eros from data provided on the Mars Global Surveyor (MGS) and the Near Earth Asteroid Rendezvous (NEAR) missions respectively,
• modeling the gravity field of another planet (Mars, using MGS), and
• using laser transponder techniques to test various aspects of relativity on solar system length scales.

It was made clear that in the majority of research objectives there is a strong dependence on absolute orbit accuracy that is made possible using laser ranging observations. As for the future, the ongoing trend towards higher accuracy, larger data volumes and the need to support more missions is expected to continue. As will be clear from the workshop summary, the laser ranging community is fully engaged to apply the results presented and discussed during the workshop. The ILRS has proven to be a very good instrument to achieve and stimulate progress on all aspects of the SLR and LLR techniques, not in the least because of its highly active working groups. The community is confident of reaching the absolute accuracy of one millimeter in a matter of years, with corresponding benefits for the international scientific community and our understanding of “System Earth”.

During the meeting, the participants had the opportunity to visit the Goddard Geophysical and Astronomical Observatory (GGAO) to view the prototype SLR2000, which is now in field-testing. Other SLR, VLBI, and GPS facilities at GGAO were also open to the attendees. The workshop’s banquet dinner was held at the impressive Smithsonian Castle Building on Thursday evening. The guests heard talks from Dr. Mary Cleave, NASA Deputy Administrator for Earth Sciences and former astronaut on future plans in NASA’s Earth Science program, Dr. Henry Plotkin, University of Maryland, Baltimore County (UMBC) Center for Advanced Studies in Photonics Research on the history of SLR, and Richard Stamm, Keeper of the Castle Collection, Smithsonian Institution, on the founding and history of the museum.

Several resolutions were made and passed in the workshop business session on Friday, October 11. Two proposals to host the next workshop in 2004 were received: one from the Real Instituto y Observatorio Armada in San Fernando, Spain, and a second, joint proposal from Electro Optic Systems (EOS) Pty. Ltd. and Geoscience Australia in Canberra, Australia. In 2004, the Observatorio will celebrate its 300th anniversary. To honor this event, the attendees voted to accept the San Fernando proposal for 2004. The Australians offered to host the laser ranging workshop in 2006; the attendees also accepted this proposal.

Ron Noomen, ILRS Analysis Coordinator
Steve Klosko, ILRS Science Coordinator
Carey Noll, Local Organizing Committee
Michael Pearlman, Local Organizing Committee
John Degnan, Local Organizing Committee

Session Summaries

Summaries from the following sessions held at the 13th International Workshop on Laser Ranging are available:

• Scientific Achievements, Applications, and Future Requirements (R. Noomen and S. Klosko)
• Laser Technology Development (K. Hamal and Howard Donovan, for Y. Gao)
• Improved or Upgraded Systems - Poster Session (F. Pierron and S. Wetzel, for Y. Fumin)
• Timing Devices (E. Samain and P. Gibbs)
• Detectors and Optical Chain Components (G. Kirchner and L. Grunwaldt)
• Automation and Control Systems (J. McGarry and F. Koidl)
• Lunar Laser Ranging (P. Shelus and J.F. Mangin)
• Station Performance Evaluation (C. Luceri and R. Wood)
• System Calibration Techniques (I. Prochazka and U. Schreiber)
• Station Operational Issues (W. Gurtner and V. Husson)
• Target Design, Signatures, and Biases (G. Appleby and G. Bianco, for V. Vasiliev)
• Atmospheric Correction and Multiwavelength Ranging (S. Riepl and E. Pavlis)
• Advanced Systems and Techniques (B. Greene and T. Murphy)

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