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13th International Laser Ranging
Workshop
"Toward Millimeter Accuracy"
Submitted
Abstracts
Scientific Achievements, Applications, and Future Requirements ( R. Noomen
and S. Klosko)
| Title | Primary Author |
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| Time-Variable Gravity Analysis Using Satellite-Laser-Ranging as a Tool for Observing Long-Term Changes in the Earth’s Systems -- INVITED | Christopher Cox |
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Christopher Cox Temporal variations in the long-wavelength geopotential have been observed using SLR for the past twenty years. The interannual trends in these estimates have generally been consistent with and attributable to post glacial rebound, in addition to a number of secondary contributors. However, since 1998, J2 began increasing. At present it is not possible to tell whether this aberration represents a change in the long-term rate of change in J2, or whether it is short term in nature. In addition to changes in the mean J2, the amplitude of the annual variation has been changing. This change signifies a large change in global mass distribution whose J2 effect clearly overshadows that of the post-glacial rebound. A number of possible causes have been considered, with oceanic mass redistribution as the leading candidate and core effects as another possible alternative. Several components of the low-degree time series show correlation to known ocean processes. These include correlations between the sectorials and the Southern Oscillation Index, as well as some level of long-term correlation between the Pacific Decadal Oscillation and the observed J2 series. While the exact cause of the recent changes in J2 may not have been formally identified, these results do indicate the usefulness of SLR as a tool to observe long-term changes in the climate. We will present our analysis of the changes in the low-degree spherical harmonics and results of our investigations into the causes. Oral paper; received August 20, 2002 |
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| The SLR Contribution to Precision Orbit Determination in the GPS Era | Scott Luthcke |
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Space Geodesy Branch, NASA GSFC Frank G. Lemoine, David D. Rowlands Nikita P. Zelensky, Teresa A. Williams Precision Orbit Determination (POD) of Low Earth Orbiting (LEO) geodetic satellites has long relied on the high accuracy and robust tracking data provided from the global Satellite Laser Ranging (SLR) network. In fact, for nearly three decades SLR has been the primary tracking data for numerous high profile geodetic satellites such as LAGEOS and TOPEX/Poseidon. Over the past decade significant advances in the Global Positioning System (GPS) itself, and GPS data processing algorithms and data distribution, have positioned this technology as the primary tracking to support POD in the new era of geodetic satellites. High profile geodetic missions such as CHAMP, JASON-1, GRACE and ICESat all carry aboard a dual frequency codeless GPS receiver as the primary POD tool. Where does this leave SLR for these modern geodetic missions? Is it anything more than just a backup to the GPS tracking data? The answer is an emphatic yes. Experience with CHAMP and JASON-1 POD has proved the SLR tracking to be an invaluable tool in the calibration and validation of the GPS orbit solutions. POD processing of GPS 1-way measurements is quite complex requiring the orbit determination of over 27 satellites and the estimation of a plethora of system parameters (e.g., clock, ambiguity biases, tropospheric scale biases). The unambiguous, direct SLR measurements provide a high accuracy absolute observation of the orbit. This characteristic has been invaluable in tuning the GPS solutions with their myriad of parameters, and validating their accuracy. Results from CHAMP and JASON-1 GPS orbit solutions will be discussed with a focus on the role and performance of the SLR tracking data. Oral presentation; received August 30, 2002 |
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| Evaluation of potential systematic bias in GNSS orbital solutions | Graham Appleby |
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NERC Space Geodesy Facility Toshimichi Otsubo In order to combine results from different space geodetic technologies it is important to explore potential systematic bias between those results. An example of such comparisons is the use of precise laser range observations to carry out independent checks on the accuracy of published orbits of a subset of the GPS and GLONASS navigational satellites. Range measurements to two GPS satellites and a subset of the GLONASS satellites obtained by the tracking network of the International Laser Ranging Service are compared in two ways with precise orbits computed by the International GPS and GLONASS Services; by direct comparison of SLR measurements to ranges computed from the microwave orbits, and by comparison of SLR-based orbits to the microwave orbits. Previous studies have shown that in such comparisons it is vital to understand both the potential for systematic range bias induced by the laser reflector arrays and the need for accurate on-satellite positions of the array phase centers. For the GLONASS satellites these parameters are now accurately known for the two different types of array currently in orbit, and the SLR results suggest that systematic orbital bias is minimal. However, for the two GPS satellites, a radial bias of some 40mm persists. Poster presentation; received August 27, 2002 |
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| Contributions of SLR to the Success of Satellite Altimeter Missions -- INVITED | Remko Scharroo |
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Delft Institute for Earth-Oriented Space Research Satellite Laser Ranging is well-known for its contributions to crustal dynamics research, to establishing the long-wavelength gravity field, and to the monitoring of changes in the major gravity field components. Lesser known is the its invaluable support in the tracking of satellites carrying a radar altimeter. SLR single-handedly saved the entire ERS-1 mission when the PRARE tracking system failed soon after launch. The Geosat Follow-On satellite was to rely fully on GPS tracking, with a laser reflector mounted at the eleventh hour only as a backup. When most of its GPS antennas failed and the receiver rendered the altimeter inoperable, again the laser ranging community came to the rescue. The ERS-2 satellite suffers from an extensive delay in the delivery of its PRARE tracking data. Only because of the rapid turn-around time of the laser ranging data, the ERS-2 altimeter data can be used for near real-time monitoring of ocean currents and El Nino events. Finally, laser ranging has been indispensable in the efforts to accurately calibrate the radar altimeters of ERS-1, TOPEX/Poseidon, and ERS-2. This presentation will highlight the vital contributions of satellite laser ranging to the orbit determination of satellites carrying a radar altimeter, to the improvement of gravity fields tailored to those missions, and to the calibration of the altimeters. Oral presentation; received September 10, 2002 |
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| SLR and the CHAMP Gravity Field Mission -- INVITED | Rolf König |
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GeoForschungsZentrum Potsdam (GFZ) Ludwig Grunwaldt, Roland Schmidt, Peter Schwintzer, Chris Reigber Presented by: Ludwig Grunwaldt The restitution of the CHAMP orbit during launch and early orbit phase in a fast and reliable manner was only possible on the basis of data from two micro wave tracking systems: the skin radar tracking and the high-low GPS-CHAMP SST tracking, yet not calibrated. Soon SLR tracking joined in and the on-board GPS data could be calibrated in the following weeks by help of the SLR data. Nowadays, during the operational phase, SLR data are used to evaluate the precise orbit recovery before solving for the gravity field. Based primarily on CHAMP observations a new class of gravity field models can be computed. The EIGEN-1S was published and its successor, the EIGEN-2S is available in a preliminary version, both with considerable improvements in comparison to former models. As such the long wave length geoid becomes recoverable from just a few months of CHAMP data only. Following this variations of the geoid will become detectable. With the CHAMP mission the fast delivery of SLR data was successfully implemented. The GRACE mission and future LEO missions take benefit thereof. The design of the laser retroreflector on-board CHAMP turned out to be very efficient and was also adopted for the GRACE satellites. Future applications of GPS receivers aboard LEO satellites will tend towards fast to real time availability of highly accurate orbits. SLR will offer a commonly accepted base for calibration and validation. For the full integration of SLR during all mission phases it will be necessary to improve the availability of short latency data or even move towards real time data streaming. Oral presentation; received September 09, 2002 |
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| Prospects for an Improved Lense-Thirring Test with SLR and the GRACE Gravity Mission -- INVITED | John Ries |
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University of Texas at Austin / Center for Space Research Richard Eanes, Byron Tapley Glenn E. Peterson The theory of General Relativity predicts several non-Newtonian effects that have been observed by experiment, but one that has not yet been directly confirmed with confidence is the Lense-Thirring precession of an orbit due to the gravitomagnetic field. Previous analyses are limited by uncertain assumptions regarding the magnitude and correlation of the errors in the low degree geopotential harmonics. Now that the joint NASA-DLR GRACE (Gravity Recovery and Climate Experiment) mission is successfully gathering data, we can examine the expected improvements in the Lense-Thirring experiment using SLR data to LAGEOS-1 and LAGEOS-2. We will also look at other direct and indirect contributions of SLR to the GRACE mission. Oral paper; received August 23, 2002 |
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| Lunar Geophysics, Geodesy, and Dynamics -- INVITED | James Williams |
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Jet Propulsion Laboratory Jean Dickey Presented by: Jean O. Dickey Experience with the dynamics and data analyses for earth and moon reveals both similarities and differences. Analysis of Lunar Laser Ranging (LLR) data provides information on the lunar orbit, rotation, solid-body tides, and retroreflector locations. Lunar rotational variations have strong sensitivity to moments of inertia and gravity field while weaker variations, including tidal variations, give sensitivity to the interior structure, physical properties, and energy dissipation. A fluid core of about 20% the moon's radius is indicated by the dissipation data. The second-degree Love numbers are detected, most sensitively k2. Lunar tidal dissipation is strong and its Q has a weak dependence on tidal frequency. Dissipation-caused acceleration in orbital longitude is dominated by tides on earth with the moon only contributing about 1%, but lunar tides cause a significant eccentricity rate. The lunar motion is sensitive to orbit and mass parameters. The very low noise of the lunar orbit and rotation also allows sensitive tests of the theory of relativity. Moon-centered coordinates of four retroreflectors are determined. Extending the data span and improving range accuracy will yield improved and new scientific results. Oral paper; received August 22, 2002 |
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| Seasonal Changes in the Icecaps of Mars from Laser Altimetry and Gravity | David Smith |
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Laboratory for Terrestrial Physics Laser altimetry of Mars' polar icecaps has provided observations of changes in the height of the icecaps due to the seasonal deposition and sublimation of carbon dioxide from the Mars atmosphere. Observations by the MOLA instrument on the Mars Global Surveyor Spacecraft (MGS) from February 1999 until June 2001 show that both poles increase in altitude by about 1 meter during winter. More snow or ice seems to be deposited at the south pole but it may be slightly deeper in the north. These seasonal icecaps appear to extend down to at least latitude 60 in each hemisphere where it may be less than 10 cm thick. During the same period precise tracking of the spacecraft by the Deep Space Network (DSN) at accuracies of better than 50 microns/sec and 3 meters in range from Earth have shown that the lowest degree coefficients in the gravity field of Mars change seasonally thus enabling the mass of carbon dioxide that is regularly exchanged between the atmosphere and the surface of the planet to be estimated. Using both the volume of material derived from the altimeter and the mass of the material derived from the changing gravity we have concluded that seasonal icecaps of Mars have densities of about 900 kg/m^3. Since the material is carbon dioxide, which has a density of nearly 1600 kg/m^3 in its solid ice form, we infer that the seasonal icecaps are probably formed of thick frost, or that it snows CO2 during Mars' winters. Oral presentation; received August 27, 2002 |
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| Future Interplanetary Laser Ranging: Science Goals and Methods -- INVITED | Ken Nordtvedt |
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Northwest Analysis Ranging to passive reflectors on the Moon has delivered frontier science measurements of gravitational theory. Tests of relativistic gravity can be carried orders of magnitude further employing laser ranging to the planets. Ranging to Mercury is discussed as example: both the science tests that might be reached and the different ways this ranging could be implemented are considered. Oral paper; received August 23, 2002 |
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| Geophysical Applications of SLR Tidal Estimates -- INVITED | John Wahr |
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University of Colorado David Benjamin Shailen Desai The largest time-varying component of the Earth's gravity field is the tidal signal, caused by a combination of the direct gravitational attraction of the Moon and Sun and the deformation of the solid Earth and ocean caused by that gravitational attraction. SLR data have been used to solve for spherical harmonic coefficients of this tidal signal over a wide range of tidal frequencies: semi-diurnal, diurnal, and long-period. The contributions from the direct luni-solar attraction are well known. If the ocean tide contributions can be independently estimated and removed, either using ocean tide models derived from altimetry or from knowledge of oceanic dynamics, the residuals can be interpreted in terms of solid Earth structure. Particularly promising has been the ability of SLR to constrain anelasticity in the earth's mantle at tidal periods. Oral presentation; received August 30, 2002 |
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| Laser Ranging Contributions to Monitoring and Interpreting Earth Orientation Changes -- INVITED | Richard Gross |
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Jet Propulsion Laboratory The groundwork for a new field in the geophysical sciences -- space geodesy -- was laid in the 1960s with the development of satellite and lunar laser ranging systems, along with the development of very long baseline interferometry systems, for the purpose of studying crustal plate motion and deformation, the Earth’s gravitational field, and Earth orientation changes. The availability of accurate, routine determinations of the Earth orientation parameters (EOPs) afforded by the launch of the LAser GEOdynamics Satellite (LAGEOS) on May 4, 1976, and the subsequent numerous studies of the LAGEOS observations, has led to a greater understanding of the causes of the observed changes in the Earth’s orientation. LAGEOS observations of the EOPs now span 26 years, making it the longest available space-geodetic series of Earth orientation parameters. Such long duration homogenous series of accurate Earth orientation parameters are needed for studying long-period changes in the Earth’s orientation, such as those caused by climate change. In addition, such long duration series are needed when combining Earth orientation measurements taken by different space-geodetic techniques. They provide the backbone to which shorter duration EOP series are attached, thereby ensuring homogeneity of the final combined series. Oral paper; received August 23, 2002 |
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| Monitoring The Origin of the TRF with Space Geodetic Techniques -- INVITED | Erricos Pavlis |
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JCET/UMBC - NASA/GSFC The origin of the Terrestrial Reference System (TRS) is realized through the adopted coordinates of its defining set of positions and velocities at epoch, constituting the conventional Terrestrial Reference Frame (TRF). Since over two decades now, these coordinates are determined through space geodetic techniques, in terms of absolute or relative positions of the sites and their linear motions. The continuous redistribution of mass within the Earth system causes concomitant changes in the Stokes^“ coefficients describing the terrestrial gravity field. Seasonal changes in these coefficients have been closely correlated with mass transfer in the atmosphere, hydrosphere and the oceans. The new gravity-mapping missions, CHAMP and GRACE, and to a lesser extent the future mission GOCE, address these temporal changes from the gravimetric point of view. For the very low degree and order terms, there is also a geometric effect that manifests itself in ways that affect the origin and orientation relationship between the instantaneous and the mean reference frame. Satellite laser ranging data to LAGEOS 1 and 2 contributed in this effort the most accurate results yet, demonstrating millimeter level accuracy for weekly averages. Other techniques, like GPS and DORIS, have also contributed and continue to improve their results with better modeling and more uniformly distributed (spatially and temporally) tracking data. We will present the results from the various techniques, assess their accuracy and compare them. Finally, we will look into potential improvements in the future, which will likely lead us to even finer resolution and higher accuracy through the constructive combination of the individual time series. Oral paper; received August 23, 2002 |
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| Absolute Earth Scale from SLR Measurements -- INVITED | Peter Dunn |
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Raytheon ITSS Since the LAGEOS-1 satellite was launched in 1976, the systematic instrument error of the best satellite laser ranging observatories has been steadily reduced. Advances in overall system accuracy, in conjunction with improved satellite, Earth, orbit perturbation and relativity modeling, now allows us to determine the value of the geocentric gravitational coefficient (GM) to less than a part per billion (ppb). This precision has been confirmed by observations of the LAGEOS-2 satellite, and is supported by results from STARLETTE, albeit at a lower level of precision. When we consider observations from other geodetic satellites orbiting at a variety of altitudes and carrying more complex retro-reflector arrays, we obtain consistent measures of scale, based upon empirically determined, satellite-dependent detector characteristics. The estimates of GM from SLR analysis fall comfortably within the ten ppb uncertainty of that determined from the most accurate alternative from lunar laser ranging observations. The adoption of a value of GM differing by a ppb would result in a difference of a few millimeters in the height definition of a near-Earth satellite. The precision of the estimate of GM from satellite laser ranging has improved by an order of magnitude in each of the last two decades, and we will discuss projected advances which will result in further refinements of this measure of Earth scale. Oral presentation; received September 10, 2002 |
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| First results of the French Transportable Laser Ranging Station during the 2002 Corsica campaign for the JASON-1 calibration and validation experiment | Joëlle Nicolas |
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Observatoire de la Côte d'Azur/CERGA Pascal Bonnefond, Pierre Exertier, and Philippe Berio After its phase of improvement and its validation with a triple laser ranging collocation experiment performed at the Grasse observatory, France at the end of 2001, the French Transportable Laser Ranging Station (FTLRS) is presently in Corsica since January 2002. It is the first campaign outside the Grasse observatory for the FTLRS in its new configuration. The aim of this campaign is the validation of the orbit and the altimeter calibration (CAL/VAL) of the JASON-1 satellite at the centimeter level. The mobile station supports the Precise Orbit Determination and geodetic reference operations. The station also participates to the same kind of experiment for the ENVISAT mission. Herein we present first the preliminary results of this campaign concerning the station positioning obtained with a combination of LAGEOS -1, -2, STELLA, and STARLETTE observations, and the comparison with the JASON-1 solution. We also summarize the contribution to the FTLRS for the JASON-1 CAL/VAL phase. This campaign is also very instructive for the next one which is scheduled in Gavdos (Crete) in 2003. Oral presentation; received August 8, 2002 |
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| Preliminary orbit determination of GRACE satellites using laser ranging data | Milena Rutkowska |
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Space Research Centre, Polish Academy of Sciences Janusz B. Zielinski Presented by: Janusz Zielinski The GRACE twin satellites provide data significantly improving the model of the Earth gravity field. Except of the autonomous GPS based orbit recovery system they are observed by the laser tracking stations. Laser data can be used for callibration and validation of other systems if the required accuracy could be attained. Presented results offer preliminary estimations of the data quality and the evolution of orbits of two spacecrafts GRACE A and B. The study is based on observations taken by the global network during the period May 5, 2002 - May 19, 2002. Oral presentation; received August 30, 2002 |
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| The Development of NASA Gravity Field Models and their Dependence on SLR – Invited | Frank Lemoine |
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Space Geodesy Branch Steven Klosko, Christopher Cox Scott LuthckeSpace Geodesy Branch Satellite laser ranging data has been an integral part of Earth gravity model development since the days of the earliest GEM (Goddard Earth Models) in the 1970's. SLR data have contributed both directly in the form of tracking of the multiplicity of satellites that have made up these solutions, and indirectly in the definition and stabilization of the terrestrial reference frame. The evolution of the SLR technology required improvements in modeling and yielded ever refined models. In this paper, we will review the contribution of SLR data, starting with the first generation laser systems in the early 1970's. The launch of Lageos-1 and its contribution will be highlighted. The intensive effort to develop an improved geopotential model prior to the launch of TOPEX/Poseidon will be reviewed. Finally we will provide some perspectives on the use of SLR data in current geopotential solutions with CHAMP data. Oral presentation; received September 30, 2002 |
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