SLR REVIEW COMMITTEE REPORT

the SLR Review Committee:

Gerhard Beutler, Astronomical Institute of Berne
Richard Eanes, University of Texas
Joseph Engeln, University of Missouri
Tom Herring, Massachusetts Institute of Technology
William Melbourne, Jet Propulsion Laboratory
Michael Pearlman, Center for Astrophysics - Executive Secretary
John Rundle, University of Colorado
Irwin Shapiro, Center for Astrophysics - Chairperson
John Wahr, University of Colorado
Carl Wunsch, Massachusetts Institute of Technology

Miriam Baltuck, Chief

Solid Earth and Natural Hazards Branch
National Aeronautics and Space Administration
Washington, DC

April 1997

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SLR REVIEW COMMITTEE REPORT

Appendix II

Science and Applications

1. Science

SLR is an important tool for the study of the long-term behavior of the Solid Earth because SLR measures range to satellite targets of great longevity. In contrast to radio tracking methods which require regular replenishment of their space segments, SLR can provide range measurements to specific targets over time intervals of decades (and probably even centuries, if desired).

The Committee agreed unanimously that SLR can make significant additional scientific contributions to many areas of NASA interest:

1. Science (listed in approximate priority order within each area)
   1. solid Earth
      1. temporal variations in tesseral and zonal harmonics of the Earth's gravity field;
      2. low and intermediate harmonics of the Earth's static gravity field;
      3. tides; and
      4. total mass;
   2. Earth surface
      1. altimeter mapping for oceanography and ice studies; and
      2. station vertical positions with respect to Earth's center of mass;
   3. Moon
      1. dissipation;
      2. core-mantle boundary;
      3. free libration; and
      4. reference-frame tie;
   4. relativity tests
      1. Lunar Laser Ranging (LLR); and
      2. Satellite Laser Ranging (SLR);
2. Precision tracking applications
   1. time transfer;
   2. calibration and validation of GPS systems;
   3. new technology satellites and propagation studies:
      1. Tether Physics and Survivability Experiment (TiPS); and
      2. Advanced Earth Observing Satellite/Retroreflector In Space (ADEOS/RIS); and
   4. emergency backup for active tracking systems; and
3. Measurement errors.

We discuss below each of these topics in turn. Because quantitative statements are dependent upon so many variables, with some not well known, our discussion of these topics is mostly qualitative. The statements on importance represent our best judgements.

Literature references have not been included, but will be provided by the Review Committee upon request.

1. Science

a. Solid Earth

i. Temporal Variations in Tesseral and Zonal Harmonics of the Earth's Gravity Field

Processes that redistribute mass, either within the Earth or on or above its surface, can cause time-varying changes in the Earth's external gravity field. Observations of time-dependent gravity is useful for learning about those processes. Existing SLR observations have been used to solve for secular changes in the very longest wavelength zonal components of the gravity field, which constrain a combination of the effects of postglacial rebound and changes in polar ice mass. These, in turn, have implications for the Earth's viscosity profile and for on-going changes in global sea level. Observed seasonal and interannual fluctuations in those same gravity coefficients have been interpreted as due to mass redistribution within the atmosphere and ocean, and to the effects of long-period solid Earth and ocean tides. Effects of hydrological processes over continental regions are also likely to be important at various periods and spatial wavelengths. Because of recent improvement in the precision of SLR measurements, the development of increasingly accurate models of the static gravity field, and the commencement (in 1992) of two-shift operations of the NASA SLR Network, it is now feasible to begin solving for non-zonal and shorter-wavelength time-dependent terms. New data should make it easier to separate the effects of competing geophysical processes. As tracking operation periods are extended with the new system improvements, and as further SLR targets are launched, SLR's capability to separate effects should continue to improve. As an example, the effects of postglacial rebound over Northern Canada and Scandinavia and of changes in the Antarctic and Greenland ice masses could all be separated reasonably effectively using components up to degree and order four, which is not an unreasonable expectation for SLR capabilities.

An optimally designed, dedicated gravity satellite mission (e.g., spacecraft-to-spacecraft tracking with a pair of nearly co-orbiting satellites) could provide time-dependent gravity results with an accuracy and a spatial resolution far exceeding those obtainable with SLR. But all such missions currently being considered have nominal lifetimes of at most five years, due to fuel requirements at the necessary low altitudes. This short lifetime prevents the separation of secular terms from decadal or other long-period terms. A continuing time series from SLR, even though it would contain far fewer and less accurate components, would help considerably in interpreting results from a dedicated gravity mission. In the longer term, a continuing SLR time series would be useful for extending the interpretation of the dedicated gravity results over the entire SLR lifetime, and for tying together gravity results from any additional dedicated gravity satellites that might be launched later on.

ii. Low and Intermediate Harmonics of the Earth's Static Gravity Field

The high quality of current models of the Earth's mean gravity field at wavelengths of 500 km and longer is primarily due to the contribution of laser ranging to a variety of satellite targets over the last 30 years. Simulations show that SLR tracking from the current global network to several new targets expected to be launched over the next several years could significantly improve future gravity-field models, and that elimination of tracking from NASA-supported SLR sites would substantially reduce the improvement. Nevertheless, without a significant redeployment of the SLR stations to achieve a more uniform distribution around the globe, the contribution of SLR is limited due to the fact that localized orbital perturbations, from gravity anomalies in regions not covered by the SLR network, are not fully sensed.

GPS tracking of TOPEX/POSEIDON and GPS/MET (a low orbiting spacecraft built for atmospheric measurements with GPS) has already been incorporated in current Earth gravity models and several new spacecraft in low orbit will carry GPS receivers in the near future. The nearly continuous tracking that GPS provides avoids the coverage problem discussed above, and as a result it is likely that future short wavelength gravity-field improvements will be dominated by the contribution of GPS, not SLR. In addition, there is a real possibility that the long sought space geodesy goal of a dedicated Earth gravity mission could be realized in the reasonably near future. Such a mission would increase the accuracy of static gravity-field models at short wavelengths well beyond that attainable with either SLR or GPS tracking (however, see below).

Although it is likely that GPS tracking and dedicated gravity missions will dominate future short wavelength gravity-field improvements, SLR will continue to play an important supporting role, especially at the longer spatial scales where temporal variations of the gravity field are large enough to be detected and to have their important information content extracted. Because the mass distribution of our planet changes over a wide variety of time scales, determination of the mean field requires a simultaneous treatment of the time variations. A dedicated gravity mission will be of short duration compared to the growing span of SLR tracking. A combination of the two sources of information will best isolate the mean field by linking the new measurements to the entire history of the SLR data. Further, errors in our knowledge of the diurnal and semidiurnal gravity-field changes due to ocean tides and thermally induced mass redistribution in the atmosphere can be a problem for a dedicated gravity mission, especially if it is in a sun-synchronous orbit. In such an orbit, the temporal variations linked to local solar time are aliased to low frequency and are thus difficult to separate from the mean field. Continued SLR tracking of targets at a variety of inclinations will allow adequate separation of the mean field from the aliased temporal variability. Finally, the high accuracy of SLR data will provide an important independent measure of the improvements in future gravity-field models.

iii. Tides

SLR observations have been used to estimate tidal contributions to the gravity field. However, ocean tides and solid-Earth tides can not be separated using the SLR data alone, but are lumped together in the solutions. One of the most useful applications of tidal estimations has been the study of the anelastic properties of the solid Earth. Ocean tide models are used to remove the (relatively small) ocean-tide effects, and the residuals are then compared with tidal models for an anelastic, oceanless Earth to determine the strength of mantle friction at tidal periods. This type of comparison is potentially important, because there are almost no other constraints on mantle friction at periods between about one hour (the longest seismic period) and the thousands of years that characterize postglacial rebound. At present, SLR results for the 18.6-year tide (the longest lunar tide) suggest that different mechanisms of mantle friction may be dominant at 18.6-years than at seismic periods. SLR results for the semi-diurnal, monthly, and fortnightly tides are now approaching the accuracies required to constrain anelasticity at those periods, as well. A more complete and confident conclusion at all periods will be possible from longer SLR time series.

Diurnal tidal observations from SLR are also nearly at the point where they could be used to address definitively a long-standing discrepancy between nutation and ground-based tidal observations, regarding the damping of a rotational oscillation involving the Earth's fluid core (the "Free Core Nutation"). Results from the two techniques exhibit systematic differences, which have so far prevented a confident identification of the dissipative mechanism. Recent SLR data may be complete and accurate enough to resolve this discrepancy, though it is unclear.

Certainly additional high-accuracy data with appropriate geographic coverage will allow for the better time-domain separation of the individual diurnal tidal frequencies needed to address this problem successfully.

iv. Total Mass

SLR tracking to Lageos has provided the most accurate determination of the Earth's gravitational coefficient (GM). The accurate determination of GM is critical in the definition of the absolute scale of the geocentric reference frame, affecting for example comparisons of SLR- and VLBI-determined station coordinates. Since each of the space geodetic techniques (e.g., SLR, DORIS, GPS, VLBI) has a different sensitivity to errors in GM, measurements of geodetic height using different combinations of techniques (at different times and places) will be corrupted differently by these errors, thus limiting scientific interpretation. Continued improvement in the SLR system will increase usefully the accuracy in the determination of the scale of the geocentric frame and hence in the scientific interpretation.

GM is currently known to 2 parts per billion (ppb), which corresponds to about 1 cm in scale uncertainty (i.e., height) for tracking stations and satellite orbits. Improving the accuracy of the absolute scale of the reference system would be valuable in defining the reference system as well as in identifying systematic errors between the various determinations. A reduction in the scale uncertainty to less than 0.5 ppb would be particularly valuable, as there are questions at the 0.7 to 1.4 ppb level regarding the scale factor between VLBI, GPS and SLR determinations of the terrestrial reference frame. Error analysis indicates that the possibility of centimeter level range biases and systematic troposphere refraction modeling errors were the significant sources of uncertainty in the determination of GM. Future efforts should be directed towards reducing the biases in the SLR data to well below the centimeter level, as well as towards validating and refining the troposphere refraction model so that it is essentially unbiased and the residual errors are mainly random from pass to pass.

b. Earth Surface

i. Altimeter Mapping for Oceanography and Ice Studies

Atmospheric-ocean interactions affect the flow and course of surface currents and produce a variety of sea-surface phenomena including waves and waterspouts. The Coriolis force constrains fluid motion to be parallel to the lines of constant pressure, flowing clockwise around northern hemisphere highs and counterclockwise around lows, with the sense of rotation being reversed in the southern hemisphere. The gravitational pull of the Sun and the Moon (and to a far lesser extent the planets) produces the familiar and predictable tides. The solid Earth, besides impeding the propagation of ocean waves and drastically affecting the course of ocean currents at the continental boundaries, can also impose its own dynamic signatures on the sea surface via, for example, occasional Earthquake-induced tsunamis.

These currents, eddies, and other dynamic features of ocean circulation create hills and valleys on the sea surface that are collectively known as ocean or sea surface topography, defined as the difference between sea level and the reference geoid. From the global topography data set one can

derive the general circulation of the ocean as well as its mesoscale variability. Ocean topography

typically varies by no more than a meter or two over distances of hundreds to thousands of kilometers, and current estimates of globally-averaged mean sea level rise vary between 1 and 3 mm per year.

Global measurements of sea surface topography over the oceans are being made by spaceborne microwave altimeters (e.g., Topex/Poseidon and ERS-2). Altimeters measure the average instantaneous distance between the spacecraft and a portion of the underlying terrain along the altimeter line-of-sight, by measuring the roundtrip transit time to the surface. However, proper interpretation of the sea surface topography from the altimeter measurements requires accurate knowledge of the satellite orbit, referenced to the Earth's center-of mass. Requirements for new programs are likely to limit errors to under the 1 cm level.

SLR should play an important role in the pursuit of these goals. Thus, although no single tracking system SLR, DORIS, or GPS has demonstrated an ability to meet this goal, preliminary GSFC analyses of SLR+DORIS+GPS combination solutions show improvements in both consistency and accuracy. Each tracking system has its own strengths and weaknesses, but with improved SLR tracking accuracy, standard errors in height determination may fall below 1 cm in combined solutions.

SLR has and will continue to be important for these reasons on present and future satellites. SLR is a major source of tracking and system calibration/validation for the altimeter aboard Topex/Poseidon and ERS-2. Both GFO-1 (Navy) scheduled for launch in March 1997 and JASON (Topex follow-on) scheduled for launch in 1998 will rely on SLR. Further, plans for Envisat-1 (European Space Agency) scheduled for launch in 1999 and NASA's Geoscience Laser Altimeter System (GLAS) scheduled for launch early in the next decade also include SLR as part of their overall tracking system. .

ii. Station Vertical Positions with Respect to Earth's Center of Mass

The need for accurate determinations of the vertical is fueled by the significant vertical crustal movements due to earthquakes and volcanoes, subsidence, and sea level changes, among others. Typically, accuracies of several millimeters are sought over time intervals of days to many years to discern these physical effects. In addition, in addressing such problems, widespread area coverage is highly desirable, even for tide gauges, so as to maximize resolution of horizontal trends in deformation profiles. Examples of phenomena where such accuracies and resolutions are important to define the dynamical behavior of the process, include postseismic earthquake deformation over time periods of weeks to years, prevolcanic inflation over time intervals of months to years, weekly and monthly monitoring of sea-level variations due to climatic, seasonal, and geophysical events, and monthly to yearly observations of subsidence in regions of active fluid or mass withdrawal.

SLR can make useful contributions to the vertical fiducial integrity of geodetic networks. However, SLR cannot provide the widespread area coverage as can GPS.

c. Moon

For lunar science, the fundamental questions that lunar laser ranging (LLR) can address fall into at least three main areas. In each one, current results are close to being able to settle important questions about the lunar interior that cannot be addressed in other ways.

i. Dissipation

The first area is dissipation of rotational energy. The tipping of the Moon's spin axis with respect to the expected direction for no dissipation gives a surprisingly low Q of about 27. Recent improvements in modeling of the dissipation indicate that roughly 40% is probably in a turbulent boundary layer at the top of a liquid core which is about 400 km in radius, and 60% in the lower mantle. This spin-axis measurement and analysis are presently the best evidence we have for a fluid core and for its size. However, as for the other areas discussed below, the results need sharpening with more and better LLR data, and further improvements in the analysis.

ii. Core-Mantle Boundary

The second area concerns the shape of the core-mantle boundary and the Love number k2 for distortion of the Moon. The apparent value of k2 from the LLR results, based on this boundary having its expected equilibrium shape, is 0.030+/-0.001. This value is substantially larger than expected from Apollo seismic results which covered depths down to about 1000 km, unless the shear velocity below that depth is very low. The effect of a nonequilibrium boundary shape has now been modeled and the data appear close to good enough to settle whether the shear velocity in the deep mantle is indeed very low or whether the difference in k2 is due to the boundary shape. More and more accurate LLR data should allow this key question to be resolved.

iii. Free Libration

The third area is free librations of the Moon. Both a 1.4 arcsec free libration in longitude with a 2.9 year period and a 74 year period elliptical wobble of the pole with semiaxes of 3 by 8 arcsec are discernible in the LLR data. Since the free librations damp with time, recent or continuing stimulating mechanisms are needed. Following a suggestion by Eckhardt, others have modelled the free longitude libration as overstimulated by resonance passage. The only presently known plausible way to excite this free wobble is turbulent motions in a fluid core. The third free libration mode is much smaller than the others, with amplitude about 0.02", and has only recently been detected. High accuracy LLR data are needed to better define the third mode and to extend the span of measurement of the 74 year period wobble.

iv. Reference-Frame Tie

The LLR measurements also determine the intersection of the Earth's equatorial plane with the plane of the Moon's orbit, and the angle between these planes. From these quantities, the equivalent quantities for the Earth's orbit are found. The resulting "dynamical equinox" and "obliquity of the ecliptic" are useful in tying the reference system for the solar system to that determined by the Earth's rotation. Since VLBI measurements tie the Earth rotation frame

tightly to an inertial frame based on very distant radio sources, the solar-system frame is tied to the same inertial frame via a combination of the different types of high-accuracy measurements. Continued LLR measurements will allow this tie to be improved and may have important benefits for future spacecraft navigation.

d. Relativity Tests

LLR can also contribute more in the future to tests of general relativity or, more generally, to tests of theories of gravitation. For gravitational physics, the main contributions involve testing Einstein's strong principle of equivalence, determining the relativistic precession of the whole lunar orbit, and looking for possible changes with time in the inherent strength of gravitational interactions. Very accurate tests of the usual equivalence principle have been done in the laboratory (standard errors of order one part in 10^11) and show that the ratio of the gravitational mass to the inertial mass is the same for materials with widely different nuclear compositions. An improved test using a satellite experiment is being considered by the NASA Office of Life and Microgravity Sciences. However, the LLR test suggested by Nordtvedt is the only one that tests whether the gravitational self-energy of a body (the Earth) has the same gravitational to inertial mass ratio as other kinds of matter/energy. (For a laboratory-sized body, the gravitational self energy is about a factor of 10^23 smaller than for the Earth.) This LLR test now shows no difference in this ratio for gravitational self-energy at the 0.1% level and could be improved further with new, high accuracy LLR measurements.

The relativistic precession of inertial frames due to the Sun is caused by the inherent curvature of space induced by the mass of the Sun. As a result, the whole lunar orbit precesses with respect to the distant stars by an extra 2 arcsec per century, compared with the predictions of classical dynamics. This effect was detected in LLR data, has now been measured by LLR to about 0.7% accuracy, and its measurement can be further improved with continued LLR observations.

Limits on possible changes in the strength of gravitational interactions with time have been established by several other kinds of measurements, including analysis of planetary radar data, radio tracking data from planetary orbiters and landers, and timing of pulsar signals. However, continued collection of LLR data will likely lead to a nearly tenfold higher accuracy than from other techniques, e.g., an accuracy of 2x10^-12 per year for checking on possible variations. Reaching this new level would be of great importance for our understanding of the evolution of the universe. Although nearly three decades of LLR measurements already are available, the improvement in the accuracy to the 3 cm level and then to the present 1 to 2 cm level occurred only in the last few years; thus continued LLR measurements are needed to reach this new level of accuracy.

One important test of general relativity feasible with SLR would involve the launching of another satellite, Lageos-III, in an orbit with an inclination supplementary to that of either Lageos-I or Lageos-II. The pair of satellites would be sensitive to the Lense-Thirring precession. The Lense-Thirring precession is the gravitomagnetic, or 'frame-dragging', effect on a satellite orbit,

in which the orbit plane is 'dragged' in the direction of the Earth's rotation. The analogous effect on a gyroscope, called the Schiff precession, is expected to be measured by the Gravity Probe B mission to better than one percent. Using the Lageos satellite pair of high-density, high-altitude satellites in orbits with supplementary inclinations, would further allow us to confirm or refute the existence of the gravitomagnetic effect, thus providing experimental support, respectively, for or against the general relativistic formulation of Mach's Principle. The supplementary inclinations provide for a precise cancellation of the most important of the Earth's gravitational perturbations, while the orbital height and high mass-to-area ratio reduce the surface forces to a level that can be effectively modeled. The most recent analyses indicate that this determination can be made to the 3-4 percent level with such a satellite pair. Another possibility for performing this test to this accuracy would be to use a low altitude pair of nearly co-orbiting satellites, with intersatellite tracking, to determine the gravitational perturbations with sufficient accuracy to obviate the need for an additional Lageos satellite. The two already in orbit could each provide a determination of the Lense-Thirring effect, thus providing some redundancy.

2. Precision Tracking Applications

a. Time Transfer

Timing laboratories using various atomic clocks are now claiming subnanosecond timing accuracies over time intervals of order one day and more, well beyond the present capability to transfer and compare time. GPS satellites, over time intervals of days, can routinely achieve 30 - 50 nsec in such time comparisons, whereas more careful experiments with GPS and microwave links via satellite can achieve corresponding accuracies of a few nanoseconds. Currently available pulsed laser techniques coupled with state-of-the-art spaceborne clocks offer improvement in the time reference scale to perhaps 20 - 50 psec for clock comparisons and for applications in cosmology, fundamental astronomy, and gravitational physics. Improved clock synchronization will also allow an increase in transmission rates for data and the implementation of more sophisticated means of security in communications, for both commercial and classified applications.

b. Calibration and Validation of GPS Systems

The inclusion of (unbiased) SLR range measurements to global positioning system (GPS) satellites in the analysis of biased radio carrier-phase measurements is expected to improve the stability and accuracy of GPS satellite orbits and, in turn, the geodetic parameters estimated using these orbits. However, although GPS satellites with retroreflectors have been in orbit since late 1994, there are still very few high quality SLR observations of these satellites that have been extensively analyzed and therefore it is not possible at present to assess the net impact that SLR will have on the determination of the orbits of GPS satellites; nonetheless, the results now available do suggest that there will be significant improvements to orbit quality with SLR data included in the analysis.

The second, and perhaps more important issue, is that of the phase reference point in GPS receivers. The difficulty arises because this point varies as a function of the azimuth and elevation angles from which the received GPS signal arrives at the antenna. Measurements of this function by three different groups in three different anechoic chambers all agree. The problem is that when the function is used in the analysis of GPS data and the results compared with those from SLR and VLBI for station heights, relative to the Earth's center-of-mass, the GPS values are systematically higher by the equivalent of a scale difference of about 16 parts per billion, about an order of magnitude in excess of the expected difference from the spread in the chamber measurements between the three independent groups. More puzzling is the fact that when the measured function is ignored in the analysis of the GPS data, the bias between GPS results and those from the other techniques virtually disappears. This critical problem will not be solvable, indeed might not have even been recognized, without an external, sufficiently accurate, standard, such as SLR, with which to compare.

c. New technology satellites and propagation studies

The global SLR network provides a major source of accurate satellite tracking for a number of spaceborne applications (see Table 1a). Two examples are:

i. Tether Physics and Survivability Experiment (TiPS)

The TiPS satellite is a proof of concept experiment carried out by the Naval Research Laboratory (NRL). The goals of the mission are to study the physics of spaceborne tethered satellite systems and to understand the survivability of tethers in an environment littered with space debris. TiPS consists of two passive end masses connected by a four kilometer tether. Each end mass has 18 faces, each with a retroreflector.

The orbital dynamics of TiPS is inferred from processing SLR observations. The orbital dynamics include the pendulum motion of the tethered system, the orbital motion of the observed end masses, the flexibility of the tether, as well as consideration of other perturbations specific to tethered systems. To achieve an understanding of the orbital behavior of TiPS, SLR data will be used to provide precise time histories of the positions of the end masses.

ii. Advanced Earth Observing Satellite / Retroreflector In Space (ADEOS/RIS)

The ADEOS/RIS satellite launched in late 1996 has as its primary mission the monitoring of global environmental changes such as maritime meteorological conditions, atmospheric ozone, and gasses that promote global warming. Eight internationally sponsored sensors located on the satellite are being used to monitor the environment. One of these sensors is the Retroreflector in Space (RIS). The RIS is a hollow cube-corner retroreflector with an effective area of 0.5 meters. The RIS is used for experiments on laser long-path absorption measurement of atmospheric trace species using LIDAR and for experiments with two color SLR equipped systems. Data from the global SLR network is necessary to provide the orbital predictions needed to support the precise pointing requirements for these experiments

d. Emergency Backup for Active Tracking Systems

For satellites equipped with retroreflectors, SLR can provide backup when active tracking systems fail. A recent example is the failure of the PRARE system aboard the European Earth Remote Sensing Satellite (ERS-1) shortly after launch in 1991. Without the intensive tracking of the global SLR network, the radar altimeter data, which continued to flow, would have been useless. With the SLR data, the full mission objectives of mapping the ocean sea surface for circulation studies were accomplished. We note that the total cost of the SLR network for 10 years is probably less than the cost of the saved ERS-1 mission.

3. Measurement errors

As geodetic measurements approach millimeter level accuracies, error sources become more difficult to diagnose solely through engineering tests and scrutiny by the analyst. SLR, as with each of the other space geodetic techniques, has its own family of error sources with their own signatures that corrupt the overall measurement. Some error sources may tend to average out over time for some geodetic observables; others, however, may be correlated significantly with the geophysical properties that we want to study. As an example, errors in calibrating temperature effects on cables can have the same periodicity as diurnal effects in Earth rotation or tides. The mix of techniques should allow us to capitalize on the differences in the nature of the error sources to understand the weaknesses of each and to combine results in the most advantageous manner, utilizing the strengths of each and at least partially circumventing the weaknesses. Such a mix may be of particular interest in addressing more elaborate models for refraction corrections for the atmosphere. GPS is far more susceptible than SLR to the temporal and spatial variations of water vapor. In the future, it may be feasible to develop sufficiently accurate profiles of the dry and wet atmosphere with sufficiently high spatial and temporal resolution to make the needed corrections. One method uses GPS receivers aboard low satellites to observe limb "soundings" of the atmosphere. Another, based on two-wavelength measurements with SLR, is sensitive to the columnar refraction through the dry atmosphere; several groups including GSFC are actively developing this technique.

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