R. Kolenkiewicz

The measurement of fault motion by satellite laser ranging

Abstract The distance between two points on opposite sides of the San Andreas Fault is being derived from laser tracking of near-earth satellites as part of an experiment to estimate the motion along the plate boundary. The two sites, at Otay Mountain near San Diego and at Quincy in northern California, are nearly 900 km apart and approximately 150 and 270 km, respectively, away from the main strike of the San Andreas Fault. The angle between the fault and the intersite vector is approximately 25°. In the fall of 1972 satellite laser tracking systems occupied these two sites, and from the data collected the relative location of the two sites was determined. The two sites were reoccupied in the fall of 1974 and again in the fall of 1976, and provided two further estimates of the relative positions of the two sites. The results of these first three measurements indicate a shortening of the intersite baseline between San Diego and Quincy at an average rate of 9 ± 3 cm/year, suggesting a much larger possible present-day motion across the fault system than expected. The main source of error in this analysis is the motion of the spacecraft which is significantly affected by unmodeled anomalies in the earths gravity field. However, major advances in our knowledge of the gravity field are expected over the next few years and as these occur the accuracy of the present results will improve.

Seasat Altimeter Calibration: Initial Results

Preliminary analysis of radar altimeter data indicates that the instrument has met its specifications for measuring spacecraft height above the ocean surface (� 10 centimeters) and significant wave height (� 0.5 meter). There is ample evidence that the radar altimeter, having undergone development through three earth orbit missions [Skylab, Geodynamics Experimental Ocean Satellite 3 (GEOS-3), and Seasat], has reached a level of precision that now makes possible its use for important quantitative oceanographic investigations and practical applications.

Polar Motion from Laser Tracking of Artificial Satellites

Measurements of the range to the Beacon Explorer C spacecraft from a single laser tracking system at Goddard Space Flight Center have been used to determine the change in latitude of the station arising from polar motion. A precision of 0.03 arc second was obtained for the latitude during a 5-month period in 1970.

Earth scale defined by modern satellite ranging observations

Recent advances in Satellite Laser Ranging (SLR) allow us to determine an improved value of the geocentric gravitational coefficient (GM) of 398600.4419 +/− .0002 km³/sec² (one sigma). This value is based on recent SLR observations of the LAGEOS I satellite; it is confirmed by observations of LAGEOS II, and is supported by results from Starlette, albeit at a lower level of precision. The information from these other satellites helps to support our claim of a conservative error estimate for GM, which amounts to about one half of a part per billion (ppb). Our determination of GM with the span of LAGEOS I data originally used to determine the current IERS92 standard (398600.4415 +/− .0008 km³/sec²) gives approximately the same value at about the same uncertainty quoted by the authors in 1992. The improved value that we suggest falls well within the two-ppb error of the current standard, but differs from it by more than the error of the new estimate. The precision of the estimate of GM from SLR observations has improved by an order of magnitude in each of the last two decades.

Determination of polar motion and earth rotation from laser tracking of satellites

Laser tracking of the Lageos spacecraft has been used to derive the position of the Earth’s pole of rotation at 5-day intervals during October, November and December 1976. The estimated precision of the results is 0.01 to 0.02 arcseconds in both x and y components, although the formal uncertainty is an order of magnitude better, and there is general agreement with the Bureau International de l’Heure smoothed pole path to about 0.02 arcseconds. Present orbit determination capability of Lageos is limited to about 25 cm rms fit to data over periods of 5 days and about 50 cm over 50 days. The present major sources of error in the perturbations of Lageos are Earth and ocean tides followed by the Earth’s gravity field, and solar and Earth reflected radiation pressure. Ultimate accuracy for polar motion and Earth rotation from Lageos after improved modeling of the perturbing forces appears to be of order ± 5 cm for polar motion over a period of about 1 day and about ± 0.2 to ± 0.3 milliseconds in U.T. for periods up to 2 or 3 months.

Polar motion and earth tides from laser tracking

The tracking of near-Earth satellites with laser systems permits the determination of the variation of latitude of the tracking station and the variation in the rotation of the Earth. The present-day capability of a single station is approximately 75 cm in latitude averaged over 6h and 0.8 ms in the length of day. When the Laser Geodynamics Satellite (Lageos) is launched, a network of laser stations is projected to be able to achieve better than 10 cm in each coordinate from less than one day of tracking. The perturbations of near-Earth satellites by solid Earth and ocean tides are now measurable and can provide new information about the Earth and oceans. The orbit perturbations have long periods (days, months) and the analysis of orbital changes are providing estimates of the amplitudes and phases of the major tidal components.

Non-conservative forces on LAGEOS I and II

Abstract The behavior of the empirically determined along-track accelerations from the LAGEOS satellites has been attributed to a variety of physical phenomena. Of these, the models for Yarkovsky thermal drag, anisotropic reflectivity and Yarkovsky-Schach drag are dependent on the spin axis orientation of the satellite. This investigation explores the utilization of these models in an attempt to recover the spin axis orientation history, particularly for the LAGEOS I satellite.

Earth scale below a part per billion from Satellite Laser Ranging

Since the LAGEOS I satellite was launched in 1976, the systematic instrument error of the best satellite laser ranging observatories has been steadily reduced to the current level of only a few millimeters. 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 II 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 somewhat more complex retro-reflector arrays, we obtain consistent measures of scale, which however must be based upon empirically determined, satellite-dependent detector characteristics. We arrive at an estimate of GM of 398600.44187 +/-.00020 km3/sec2, which lies within the 2 ppb uncertainty of the current standard, but differs from it by more than the error of the new estimate. Both the current standard and our recommended value fall comfortably within the ten ppb uncertainty of that determined from the most accurate alternative from lunar laser ranging observations. 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.

Geodetic Applications of Laser Ranging

The measurement of intersite distances with laser ranging to satellites has been demonstrated during the last few years for distances of several hundred to several thousand kilometres with precisions of a few tens of centimetres. These techniques are now being tested across the San Andreas fault in California where it is hoped plate motion will be observable after several years of measurements. The first measurements, between sites in southern and northern California, were made in 1972 and repeated again in 1974 with agreement between the baselines for each of the two years at the 10 cm level. The next measurements are planned for the summer of 1976. The results of these and related experiments will be described together with simulations of the projected capability using the high altitude Lageos satellite. General plans for future experiments will be described.

Satellite altimeter calibration techniques

Abstract During the next five years several satellites carrying high accuracy radar altimeters are scheduled for launch. These include the European Space Agency ERS-1 satellite and the jointly sponsored NASA/CNES TOPEX/POSEIDON satellite. In-flight height calibration and height stability verification for both satellites are planned, and studies have been conducted to ascertain the calibration techniques which can most effectively satisfy the mission requirements. All of the proposed calibration techniques require at least one laser to provide the satellite height reference. For most techniques, only one laser in the calibration area is utilized; in this instance the satellite groundtrack must pass relatively close (within 20 km) to the laser site in order to obtain an orbit height uncertainty of less than one cm. The currently proposed calibration techniques for over-water calibration include: (1) tide gauge on a tower at-sea and a nearby laser; (2) laser and tide gauge on an island with offshore satellite pass and geoid tie between satellite groundtrack and laser, (3) tide gauge on a tower at-sea with satellite positioning from multiple lasers and a Global Positioning System (GPS) tie between the lasers and tide gauge; and (4) laser and tide gauge on a tower at-sea. Error budgets for these techniques have been developed based on state-of-the art laser tracking systems. For a single pass, these budgets have been found to have one sigma height uncertainties in the 2.8 to 4.9 cm range.