J. W. Robbins

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.

Crustal deformation around the Gulf of California

A number of Satellite Laser Ranging (SLR) sites in the southwest United States and Mexico, in operation for over ten years, have been supporting the global laser network measuring tectonic plate motion and providing information for studies of regional crustal deformation. Observations of the Laser Geodetic Satellite (LAGEOS) collected in 1994 by the transportable satellite laser ranging system, TLRS-4, at two sites on the Baja peninsula now provide the means to extend the network of fixed stations at Monument Peak and Otay Mountain in southern California and Mazatlan on mainland Mexico. After the third SLR occupation of Ensenada, its estimated site motion exhibits nearly the full plate rate predicted by the NNR-NUVEL1A model for a location on the Pacific plate. At the southern tip of the Baja, the motion of Cabo San Lucas has an azimuth that is more westerly than that expected from Pacific modeled motion. This discrepancy in azimuth, in conjunction with the slower SLR recovered velocity for Mazatlan, results in an apparent 6 mm/yr faster spreading rate across the mouth of the Gulf of California than that predicted by the NUVEL-1A model. The velocities of the Monument Peak and Otay Mountain sites show the expected long-term difference from Pacific plate motion, due to their proximity to the San Andreas Fault system.

The role of laser determined orbits in geodesy and geophysics

Abstract Since the US National Aeronautics and Space Administration (NASA) launch of the LAGEOS (LAser GEOdynamics Satellite) in May of 1976, a wealth of information enhancing the knowledge of geodesy and geophysics has become available to the scientific community. Both the quality and quantity of data have improved. The precision of satellite laser ranging (SLR) is now at the sub-centimeter level, and cooperation with other nations in the purchase and deployment of SLR systems and laser satellites has led to an extensive SLR data base. Scientists are now able to make precise estimates of the product of the gravitational constant and the Earths mass (GM), polar motion, Earth rotation, laser station coordinates, distances between stations, and tectonic plate motions. SLR also contributes strongly to improved estimates of the Earths gravitational field. Recent NASA solutions using the laser ranging data indicate the value of GM is 398600.4408 ± 0.0006 km3/s2. The one sigma precision of the other geodetic parameters obtained are on average 1.9 marc sec polar motion, 0.09 ms length of day, 35 mm center of mass geodetic positioning, 20 mm baselines, and 5 mm/yr tectonic plate rates. The precision of a number of these quantities has been confirmed by comparison of an independent data set obtained by very long baseline interferometry antennas located near several of the SLR sites.

Tracking strategies for laser ranging to multiple satellite targets

Abstract By the middle of the decade, several new laser geodynamic satellites will be launched to join the current constellation comprised of LAGEOS (US), Starlette (France), Ajisai (Japan), and Etalon I and II (USSR). The satellites to be launched, LAGEOS II and III (US & Italy), and Stella (France), will be injected into orbits that differ from the existing constellation so that geodetic and gravimetric quantities are sampled to enhance their resolution and accuracy. An examination of various possible tracking strategies adopted by the network of laser tracking stations has revealed that the recovery of precise geodetic parameters can be obtained over shorter intervals than is currently obtainable with the present constellation of satellites. This is particularly important in the planning of mobile laser tracking operations, given a network of permanently operating tracking sites. Through simulations, it is shown that laser tracking of certain satellite passes, pre-selected to provide optimal sky-coverage, provides the means to acquire a sufficient amount of data to allow the recovery of 1 cm station positions.

Deformation in the Pacific Basin from Lageos

Measurements to LAGEOS have provided the means to determine the relative positions as a function of time for six laser tracking sites in the Pacific Basin. These relative positions, and the determined relative motions, have been used to generate a motion model that is compared to geological predictions. The motion of the central station in this sub-network (from the global SLR network) on Maui, Hawaii agrees well with that suggested by Minster and Jordan’s AM0–2 (1978) model when considered relative to the stations on the North American continent. The station at Yarragadee, Australia provides the longest record of continuous tracking (9 years). Its motion suggests a rate which is 11 mm/year slower than the 74 mm/year given by the AM0–2 prediction for the Austro-Indian plate at that location. The observed motion of Simosato, Japan does not conform to that predicted by the AM0–2 model for the Pacific, North American, or the Eurasian plate. The station at Huahine, in the Society Islands, appears to be moving south-west of the direction expected from the AM0–2 model by approximately 9° at a rate 11 mm/yr faster than AM0–2. The motion of Easter Island (on the Nazca plate) is expected to be better resolved when the 1988–1989 occupation is completed. The observed westward movement of Monument Peak in Southern California is an outcome of the so called “San Andreas anomaly” and provides an important link to sites located on the stable North American continent.

Plate Motions and Deformation from Lageos

LAGEOS laser ranging observations collected by the global tracking network between May 1976 and December 1988 have been analysed to yield a comprehensive geodetic parameter solution. Some of the stations in the participating network have a continuous tracking record over the full LAGEOS mission lifetime and can be used to monitor positions in a limited network for over 12 years. However, the introduction of several new stations of improved precision has allowed determination of relative positions to centimeter accuracy for each quarter of as year solution since the beginning of 1980. A nine year history of these three-dimensional positions yields horizontal inter-station baseline rates to an accuracy of a few mm/yr as well as vertical displacement rates to similar accuracy. Comparisons of the measurements of plate motion between sites centrally located within the plates with those predicted by current geological models agree to better than 9 mm/yr RMS. The phenomena of post-glacial rebound is considered as an explanation for the vertical motion.