Peter J. Dunn

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.

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.

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.

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.

Contemporary plate motions from Lageos - A decade later

Abstract Reduction of a decades worth of Lageos Satellite Laser Ranging (SLR) is providing new insights into contemporary plate kinematics. Globally, the SLR results have largely confirmed the plate motion models developed from geologic evidence. Analysis of the data from 12 base stations finds all interstation SLR rates having a linear cross correlation of .91 with the Minster and Jordan geologic model. To within their uncertainties, the time scales of the geologic and SLR models are found to be in agreement indicating that globally, the tectonic rates are linear over time scales of 1 to 10 million years. Regionally, SLR data exclusively has been used to develop a model of the absolute station motions for observing sites within the Western United States. The observed intersite motion of the two stations comprising the San Andreas Fault Experiment appears to be non-linear over the last decade, with the relative motion between these sites changing from −6 to −2 cm/year during the last four years. The results achieved with SLR are complemented and largely confirmed by those achieved with other space technologies. It is clear that Satellite Laser Ranging has reached a new level of maturity. After passing through the threshold of confirming the global nature of plate kinematics, research is now focusing on the development of models for the effective utilization of the constraints provided by space geodesy. These constraints will assist in our understanding of the mechanisms which drive tectonic motions and cause a complex picture of strain accumulation at the plate boundaries.

Geopotential resonance in a Landsat orbit

When the orbit of the Landsat I spacecraft was liberated to natural forces, the loss of observations to the remote sensing community was balanced by a modest gain for geodesy. The orbit’s long ground-track repeat period of eighteen days gives rise to a shallow resonance with fourteenth, twenty-eighth and forty-second order terms in the geopotential. A single continuous span of twenty-four days of Unified S-Band tracking data, collected at a single station in 1976, has been analyzed to define constraints on the dominant resonance terms of these orders and of fourteenth-order fringe resonance effects depending on the eccentricitye≈.002. Tracking observations from other stations collected during 1974 and 1975 gave essentially the same results, which provided error estimates for the lumped resonance coefficients. The application of the resonance model can considerably improve the definition and prediction of the Landsat 1 orbit. Direct numerical estimates of the influence coefficients in the resonance constraint equations were made to confirm the accuracy of analytical expressions which allow the equations to be applied to geopotential fields of arbitrarily high degree and order. Several recently derived gravity fields were tested against the Landsat resonance constraints and their comparative agreement is discussed.

The next generation of satellite laser ranging systems

Satellite laser ranging (SLR) stations in the International Laser Ranging Service (ILRS) global tracking network come in different shapes and sizes and were built by different institutions at different times using different technologies. In addition, those stations that have upgraded their systems and equipment are often operating a complementary mix of old and new. Such variety reduces the risk of systematic errors across all ILRS stations, and an operational advantage at one station can inform the direction and choices at another station. This paper describes the evolution of the ILRS network and the emergence of a new generation of SLR station, operating at kHz repetition rates, firing ultra-short laser pulses that are timestamped by epoch timers accurate to a few picoseconds. It discusses current trends, such as increased automation, higher repetition rate SLR and the challenges of eliminating systematic biases, and highlights possibilities in new technology. In addition to meeting the growing demand for laser tracking support from an increasing number of SLR targets, including a variety of Global Navigation Satellite Systems satellites, ILRS stations are striving to: meet the millimetre range accuracy science goals of the Global Geodetic Observing System; make laser range measurements to space debris objects in the absence of high optical cross-sectional retro-reflectors; further advances in deep space laser ranging and laser communications; and demonstrate accurate laser time transfer between continents.

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.

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.