Satellite laser ranging (SLR) to LAGEOS acquired during the period 1978–1988 has been analyzed to yield estimates of tectonic motion for 22 laser tracking stations situated on seven major plates. The analysis is based on the precise modeling of the orbit dynamics of LAGEOS and includes the determination of other geodynamic and nonconservative force model parameters involved in the orbit determination problem at the centimeter level. Site velocities were recovered from station positions determined each calendar quarter using a network adjustment procedure which maintains the reference frame. Within the stable interior of tectonic plates, the recovered motions indicate good agreement with existing estimates of stress fields, particularly across northern Europe, the eastern Pacific, and in the Basin and Range province of North America. A comparison of the intersite motions for stations centrally located on separate plates between SLR and those implied by the NUVEL 1 geologic motion model yield a strong positive correlation of 0.989 and a 3–7% scale difference which may be attributable to recent changes in relative velocities or geologic time scale uncertainties. Sites located at or within 250 km of convergent plate boundaries have additional components of motion, indicating a need for further tectonic modeling beyond that provided by simple rigid plate motions. For lines crossing the north Atlantic, the San Andreas fault, and within the Basin and Range province, the geodesic rates determined by SLR are in good agreement with those determined by very long baseline interferometry, an alternative space geodetic technique.

We present estimates for the mean bias of the TOPEX/POSEIDON NASA altimeter (ALT) and the Centre National d'Etudes Spatiales altimeter (SSALT) using in situ data gathered at Platform Harvest during the first 36 cycles of the mission. Data for 21 overflights of the ALT and six overflights of the SSALT have been analyzed. The analysis includes an independent assessment of in situ measurements of sea level, the radial component of the orbit, wet tropospheric path delay, and ionospheric path delay. (The sign convention used in this paper is such that, to correct the geophysical data record values for sea level, add the bias algebraically. Unless otherwise stated, the uncertainty in a given parameter is depicted by ±σx, where σx is the sample standard deviation of x about the mean.) Tide gauges at Harvest provide estimates of sea level with an uncertainty of ±1.5 cm. The uncertainty in the radial component of the orbit is estimated to be ±1.3 cm. In situ measurements of tropospheric path delay at Harvest compare to within ±1.3 cm of the TOPEX/POSEIDON microwave radiometer, and in situ measurements of the ionospheric path delay compare to within −0.4±0.7 cm of the dual‐frequency ALT and 1.1±0.6 cm of Doppler orbitography and radiopositioning integrated by satellite. We obtain mean bias estimates of −14.5±2.9 cm for the ALT and +0.9±3.1 cm for the SSALT (where the uncertainties are based on the standard deviation of the estimated mean σx–/y , which is derived from sample statistics and estimates for errors that cannot be observed). These results are consistent with independent estimates for the relative bias between the two altimeters. A linear regression applied to the complete set of data shows that there is a discernable secular trend in the time series for the ALT bias estimates. A preliminary analysis of data obtained through cycle 48 suggests that the apparent secular drift may be the result of a poorly sampled annual signal.

Four campaigns to acquire Satellite Laser Ranging (SLR) measurements at sites in the Mediterranean region have been completed. These measurements to the LAGEOS satellite, made largely by mobile systems, cover a time span beginning in November 1985 and ending in June 1993. The range data from 18 sites in the central and eastern Mediterranean have been simultaneously analyzed with data acquired by the remainder of the global laser tracking network. Estimates of horizontal motion were placed into a regional, northern Europe‐fixed, kinematic reference frame. Uncertainties are on the order of 5 mm/yr for sites having at least four occupations by mobile systems and approach 1 mm/yr for permanently located sites with long histories of tracking. The resulting relative motion between sites in the Aegean exhibit characteristics of broadly distributed pattern of radial extension, but at rates that are about 50% larger than those implied from studies of seismic strain rates based on seismicity of magnitude 6 or greater across the region. The motion estimated for sites in Turkey exhibit velocity components associated with the westward motion of the Anatolian Block relative to Eurasia. These results provide a present‐day “snapshot” of ongoing deformational processes as experienced by the locations occupied by SLR systems.

The editors of the special issue want to thank the individuals and institutions involved in the preparation of the LAGEOS special issue. We particularly appreciate the efforts of the a. uthors, who submitted high-quality reports of their research, and the reviewers, who provided insightful comments and suggestions. Gerald Schubert, editor of JGR-Solid Earth and Planets, provided editorial and administrative expertise as well as general guidance throughout the preparation of the special issue. Marsha Berkowitz, editor's assistant, provided the logistical management for processing the papers through the review cycle. Michael Connolly, the production coordinator of the American Geophysical Union, directed the business aspects of producing the special issue.

S U M M A R YAn analysis of Satellite Laser Ranging (SLR) data to the LAGEOS satellite has yielded improved estimates of the horizontal motion for a subset of 34 tracking sites within the global tracking network. The analysis, called SL8.3, utilized data acquired between 1980 January and 1993 June by the global network composed of 71 sites. The solution design provides for the simultaneous estimation of site positions and their velocities within a pre-defined kinematic frame. The solution is statistically rigorous and retains the full correlation information content. Least-squares estimates of relative poles of rotation, which are used to model the motion of one plate relative to another, were made based on the SLR estimated velocities for sites known to be well away from deformation zones. The resulting SLR-based relative rotation poles differ slightly from those of NUVEL-1, but in general, indicate that the magnitude of the SLR implied velocities is slower than those implied by NUVEL-1, consistent with the 4-5 per cent slowing in relative spherical rates noted in earlier comparisons. Spherical rates between sites in western North America support models of extension in the Basin and Range Province and the rotation of the Sierra Nevada microplate. An analysis of the spherical rates crossing the North Atlantic shows that SL8.3 estimated extension between North America-Eurasia sites is generally smaller than those implied by NUVEL-1; meanwhile SL8.3 rates between North America-Africa sites are in better agreement with NUVEL-1, although they are not so well determined. The maintenance and ongoing monitoring of global SLR site kinematics provides a well-defined global reference which will aid in combination global kinematic solutions where information from other technologies are merged (e.g. Very Long Baseline Interferometry and Global Positioning System) and in providing the context for densification studies of regional kinematics derived from terrestrial and Global Positioning System observations.