We have been developing a system for detecting seafloor crustal movement by combining kinematic GPS and acoustic ranging techniques. A linear inversion method is adopted to determine the position of seafloor stations from coordinates of a moving survey vessel and measured travel times of acoustic waves in seawater. The positioning accuracy is substantially improved by estimating the temporal variation of the acoustic velocity structure. We apply our method to the ranging data acquired at the seafloor reference point, MYGI, located off Miyagi Prefecture, in northeast Japan, where a huge earthquake is expected to occur in the near future. A time series of horizontal coordinates of MYGI obtained from seven campaign observations for the period 2002-2005 exhibits a linear trend with a scattering rms of about 2 cm. A linear fit to the time series gives an intraplate crustal velocity of more than several centimeters per year towards the WNW, which implies strong interplate coupling around this region. The precision of each campaign solution was examined at MYGI and other seafloor reference points along the Nankai Trough through comparison of independent one-day subset solutions within the campaign. The resultant repeatability looks to be well-correlated with the temporal and spatial stability of the acoustic velocity structure in the seawater depending on the region as well as the season.

A major new computation of a terrestrial gravitational field model has been performed by the Geodynamics Branch of Goddard Space Flight Center (GSFC). In the development of this new model, designated Goddard Earth Model GEM‐T1, the design decisions of the past have been reassessed in light of the present state of the art in satellite geodesy. With GEM‐T1 a level of internal consistency has been achieved which is superior to any earlier Goddard Earth Model. For the first time a simultaneous solution has been made for spherical harmonic parameters of both invariant and tidal parts of the gravitational field. The solution of this satellite model to degree 36 is a major factor accounting for its improved accuracy. The addition of more precise and previously unused laser data and the introduction of consistent models were also accomplished with GEM‐T1. Another major factor allowing the creation of this model was the redesign and vectorization of our main software tools (GEODYN II and SOLVE) for the GSFC Cyber 205 computer. In particular, the high‐speed advantage (50:1), gained with the new SOLVE program, made possible an optimization of the weighting and parameter estimation scheme used in previous GEM models resulting in significant improvement in GEM‐T1. The solution for the GEM‐T1 model made use of the latest International Association of Geodesy reference constants, including the J2000 Reference System. It provided a simultaneous solution for (1) a gravity model in spherical harmonics complete to degree and order 36; (2) a subset of 66 ocean tidal coefficients for the long‐wavelength components of 12 major tides. This adjustment was made in the presence of 550 other fixed ocean tidal terms representing 32 major and minor tides and the Wahr frequency dependent solid earth tidal model; and (3) 5‐day averaged Earth rotation and polar motion parameters for the 1980 period onward. GEM‐T1 was derived exclusively from satellite tracking data acquired on 17 different satellites whose inclinations ranged from 15° to polar. In all, almost 800,000 observations were used, half of which were from third generation (<5 cm) laser systems. A calibration of the model accuracies has been performed showing GEM‐T1 to be a significant improvement over earlier GSFC “satellite‐only” models based purely on tracking data for both orbital and geoidal modeling applications. For the longest wavelength portion of the geoid (to 8×8), GEM‐T1 is a major advancement over all GEM models, even those containing altimetry and surface gravimetry. The radial accuracy for the anticipated TOPEX/POSEIDON orbit was estimated using the covariances of the GEM‐T1 model. The radial errors were found to be at the 25‐cm rms level as compared to 65 cm found using GEM‐L2. This simulation evaluated only errors arising from geopotential sources. GEM‐L2 was the best available model for TOPEX prior to the work described herein. A major step toward reaching the accuracy of gravity modeling necessary for the TOPEX/POSEIDON mission has been achieved.

Abstract. The influence of the ionosphere can be one of the main obstacles to GPS carrier phase ambiguity resolution in real-time, particularly over long baselines. This is important to all users of GPS requiring sub-decimeter positioning, perhaps in real time, especially with high geomagnetic activity or close to the Solar Maximum. Therefore, it is desirable to have a precise estimation of the ionospheric delay in realtime, to correct the data. In this paper we asses a real-time tomographic model of the ionosphere created using dualfrequency phase data simultaneously collected with the receivers of a network of stations in the USA and Canada, with separations of 400-1000 km, during a period of high geomagnetic activity (Kp-6). When the tomographic ionospheric correction is included, the resolution on-the-fly (OTF) of the widelane double-differenced ambiguities at the reference stations is nearly 100% successful for satellite elevations above 20 degrees, while the resolution of the L•, L2 ambiguities at the rover is typically more than 80% successful.

The present study shows that patients with mild AD evidenced marked alterations in eye movement behavior during reading, even at early stages of the disease. Hence, evaluation of eye movement behavior during reading might provide a useful tool for a more precise early diagnosis of AD and for dynamical monitoring of the pathology.

Analytical orbit perturbation theory suggests that the errors in the ephemerides of the Global Positioning System (GPS) satellites should be mostly resonant effects that can be corrected by adjusting a few parameters in a simple empirical acceleration formula, despite the complexity of their causes (mismodeling of gravity, radiation pressure, etc.), at least for arcs free from orbit maneuvers. This theoretical conclusion has been tested with simulations and actual data analyses. For these analyses, data from the Spring 1985 Experiment have been used to calculate improved ephemerides; in turn, these ephemerides have been used in the estimation of the coordinates of stations within the continental United States, previously positioned with very long baseline interferometry (VLBI). The agreement between the VLBI and the GPS results gives a measure of orbit quality. The outcome of this test supports the idea that the errors are mostly of a resonant nature. The principles described here are potentially applicable to the computation of the precise ephemerides of other spacecraft, such as geosynchronous relay satellites and oceanographic satellites, which are usually in highly resonant, nearly circular, repeating orbits.