Geophysical parameters from the analysis of laser ranging to starlette

Starlette Satellite Laser Ranging (SLR) data were used, along with several other satellite data sets, for the solution of a preliminary gravity field model for TOPEX, PTGF1. A further improvement in the earth gravity model was accomplished using data collected by 12 satellites to solve another preliminary gravity model for TOPEX, designated PTGF2. The solution for the Earth Rotation Parameter (ERP) was derived from the analysis of SLR data to Starlette during the MERIT Campaign. Starlette orbits in 1976 and 1983 were analyzed for the mapping of the tidal response of the earth. Publications and conference presentations pertinent to research are listed

Altimeter measurements for the determination of the Earth's gravity field

Progress in the following areas is described: refining altimeter and altimeter crossover measurement models for precise orbit determination and for the solution of the earth's gravity field; performing experiments using altimeter data for the improvement of precise satellite ephemerides; and analyzing an optimal relative data weighting algorithm to combine various data types in the solution of the gravity field

Geophysical parameters from the analysis of laser ranging to Starlette

The results of geodynamic research from the analysis of satellite laser ranging data to Starlette are summarized. The time period of the investigation was from 15 Mar. 1986 to 31 Dec. 1991. As a result of the Starlette research, a comprehensive 16-year Starlette data set spanning the time period from 17 Mar. 1975 through 31 Dec. 1990, was produced. This data set represents the longest geophysical time series from any geodetic satellite and is invaluable for research in long-term geodynamics. A low degree and order ocean tide solution determined from Starlette has good overall agreement with other satellite and oceanographic tide solutions. The observed lunar deceleration is -24.7 +/- 0.6 arcsecond/century(exp 2), which agrees well with other studies. The estimated value of J2 is (-2.5 +/- 0.3) x 10(exp -11) yr(exp -1), assuming there are no variations in higher degree zonals and that the 18.6-year tide is fixed at an equilibrium value. The yearly fluctuations in the values for S(sub a) and S(sub sa) tides determined by the 16-year Starlette data are found to be associated with changes in the Earth's second degree zonal harmonic caused primarily by meteorological excitation. The mean values for the amplitude of S(sub a) and S(sub sa) variations in J2 are 32.3 x 10(exp -11) and 19.5 x 10(exp -11), respectively; while the rms about the mean values are 4.1 x 10(exp -11) and 6.3(10)(exp -11), respectively. The annual delta(J2) is in good agreement with the value obtained from the combined effects of air mass redistribution without the oceanic inverted-barometer effects and hydrological change. The annual delta(J3) values have much larger disagreements. Approximately 90 percent of the observed annual variation from Starlette is attributed to the meteorological mass redistribution occurring near the Earth's surface

Altimeter measurements for the determination of the Earth's gravity field

The ability of satellite-borne radar altimeter data to measure the global ocean surface with high precision and dense spatial coverage provides a unique tool for the mapping of the Earth's gravity field and its geoid. The altimeter crossover measurements, created by differencing direct altimeter measurements at the subsatellite points where the orbit ground tracks intersect, have the distinct advantage of eliminating geoid error and other nontemporal or long period oceanographic features. In the 1990's, the joint U.S./French TOPEX/POSEIDON mission and the European Space Agency's ERS-1 mission will carry radar altimeter instruments capable of global ocean mapping with high precision. This investigation aims at the development and application of dynamically consistent direct altimeter and altimeter crossover measurement models to the simultaneous mapping of the Earth's gravity field and its geoid, the ocean tides and the quasi-stationary component of the dynamic sea surface topography. Altimeter data collected by SEASAT, GEOS-3, and GEOSAT are used for the investigation

Gravity field determination and error assessment techniques

Linear estimation theory, along with a new technique to compute relative data weights, was applied to the determination of the Earth's geopotential field and other geophysical model parameters using a combination of satellite ground-based tracking data, satellite altimetry data, and the surface gravimetry data. The relative data weights for the inhomogeneous data sets are estimated simultaneously with the gravity field and other geophysical and orbit parameters in a least squares approach to produce the University of Texas gravity field models. New techniques to perform calibration of the formal covariance matrix for the geopotential solution were developed to obtain a reliable gravity field error estimate. Different techniques, which include orbit residual analysis, surface gravity anomaly residual analysis, subset gravity solution comparisons and consider covariance analysis, were applied to investigate the reliability of the calibration

The Study of Effects of Time Variations in the Earth's Gravity Field on Geodetic Satellites

The temporal variations in the Earth's gravity field are the consequences of complex interactions between atmosphere, ocean, solid Earth, hydrosphere and cryosphere. The signal ranges from several hours to 18.6 years to geological time scale. The direct and indirect consequences of these variations are manifested in such phenomena as changes in the global sea level and in the global climate pattern. These signals produce observable geodetic satellites. The primary objectives of the proposed effects on near-Earth orbiting investigation include (1) the improved determination of the time-varying gravity field parameters (scale from a few hour to 18.6 year and secular) using long-term satellite laser rs ranging (SLR) observations to multiple geodetic satellites, and (2) the enhanced understanding of these variations with their associated meteorological and geophysical consequences

Geodynamic implications of temporal gravity changes over Tibetan Plateau

The Tibetan Plateau is one of the most geologically dynamic systems and the highest plateau in the world with ongoing three-dimensional crustal deformation. The Plateau is uplifting and deforming horizontally as observed by present-day global navigation satellite system (GNSS) and repeated leveling measurements. Crustal mass is conservative and less dense than the mantle, thus the horizontal shortening must be accompanied by crustal thickening and horizontal extrusion. According to the level of isostatic compensation, the thickening is partitioned into topographic uplift and Moho deepening. Here, we investigate the mass change induced gravity signal observed at or near the crust, and discuss whether this signal could be detected using terrestrial or satellite gravity observations. We set up a model for the Tibetan Plateau crustal thickening and calculate the expected gravity signal. The predictions are then compared with the present-day gravity changes observed by GRACE and with published in situ absolute gravity rates. We conclude that the crustal thickening signal cannot be neglected and that it contributes significantly to the observed signal. Those studies with focus on the mountain glacier and hydrologic mass fluxes should be aware that, if neglected, the crustal signal could introduce a significant bias. The observations give a positive gravity rate over central Tibetan Plateau, unexplained by the hydrologic or cryospheric signals, and a negative rate over the Himalayas and at its foothill, which is attributable to terrestrial hydrologic signals including human depletion of groundwater. Our model shows that the positive gravity rate could be explained by elevation uplift, and a stable or upwelling Moho. The negative gravity change signal is due primarily to the strong elevation-gradient at the foothill of the Himalayas, and to an uplift accompanied by crustal thickening and Moho lowering. The estimated gravity rates can be used when defining the requirements on future gravity missions, as the tectonic signal should be resolved in order to improve its separation from hydrologic and/or cryospheric processes generating a gravity change

Attention-deficit/hyperactivity disorder (ADHD) in cultural context do parents in Hong Kong and the United Kingdom adopt different thresholds when rating symptoms, and if so why?

Objectives Attention-deficit/hyperactivity disorder (ADHD) prevalence is similar across world regions. However, because informants' decision thresholds may vary between regions, these similarities may mask regional variations in actual ADHD behaviours. We tested this by comparing the relationship between informant's ratings and children's measured activity in United Kingdom (UK) and Hong Kong (HK) and then explored whether any national differences in endorsement thresholds discovered are linked to cultural variations in parenting factors. Methods Parents rated the 18 ADHD symptoms in 112 three-to-five-year-old children stratified for ADHD symptom levels (49 girls and 63 boys; 55 from the UK and 57 from HK) and completed some parenting questionnaires. Children's task-related activity was measured using actometers. Results In both groups, measured activity was positively correlated with hyperactivity/impulsivity (r = 0.44HK; r = 0.41UK). While HK children were less active than UK children (p < 0.01), HK parents rated their children as more hyperactive/impulsive and inattentive (ps < 0.05). The lower rating threshold indicated by this pattern in HK parents were explained by their higher child-related stress levels. Conclusions UK and HK parents operated different ADHD symptom endorsement thresholds. The link between these and child-related stress may mark a more general role of cultural pressure for child conformity and school achievement in HK