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Satellite accelerometer measurements of atmospheric density have provided significant improvement in our understanding of the structure and dynamics of the lower thermosphere. Derivation of accurate data with this technique requires removal of instrument bias from the total sensor output. The ROCA (Rotatable Calibration Accelerometer) experiment was flown to provide and orbital calibration capability on the three-axis stabilized S3-4 satellite. The ROCA sensitive axis could be operated in either of two orientations selectable by ground command. For density measurement (normal operating mode) the sensitive axis was aligned with the satellite velocity vector. For direct measurement of bias, the sensitive axis was aligned perpendicular to the velocity vector. Utilization of the inflight calibration technique showed a dependence of the bias upon the instrument operating temperature. Removal of the bias-temperature component from the total acceleration signal obtained in the normal operating mode permits derivation of accurate density data. Measurements of atmospheric density were obtained during approximately 600 orbits over a five month period. The resulting ROCA data will be utilized for improved satellite ephemeris computations and for detailed studies of the lower thermosphere, particularly those related to energy inputs at high latitudes. (Author).
A new satellite triaxial accelerometer system has been developed. This instrument has been flight-tested on three-axis stabilized satellites and has demonstrated the capability to accurately measure accelerations in the satellite's in-track, cross-track, and radial directions. The in-track data provide direct determination of atmospheric density. These data, obtained during a period of high solar flux, supplement the extensive set of measurements obtained by other accelerometer experiments during low solar flux conditions. The cross-track and radial results permit, for the first time, large-scale measurements of the zonal and, possibly, radial components of neutral atmospheric winds. These simultaneous wind and density data provide an extremely valuable input for the understanding of dynamic processes in the atmosphere and for the improvement of atmospheric models.
A satellite experiment has obtained nadir-viewing atmospheric radiance measurements in the 1100 to 2900 angstrom wavelength region from the polar orbiting satellite S3-4. This report describes the instrumentation, measurement techniques, and initial results. The observed emission is from airglow, solar scatter, and auroral regions. Spectral and spatial measurements previously unavailable have been made for the nitrogen Lyman-Birge-Hopfield emission, vacuum ultraviolet aurora, hydrogen geocorona, tropical ultraviolet airglow, twilight fluorescence nitric oxide spectrum, and night oxygen Herzberg band emission. Low values of the day and night background radiance values have been found; these values can be applied to improved missile detection techniques. Spatial resolution down to about one kilometer will enable possible variability to be sought. Observations were made in the period March to September 1978. Correlations with other geophysical factors such as solar flux, magnetic index, and seasonal variations will be made after the data base from the experiment is completed.
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Atmospheric density measurements obtained by the satellite accelerometer experiment provide data over a wide range of solar and geophysical conditions. These results are used in a preliminary evaluation of several atmospheric models. The model accuracies are compared by their mean values and standard deviations relative to the accelerometer data. Sources of model uncertainties and problems in reducing them are described. Long-term programs involving coordinated measurements, analyses of available data and theoretical studies are required along with development of more accurate indicators of solar and geomagnetic activity before models can show significant improvement. Keywords include: Satellite, Accelerometers, Atmospheric density measurements, Thermospheric model evaluations, Thermospheric density variability.