Abstract:

Evaluation of flight data from the Upper Atmosphere Research Satellite shows that the solar-disk tracking sub-system within the Halogen Occultation Experiment responds to and completely accommodates cyclic disturbances from the spacecraft solar-array vibration and continuous scanning motions by the High-Resolution Doppler Imager instrument. A fine sun sensor within the Halogen Occultation Experiment senses the top egde of the solar disk as the primary tracking control input. Data streams from such sensing show a characteristic frequency, which is principally determined by the apparent solar motion. Power-spectral-density renderings of sun sensor data streams show the distrubing frequencies as sideband peaks around a central peak at the characteristic frequency. The sidebands are analogous to frequency modulation of a carrier frequency. A mathematical model developed for this analysis demonstrates the factors that influence the frequency-modulated measurements. Modeling analyses verified the frequency content and allowed estimates of local relative amplitudes of the principal distrubances imparted to the spacecraft. The model can predict mutual interactions between instruments as well as the influence of structural excitations upon instruments. This analysis offers a means toward evaluating potentials for adverse ineteractions in cases where a spacecraft carries closed-loop instruments that track fixed soar or stellar targets; such interactions can become crucial if tracking requirements have small margins of error.