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The HST uses an on-board ephemeris of its orbit to control pointing of its high gain antennas, perform momentum unloading with the magnetic torquer bars, and apply parallax corrections when observing solar system objects, among other uses. The flight software uses a simple series expansion of Kepler's equation for two bodies. The coefficients for the ephemeris are normally updated every other day, based on recent, measured orbital positions and velocities. These parameters are archived as FITS keywords with the Standard Header Packet (SHP) file for each HST observation. Thus, the observer can perform a variety of calibration and characterization calculations relating to the orbital position and velocity of the telescope. Such computations include geocentric positions for limb-scattered light and geocoronal emission studies, geomagnetic coordinates for cosmic-ray noise analyses and geomagnetically induced image motion correction, and Doppler shifts to align rapid read out spectra. We will present the methodology and accuracy of these calculations, and show examples of the orbital phenomena that can be modeled.
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