Calculating the Position and Velocity Components of {\it HST}
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**Session 3 -- HST Performance and Process**
*Display presentation, Monday, 30, 1994, 9:20-6:30*

## [3.08] Calculating the Position and Velocity Components of {\it HST}

*T.B. Ake (CSC/GSFC)*
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.

**Monday
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