Observed Outgassing Acceleration of GPS Block IIR Spacecraft

Details Observed Outgassing Acceleration of GPS Block IIR Spacecraft Observed Outgassing Acceleration of GPS Block IIR Spacecraft

Jul 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007dda....38.1401s&link_type=abstract

American Astronomical Society, DDA meeting #38, #14.01

Astronomy and Astrophysics

Astronomy

Scientific paper

For each GPS satellite, USNO daily fits a trajectory through a 4 to 6 day span of the most recent IGS Rapid orbit positions. The least-squares trajectory solution finds initial values for 6 orbital elements plus 6 corrections to the solar radiation force (SRF) model. During the first 3-6 months after launch into orbit, outgassing of the GPS spacecraft materials produces a reaction force on the spacecraft which the GYPSY-OASIS software absorbs into the SRF model corrections. Comparison of the "corrected" SRF model during the outgassing interval with the SRF model obtained 6 to 12 months later, when outgassing is negligible and the Sun direction makes the same range of angles to the orbit plane, allows us to evaluate certain weighted integrals around the orbit of the outgassing acceleration. These integrals essentially give the Fourier components that produce secular perturbations to the orbit.
We can mathematically reproduce some of these integrals by a constant (over the orbit period) acceleration along the spacecraft D, B, and Y axes. From observations of three GPS Block IIR spacecraft, we find that the acceleration magnitude decreases approximately exponentially with time, but the "time constant" for decay often increases by a factor of 2 during the 2 to 3 months before the outgassing acceleration becomes imperceptible. The D axis acceleration component is largest, with an initial value 3 nm/s2 (away from the Sun) which decays with time constants ranging from 7 to 13 days. The B and Y axis components show much different time constants which become as long as 38 days. These differences indicate possible outgassing of several different substances, each with a unique time constant.

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