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Optimising the Gaia scanning law for relativity experiments

Published online by Cambridge University Press:  06 January 2010

Jos de Bruijne
Affiliation:
Research and Scientific Support Department of the European Space Agency, European Space Research and Technology Centre, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
Hassan Siddiqui
Affiliation:
Science Operations Department of the European Space Agency, European Space Astronomy Centre, Villanueva de la Cañada, 28692 Madrid, Spain
Uwe Lammers
Affiliation:
Science Operations Department of the European Space Agency, European Space Astronomy Centre, Villanueva de la Cañada, 28692 Madrid, Spain
John Hoar
Affiliation:
Science Operations Department of the European Space Agency, European Space Astronomy Centre, Villanueva de la Cañada, 28692 Madrid, Spain
William O'Mullane
Affiliation:
Science Operations Department of the European Space Agency, European Space Astronomy Centre, Villanueva de la Cañada, 28692 Madrid, Spain
Timo Prusti
Affiliation:
Research and Scientific Support Department of the European Space Agency, European Space Research and Technology Centre, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
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Abstract

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Gaia is ESA's upcoming astrometry mission, building on the heritage of its predecessor, Hipparcos. The Gaia nominal scanning law (NSL) prescribes the ideal attitude of the spacecraft over the operational phase of the mission. As such, it precisely determines when certain areas of the sky are observed. From theoretical considerations on sky-sampling uniformity, it is easy to show that the optimum scanning law for a space astrometry experiment like Gaia is a revolving scan with uniform rotation around the instrument symmetry axis. Since thermal stability requirements for Gaia's payload require the solar aspect angle to be fixed, the optimum parallax resolving power is obtained by letting the spin axis precess around the solar direction. The precession speed has been selected as compromise, limiting the across-scan smearing of images when they transit the focal plane, providing sufficient overlap between successive “great-circle” scans of the fields of view, and guaranteeing overlap of successive precession loops. With this scanning law, with fixed solar-aspect angle, spin rate, and precession speed, only two free parameters remain: the initial spin phase and the initial precession angle, at the start of science operations. Both angles, and in particular the initial precession angle, can be initialized following various (programmatic) criteria. Examples are optimization/fine-tuning of the Earth-pointing angle, of the number and total duration of Galactic-plane scans, or of the ground-station scheduling. This paper explores various criteria, with particular emphasis on the opportunity to optimise the scanning-law initial conditions to “observe” the most favorable passages of bright stars very close to Jupiter's limb. This would allow a unique determination of the light deflection due to the quadrupole component of the gravitational field of this planet.

Type
Contributed Papers
Copyright
Copyright © International Astronomical Union 2010

References

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