Determining PPN gamma with Gaia's Astrometric Core Solution

Details Determining PPN gamma with Gaia's Astrometric Core Solution Determining PPN gamma with Gaia's Astrometric Core Solution

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009iau...261.1603h&link_type=abstract

American Astronomical Society, IAU Symposium #261. Relativity in Fundamental Astronomy: Dynamics, Reference Frames, and Data Ana

Astronomy and Astrophysics

Astronomy

Scientific paper

Gaia is the ESA space astrometry mission due for launch in December 2011. Its objective is to map a large part of our Galaxy and its surroundings by simultaneous positional, photometric and spectroscopic measurements. In addition to its huge output of fundamental astrometric and astrophysical data, it will provide stringent new tests of general relativity.
One of the largest relativistic effects observed via Gaia's measurements is the gravitational light bending due to the Sun and major planets. This opens the possibility of accurately measuring the parameter γ of the Parameterized Post-Newtonian (PPN) formulation, which is of key importance to fundamental physics. By analyzing the astrometric residuals, which compare the observations to predictions based on general relativity, a constraint on PPN γ could be obtained which is significantly better than today's best estimate from the Cassini mission of 2x10-5.
The effects of introducing global unknowns into Gaia's Astrometric Global Iterative Solution (AGIS) are not well understood and the convergence properties of the solution may be affected in undesirable ways. For this reason we have implemented the algorithm to determine PPN γ within our simulation software, known as AGISLab. This light-weight and scalable simulation tool allows to investigate the convergence properties of the solution when PPN γ is included together with the astrometric and attitude updates.
In particular we have considered the statistical correlation between PPN γ and the stellar parallaxes which slows the convergence of the iterative solution. By introducing a further global pseudo parameter, equivalent to a common parallax shift but constrained to zero, this correlation is eliminated resulting in an improved convergence and a better estimate of the standard error of PPN γ. Preliminary results are presented based on simulated data, which include only the deflection of light by the Sun.

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