The Time Correlation Approach for the Gaia Astrometry Mission

Abstract

Gaia is an ESA science cornerstone mission, which relies on the proven principles of ESA's Hipparcos mission to solve one of the most difficult yet deeply fundamental challenges in modern astronomy: to create an extraordinarily precise three-dimensional map of about one billion stars aiming at star magnitudes down to 20 throughout our Galaxy and beyond. Gaia is due to be launched in 2013 on Soyuz-Fregat, from French Guyana, and will be operated by ESOC. To achieve Gaia’s challenging mission goals, the spacecraft and ground segment contain a number of novel design solutions. One of the major challenges of Gaia is the high accuracy requirements for the performance of the correlation of the satellite OBT & SCET times to the ground time scale UTC. The Gaia mission has for science purposes (may be could be slightly developed: minor: accuracy needed for minor planet orbit determination at the level of 150m) the maximum allowable RMS error in the correlation of 2μs. The figure of 2μs will be achievable some weeks (about six) after acquisition when the reconstructed orbit is used and orbital dependent relativistic corrections are applied. The paper will provide an overview of the time correlation approach and details the significant and measurable delays that have to be determined/checked for each time packet in order to allow correlation between the (science) on-board packet generation time (OBT) and the ground time stamping (UTC) when the packet are received. This paper will also describes all errors involved in the final accuracy of the time correlation itself (e.g. errors due to propagation outside of coverage times and effects of the time packet generation) and describes in detail how time correlation is performed at the Operations Control Centre.