GNC 2008
7th International ESA Conference on Guidance, Navigation & Control Systems 2-5 June 2008, Tralee, County Kerry, Ireland THE GAIA ATTITUDE & ORBIT CONTROL SYSTEM P. Chapman1, T. Colegrove1, D. Di Filippantonio1, A. Walker-Deemin1, A. Davies1, J. Myatt2, E. Ecale3, B. Girouart4 1 Astrium Ltd (United Kingdom); 2 Tessella plc (United Kingdom); 3Astrium SAS (France); 4 ESTEC/ESA (Netherlands)
[email protected]
ABSTRACT The Gaia spacecraft is an ESA astronomy mission to investigate the structure and evolution of the galaxy by measuring the positions of one billion stars to unprecedented accuracy [1]. Gaia is being developed by Astrium SAS (prime), with Astrium Ltd developing the electrical service module, including the AOCS. The challenges for the AOCS design are to provide attitude control within a restricted pointing domain, due to thermal constraints, and to provide a fine pointing mode with relative pointing error (RPE) of a few milli-arcseconds (mas). The fine pointing mode needs to use the scientific payload as an AOCS sensor in order to measure angular rate to the accuracy required. New proportional cold gas thrusters are used to provide actuation with micro-N noise. This paper presents an overview of the AOCS design. THE MISSION The prime objective of Gaia is to provide Astrometric measurements of all (approx 1 billion) objects to magnitude 20, with multi-colour multi-epoch photometry and radial velocities for objects brighter than magnitude 16-17. The main scientific instrument (“ASTRO”) measurements will allow a final catalogue to be prepared to better than 10 microarcseconds accuracy at magnitude 15. The experiment operations principle is for a continuous scanning of the sky on great-circles with constant inclination to the Sun, as was the case for Gaia’s predecessor mission Hipparcos. The dual field of view ASTRO instrument entirely relies on the AOCS to provide the continuous scanning motion that is the basis of the science data gathering process. The spacecraft attitude control must follow the theoretical “Scan Law” [2] (see Fig. 1) to within 30 arcseconds in absolute pointing error, with a mean rate error over any 10 second period of only 2 milli-arcseconds/second.
Fig. 1. The Gaia Scan Law
Fig 2. Artists view of the Gaia spacecraft, following sunshield deployment
Gaia will be launched on Soyuz-Fregat from Kourou in 2011, directly injected into a transfer trajectory towards the Earth/Sun L2 Lagrange point. Soon after separation from the Fregat upper stage the AOCS will acquire a sun-pointing attitude. This is followed by deployment of the Deployable Sunshield Assembly (DSA). From this point onwards the +X side of the DSA lies in permanent shadow, to protect the telescopes. During transfer to L2 the spacecraft settles to thermal equilibrium, and the spacecraft commissioned. A delta-v manoeuvre inserts the spacecraft into a Lissajous orbit centred at L2, which maintains communications aspect angles and avoids eclipses by the earth. For the following
GNC 2008
7th International ESA Conference on Guidance, Navigation & Control Systems 2-5 June 2008, Tralee, County Kerry, Ireland 5 years the spacecraft follows the scan law, spinning with a 6 hour period whilst the spin axis is precessed about the sun-line. The optical payload repeatedly scans every point on the celestial sphere, allowing the CCD detectors to collect position measurements which will ultimately be processed to obtain a scientific database of results. PRIMARY AOCS REQUIREMENTS AND CHALLENGES The main challenges are the fine pointing requirements and the consequences of the sun illumination constraints, which define the FDIR requirements. The pointing requirements are expressed in terms of Along scan (AL, in direction of spin sense) and Across scan (AC, the directions normal to the payload LOS directions). The Normal Mode (NM) fine pointing requirements for Relative Pointing Error (RPE), Mean rate Error (MRE), and Rate Measurement Error (RME) are shown in Table 1. The attitude high frequency disturbance (AHFD) has a complex definition, which is not elaborated here; it effectively defines a limit for the Micro-Propulsion System (MPS) torque noise PSD, but otherwise has little interaction with the NM design. The PSD defined by the AL AHFD constrains the combination of MPS thruster configuration (moment arms) and thrust error PSD. NM additionally has more conventional pointing requirements of APE< 30” and AME
