Physically adequate reference system of a massless observer and relativistic description of the GAIA attitude
A relativistic definition of the physically adequate Center -of-Mass Reference System (CoMRS) is suggested as local PPN reference system of a massless observ er. The interrelations between the suggested definitions and the IAU Resolutions (2000) on relativity are elucidated. The tetrad representation of the CoMRS at its origin is also explicated. It is demonstrated how to us e that tetrad representation to calculate the relation between the observed direction of a light ray and the corresp onding coordinate direction in the BCRS. It is argued that the kinematically non-rotating CoMRS is the nat ural choice of a reference systeme where the attitude of the observer (e.g. of the GAIA satellite) should be modeled. The relativistic equations of rotational motion of a satellite relative to its CoMRS are briefly discus sed. A simple algorithm for the relativistic satellite attitude description is proposed.