A new method of analysis and reduction of VLBI observations

Abstract

In the classical algorithm of analysis of VLBI observations, the equatorial coordinates of the radio sources, the coordinates of the stations, the Earth Orientation Parameters (EOP) and, eventually, the corrections to the position of the celestial pole given by the IAR 1976/1980 models are simultaneously adjusted. We have explored a new method of constructing a celestial frame independently of the terrestrial frame and of the Earth's orientation in space. The new strategy of observation consists on "ideally" observing two extragalactic radio sources simultaneously with tho pairs of radiotelescopes, each on at the extremities of a VLBI baseline. On the basis of the classical VLBI observables - the delay and the the delay rate - we have constructed a new observable: the arc between a pair of sources. A set of these new observables leads to spherical coordinates of the radio sources relative to an arbitrary plane and origin. We have started with the simple case, where the delay and the delay rate are entirely due to the VLBI geometry. Then, we have considered all their components. In all the cases, the effects of polar motion, UT1-UTC, precession and nutation are neglected. We have concluded that it is possible to obtain a set of equations of observations, independently of the baseline orientation and the vector of angular rotation velocity of the Earth. Adopting a reference system attached to the baseline (s), we can construct an ew observable, the arc between a pair of radio sources simultaneously observed. The theoretical precision of the method, estimated from the terms neglected in the developments, is at the level of 0.0004" when the observations are performed with a two-element interferometer and the reference system is attached to the unique baseline and to the direction of the vector of rotation velocity of the Earth. When the reference system is defined only by two baseline vectors (case of the three-element interferometer), and only the delay "tau" is considered in the expression of the arc, the theoretical precission is "10^3" times higher.