  A crucial part of a space mission for very-long baseline interferometery
(VLBI), which is the technique capable of providing the highest resolution
images in astronomy, is orbit determination of the mission's space radio
telescope(s). In order to successfully detect interference fringes that result
from correlation of the signals recorded by a ground-based and a space-borne
radio telescope, the propagation delays experienced in the near-Earth space by
radio waves emitted by the source and the relativity effects on each
telescope's clock need to be evaluated, which requires accurate knowledge of
position and velocity of the space radio telescope. In this paper we describe
our approach to orbit determination (OD) of the RadioAstron spacecraft of the
RadioAstron space-VLBI mission. Determining RadioAstron's orbit is complicated
due to several factors: strong solar radiation pressure, a highly eccentric
orbit, and frequent orbit perturbations caused by the attitude control system.
We show that in order to maintain the OD accuracy required for processing
space-VLBI observations at cm-wavelengths it is required to take into account
the additional data on thruster firings, reaction wheel rotation rates, and
attitude of the spacecraft. We also investigate into using the unique orbit
data available only for a space-VLBI spacecraft, i.e. the residual delays and
delay rates that result from VLBI data processing, as a means to evaluate the
achieved OD accuracy. We present the results of the first experience of OD
accuracy evaluation of this kind, using more than 5,000 residual values
obtained as a result of space-VLBI observations performed over 7 years of the
RadioAstron mission operations.
