  In this paper we discuss the characteristics and operation of Astro Space
Center (ASC) software FX correlator that is an important component of
space-ground interferometer for Radioastron project. This project performs
joint observations of compact radio sources using 10 meter space radio
telescope (SRT) together with ground radio telescopes at 92, 18, 6 and 1.3 cm
wavelengths. In this paper we describe the main features of space-ground VLBI
data processing of Radioastron project using ASC correlator. Quality of
implemented fringe search procedure provides positive results without
significant losses in correlated amplitude. ASC Correlator has a computational
power close to real time operation. The correlator has a number of processing
modes: "Continuum", "Spectral Line", "Pulsars", "Giant Pulses","Coherent".
Special attention is paid to peculiarities of Radioastron space-ground VLBI
data processing. The algorithms of time delay and delay rate calculation are
also discussed, which is a matter of principle for data correlation of
space-ground interferometers. During 5 years of Radioastron space radio
telescope (SRT) successful operation, ASC correlator showed high potential of
satisfying steady growing needs of current and future ground and space VLBI
science. Results of ASC software correlator operation are demonstrated.
