ИСТИНА

A unique test of general relativity is possible with the space radio telescope RadioAstron. The ultra-stable on-board hydrogen maser frequency standard and the highly eccentric orbit make RadioAstron an ideal instrument for probing the gravitational redshift effect. The large gravitational potential variation, occurring on the time scale of $\sim$24 hr, causes a large variation of the on-board H-maser clock rate, which can be detected via comparison with frequency standards installed at various ground radio astronomical observatories. In order to perform this comparison the RadioAstron spacecraft is tracked by ground radio telescopes equipped with 8.4 or 15 GHz receivers, and its downlink signal is recorded by standard VLBI equipment. The well-established algorithms, developed originally for PRIDE (Planetary Radio Interferometry and Doppler Experiment) are used to accurately recover the frequency of the recorded signal, while the 1st-order Doppler shift and atmospheric disturbances are compensated for by a technique similar to that of phase referencing and based on interleaving 1-way and 2-way modes of operation of the spacecraft radio links. Additional tropospheric and ionospheric calibration are obtained from on-site WVR (Water Vapour Radiometer) and GNSS (Global Navigation Satellite Sys-tems) receivers available at the majority of the EVN and IVS stations par-ticipating in the experiment. We expect the accuracy of the test to reach a value of $\sim 2\times10^{-5}$, thus improving the currently best result of the Gravity Probe A mission by a factor of 10.