The Einstein Equivalence Principle (EEP) is a cornerstone of general relativity and predicts the existence of gravitational redshift. We report on new results of measuring this shift with RadioAstron (RA), a space very long baseline interferometry (VLBI) spacecraft launched into an evolving high eccentricity orbit around Earth with geocentric distances reaching 353 000 km. The spacecraft and ground tracking stations at Pushchino, Russia, and Green Bank, USA, were each equipped with a hydrogen maser frequency standard allowing a possible violation of the predicted gravitational redshift, in the form of a violation parameter ɛ, to be measured. By alternating between RA’s frequency referencing modes during dedicated sessions between 2015 and 2017, the recorded downlink frequencies can essentially be corrected for the non-relativistic Doppler shift. We report on an analysis using the Doppler-tracking frequency measurements made during these sessions and find ε = ( 2.1 ± 3.3 ) × 10 − 4 . We also discuss prospects for measuring ɛ with a significantly smaller uncertainty using instead the time-domain recordings of the spacecraft signals and envision how 10⁻⁷ might be possible for a future space VLBI mission.

An upgraded version of the combined optoacoustic gravitational radiation detector OGRAN is considered. This underground setup located at the Baksan Neutrino Observatory is designed to search for collapsing stars in the Galaxy in conjunction with the neutrino telescope BUST. Both instruments possess the sensitivity sufficient for registering collapses in our Galaxy as rare events with an average rate of 0.03 events per year. Observations are carried out in the form of continuous synchronous monitoring of the astrophysical background through both recording channels. Strict requirements are imposed on the systems for maintaining the operating modes of both installations. For the gravity detector, the problem is nontrivial owing to the complexity of its automatic control systems and fine-tuning of the operating point. For this reason, the method and technique for maintaining the OGRAN detector in monitoring mode is described in detail. Also, the characteristics of the OGRAN detector after modernization are briefly presented.

The possibility of compensating the atmospheric influence on precision measurement of gravitational ‘‘redshift’’ using the Earth orbiting satellites is discussed. For a communication radio signal propagating from a ground-based tracking station to a spacecraft and back, the interaction with the ionosphere and the troposphere produces a undesirable contribution to the total frequency shift which must be removed by specific data processing. In the paper we present a brief overview of atmospheric hindrances with estimation of their magnitudes. Then, the algorithms for filtering the gravitational effects against the atmospheric noise background are considered. The efficiency of such filtering is illustrated using the example of gravitational measurements with Spectr-R satellite in the RadioAstron mission.

In this paper, we estimate efficiency of a conceivable Euro-Asian network of gravitational wave (GW) interferometers that might be realized having in mind a plan of construction of third generation interferometer in Novosibirsk region. Subsequently, some network would be composed, including four GW detectors. Among them there are the already active interferometers VIRGO (Italy) and KAGRA (Japan), Indian interferometer under construction—LIGO India and the interferometer in Siberia mentioned above. The quality of network in question is considered on the base of typical numerical criteria of efficiency for detecting GW signals of known structure—radiation of relativistic binary coalescence.

A method has been developed for precision measurement of the gravitational frequency shift of communication radio signals between a spacecraft and a ground tracking station, based on the maximum likelihood principle, using the Rao-Cramer limit estimates for the kinematic parameters associated with orbital motion. Numerical illustrations of the efficiency of the method are presented using the example of data obtained in experiments with the Spektr-R satellite as part of the VLBI system in the Radioastron mission. A compensatory “on-line” technique for suppressing Doppler and atmospheric noise has been implemented due to the presence of two modes of communication in gravity sessions: unidirectional (1w) and looped (2w). Recipes for reducing the magnitude of systematic errors are discussed.

Experiments were performed with Fabry–Perot optical resonators in vacuum at low temperatures. Mirrors were applied on substrates of various optical materials. An infrared laser with a wavelength of 1.064 microns was used. The pump power at the maximum could reach 450 mW. The evolution of the optical properties of the FP cavity was traced in the temperature range 300–10 K. The main parameters measured were the integral characteristics of the FP resonances–sharpness (finesse) and contrast of interference. Three types of substrates were tested: a sitall, an optical glass with ultra low thermal expansion (ULE); sapphire; and calcium fluoride. During cooling, the degradation of the integral characteristics of the FP cavity was observed for the sitall mirrors due to the loss of the properties of ULE, and for sapphire mirrors due to the birefringence effect. The satisfactory constancy of the integral characteristics of the FP resonator on calcium fluoride was demonstrated in the entire temperature range studied.