The Russian Academy of Sciences and Federal Space Agency, together with the participation of many international organizations, worked toward the launch of the RadioAstron orbiting space observatory with its onboard 10-m reflector radio telescope from the Baikonur cosmodrome on July 18, 2011. Together with some of the largest ground-based radio telescopes and a set of stations for tracking, collecting, and reducing the data obtained, this space radio telescope forms a multi-antenna groundspace radio interferometer with extremely long baselines, making it possible for the first time to study various objects in the Universe with angular resolutions a million times better than is possible with the human eye. The project is targeted at systematic studies of compact radio-emitting sources and their dynamics. Objects to be studied include supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei, stellar-mass black holes, neutron stars and hypothetical quark stars, regions of formation of stars and planetary systems in our and other galaxies, interplanetary and interstellar plasma, and the gravitational field of the Earth. The results of ground-based and inflight tests of the space radio telescope carried out in both autonomous and ground-space interferometric regimes are reported. The derived characteristics are in agreement with the main requirements of the project. The astrophysical science program has begun.
Frequency measurements versus microwave power (P,x), lamp temperature (TL), and cell temperature (T,) have been made on a passive Rb87 frequency standard. The lamp and integrated cell, upon which the measurements were made, are typical of the majority of the commercial rubidium standards in the field. This is the first report of the frequency dependence upon PVh.
Original proposals and experiments on gravitation and fundamental metrology on the space station are described. These experiments were formulated in the Metrology and Gravitation Science Team, in two ESA industrial study contracts, on microsatellites and on time and frequency science, within the space station scenario. Although limited by the design constraints of the space station, the experiments range from clock-based tests on special and general relativity to, with additional infrastructure, the equivalence principle and the detection of gravitational waves. Supporting technology, such as damping systems and microgravity cooled atom clocks, is also described. Finally, the major scientific goals, the experiments, hardware and the status are summarized. This work represents the first coordinated attempt, at least within the European space programmes, to consider experiments on relativity and fundamental physics without resorting to experiment dedicated space missions. For details on specific issues a large bibliography is referred to.
For the Galileo system it is required that a space clock time prediction be performed, covering the time interval (Tp) between two uploads. The time prediction accuracy of the space clock is therefore an important issue. The predictability of the Space Passive Hydrogen Maser (S-PHM) time error is evaluated by the RMS of the predicted time errors at the prediction time Tp: ΔTRMS(Tp). A linear prediction model is used, corresponding to the absence of a frequency drift. The results show that: (1) the RMS time error can be evaluated from a priori knowledge of the clock's Allan deviation; (2) conversely, it is possible to extract the Allan deviation from the measurement of ΔTRMS versus Tp; (3) the modelling of ΔTRMS(Tp) of S-PHM based on the white frequency and flicker frequency noises appears to be particularly accurate: the difference between the model fit and the measured prediction accuracy is ⩽10 ps RMS for Tm = 24 h; (4) for Tp = 4 h, the performance of the S-PHM is a factor of 4.5 better than the performance of the space rubidium frequency standard (S-RAFS). This has a dramatic effect on the probability of the Signal In Space Accuracy: assuming a Gaussian distribution the probability of a predicted time error ΔT⩽1.5 ns for Tp = 4 h is 89% for the S-RAFS, while the same time error constitutes an absolute upper bound for the Galileo S-PHM.
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