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A test of a cornerstone of general relativity, the gravitational redshift effect, is currently being conducted with the RadioAstron spacecraft, which is on a highly eccentric orbit around Earth. Using ground radio telescopes to record the spacecraft signal, synchronized to its ultra-stable on-board H-maser, we can probe the varying flow of time on board with unprecedented accuracy. The observations performed so far, currently being analyzed, have already allowed us to measure the effect with a relative accuracy of $4times10^{-4}$. We expect to reach $2.5times10^{-5}$ with additional observations in 2016, an improvement of almost a magnitude over the 40-year old result of the GP-A mission.
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RadioAstron satellite admits in principle a testing the gravitational redshift effect with an accuracy of better than $10^{-5}$. It would surpass the result of Gravity Probe A mission at least an order of magnitude. However, RadioAstrons communications and frequency transfer systems are not adapted for a direct application of the non relativistic Doppler and troposphere compensation scheme used in the Gravity Probe A experiment. This leads to degradation of the redshift test accuracy approximately to the level 0.01. We discuss the way to overcome this difficulty and present preliminary results based on data obtained during special observing sessions scheduled for testing the new techniques.
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. Large gravitational potential variation, occurring on the time scale of $sim$24 hr, causes 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. The experiment requires specific on-board hardware operating modes and support from ground radio telescopes capable of tracking the spacecraft continuously and equipped with 8.4 or 15 GHz receivers. Our preliminary estimates show that $sim$30 hr of the space radio telescopes observational time are required to reach $sim 2times10^{-5}$ accuracy in the test, which would constitute a factor of 10 improvement over the currently achieved best result.
Gravitational wave (GW) detection in space probes GW spectrum that is inaccessible from the Earth. In addition to LISA project led by European Space Agency, and the DECIGO detector proposed by the Japan Aerospace Exploration Agency, two Chinese space-based GW observatories -- TianQin and Taiji -- are planned to be launched in the 2030s. TianQin has a unique concept in its design with a geocentric orbit. Taijis design is similar to LISA, but is more ambitious with longer arm distance. Both facilities are complementary to LISA, considering that TianQin is sensitive to higher frequencies and Taiji probes similar frequencies but with higher sensitivity. In this Perspective we explain the concepts for both facilities and introduce the development milestones of TianQin and Taiji projects in testing extraordinary technologies to pave the way for future space-based GW detections. Considering that LISA, TianQin and Taiji have similar scientific goals, all are scheduled to be launched around the 2030s and will operate concurrently, we discuss possible collaborations among them to improve GW source localization and characterization.
The Low Frequency Array (LOFAR) is a new generation of electronic radio telescope based on aperture array technology and working in the frequency range of 30-240 MHz. The telescope is being developed by ASTRON, and currently being rolled-out across the Netherlands and other countries in Europe. The plan is to build at least 36 stations in the Netherlands (with baseline lengths of up to 100 km), 5 stations in Germany, and 1 station in each of Sweden, France and the UK. With baseline lengths of up to 2000 km, sub-arcsecond resolution will be possible at the highest frequencies. The Key Science Projects being addressed by the project include: deep, wide-field cosmological surveys, transients, the epoch of re-ionisation and cosmic ray studies. We present the current status of the project, including the development of the super-core in Exloo and the completion of the first 3 stations. First fringes from these stations is also presented.
We investigate the performance of the upcoming ACES (Atomic Clock Ensemble in Space) space mission in terms of its primary scientific objective, the test of the gravitational redshift. Whilst the ultimate performance of that test is determined by the systematic uncertainty of the on-board clock at 2-3 ppm, we determine whether, and under which conditions, that limit can be reached in the presence of colored realistic noise, data gaps and orbit determination uncertainties. To do so we have developed several methods and software tools to simulate and analyse ACES data. Using those we find that the target uncertainty of 2-3 ppm can be reached after only a few measurement sessions of 10-20 days each, with a relatively modest requirement on orbit determination of around 300 m.
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