The Dark Ages and Cosmic Dawn are largely unexplored windows on the infant Universe (z ~ 200–10). Observations of the redshifted 21-cm line of neutral hydrogen can provide valuable new insight into fundamental physics and astrophysics during these eras that no other probe can provide, and drives the design of many future ground-based instruments such as the Square Kilometre Array (SKA) and the Hydrogen Epoch of Reionization Array (HERA). We review progress in the field of high-redshift 21-cm Cosmology, in particular focussing on what questions can be addressed by probing the Dark Ages at z > 30. We conclude that only a space- or lunar-based radio telescope, shielded from the Earth’s radio-frequency interference (RFI) signals and its ionosphere, enable the 21-cm signal from the Dark Ages to be detected. We suggest a generic mission design concept, CoDEX, that will enable this in the coming decades.

Having an accurate calibration method is crucial for any scientific research done by a radio telescope. The next generation radio telescopes such as the Square Kilometre Array (SKA) will have a large number of receivers which will produce exabytes of data per day. In this paper we propose new direction-dependent and independent calibration algorithms that, while requiring much less storage during calibration, converge very fast. The calibration problem can be formulated as a non-linear least square optimization problem. We show that combining a block-LDU decomposition with Gauss-Newton iterations produces systems of equations with convergent matrices. This allows significant reduction in complexity per iteration and very fast converging algorithms. We also discuss extensions to direction-dependent calibration. The proposed algorithms are evaluated using simulations.

For subspace estimation with an unknown colored noise, Factor Analysis (FA) and its extensions, denoted as Extended FA (EFA), are good candidates for replacing the popular eigenvalue decomposition (EVD). Finding the unknowns in (E)FA can be done by solving a non-linear least square problem. For this type of optimization problems, the Gauss-Newton (GN) algorithm is a powerful and simple method. The most expensive part of the GN algorithm is finding the direction of descent by solving a system of equations at each iteration. In this paper we show that for (E)FA, the matrices involved in solving these systems of equations can be diagonalized in a closed form fashion and the solution can be found in a computationally efficient way. We show how the unknown parameters can be updated without actually constructing these matrices. The convergence performance of the algorithm is studied via numerical simulations.

The radio sky at frequencies below ∼30 MHz is virtually unobservable from Earth due to ionospheric disturbances and the opaqueness of the ionosphere below ∼10MHz, and also due to strong terrestrial radio interference. Deploying a radio observatory in space would open up this largely unexplored frequency band for science in astronomy, cosmology, geophysics, and space science. A Chinese-European team is proposing an ultra long wavelength (ULW) radio interferometer mission DSL (Discovering the Sky at the Longest Wavelengths). The proposed radio interferometer will be deployed in low-altitude lunar orbit, exploiting the radio quietness of the lunar far side. DSL will consist of a mother-spacecraft for data transport and control, plus eight small micro-satellites each equipped with three orthogonal dipoles. These satellites form a virtual distributed observatory with adjustable baselines, allowing different scientific observation strategies. The satellites are configured in a flexible quasi-linear array in nearly identical orbits, guaranteeing low relative drift rates. Short orbital periods and orbit precession ensure quick filling of the interferometric spatial frequency (u, v, w) space, enabling high quality imaging. The science themes considered for the DSL mission include pioneering studies of the unknown and exploratory science such as the search for signatures of the cosmological Dark Ages, complementing current (e.g. LOFAR) and future SKA telescope searches; full-sky continuum survey of discrete sources, including ultra-steep spectrum extragalactic sources, pulsars, and transients (galactic and extragalactic); full-sky map of continuum diffuse emission; solar-terrestrial physics, planetary sciences, and cosmic ray physics. The main frequency band covered is 1-30 MHz extending down to 0.1 MHz, and up to about 50 MHz for cross-referencing with ground-based instruments. DSL will support a variety of observational modes, including broad-band spectral analysis for Dark Ages, radio interferometric cross-correlations for imaging, and flexible raw data downlink capability. Data processing will be performed at radio astronomy science data centres in Europe and China.