Space-based aperture array for ultra-long wavelength radio astronomy

Experimental Astronomy, Dec 2015

The past decade has seen the advent of various radio astronomy arrays, particularly for low-frequency observations below 100 MHz. These developments have been primarily driven by interesting and fundamental scientific questions, such as studying the dark ages and epoch of re-ionization, by detecting the highly red-shifted 21 cm line emission. However, Earth-based radio astronomy observations at frequencies below 30 MHz are severely restricted due to man-made interference, ionospheric distortion and almost complete non-transparency of the ionosphere below 10 MHz. Therefore, this narrow spectral band remains possibly the last unexplored frequency range in radio astronomy. A straightforward solution to study the universe at these frequencies is to deploy a space-based antenna array far away from Earths’ ionosphere. In the past, such space-based radio astronomy studies were principally limited by technology and computing resources, however current processing and communication trends indicate otherwise. Furthermore, successful space-based missions which mapped the sky in this frequency regime, such as the lunar orbiter RAE-2, were restricted by very poor spatial resolution. Recently concluded studies, such as DARIS (Disturbuted Aperture Array for Radio Astronomy In Space) have shown the ready feasibility of a 9 satellite constellation using off the shelf components. The aim of this article is to discuss the current trends and technologies towards the feasibility of a space-based aperture array for astronomical observations in the Ultra-Long Wavelength (ULW) regime of greater than 10 m i.e., below 30 MHz. We briefly present the achievable science cases, and discuss the system design for selected scenarios such as extra-galactic surveys. An extensive discussion is presented on various sub-systems of the potential satellite array, such as radio astronomical antenna design, the on-board signal processing, communication architectures and joint space-time estimation of the satellite network. In light of a scalable array and to avert single point of failure, we propose both centralized and distributed solutions for the ULW space-based array. We highlight the benefits of various deployment locations and summarize the technological challenges for future space-based radio arrays.

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Space-based aperture array for ultra-long wavelength radio astronomy

Exp Astron Space-based aperture array for ultra-long wavelength radio astronomy Raj Thilak Rajan 0 1 2 3 4 Albert-Jan Boonstra 0 1 2 3 4 Mark Bentum 0 1 2 3 4 Marc Klein-Wolt 0 1 2 3 4 Frederik Belien 0 1 2 3 4 Michel Arts 0 1 2 3 4 Noah Saks 0 1 2 3 4 Alle-Jan van der Veen 0 1 2 3 4 0 Telecommunications Engineering, University of Twente , Enschede , The Netherlands 1 Research and Development, Technical University of Delft , Delft , The Netherlands 2 Research and Development, Netherlands Institute for Radio Astronomy (ASTRON) , Dwingeloo , The Netherlands 3 Airbus Defence , Space, Friedrichshafen , Germany 4 Radboud University , Nijmegen , The Netherlands The past decade has seen the advent of various radio astronomy arrays, particularly for low-frequency observations below 100 MHz. These developments have been primarily driven by interesting and fundamental scientific questions, such as studying the dark ages and epoch of re-ionization, by detecting the highly redshifted 21 cm line emission. However, Earth-based radio astronomy observations at frequencies below 30 MHz are severely restricted due to man-made interference, ionospheric distortion and almost complete non-transparency of the ionosphere below 10 MHz. Therefore, this narrow spectral band remains possibly the last unexplored frequency range in radio astronomy. A straightforward solution to study the universe at these frequencies is to deploy a space-based antenna array far away from Earths' ionosphere. In the past, such space-based radio astronomy studies were principally limited by technology and computing resources, however current processing and communication trends indicate otherwise. Furthermore, successful space-based missions which mapped the sky in this frequency regime, such as the lunar orbiter RAE-2, were restricted by very poor spatial resolution. Recently concluded studies, such as DARIS (Disturbuted Aperture Array for Radio Astronomy In Space) have - shown the ready feasibility of a 9 satellite constellation using off the shelf components. The aim of this article is to discuss the current trends and technologies towards the feasibility of a space-based aperture array for astronomical observations in the Ultra-Long Wavelength (ULW) regime of greater than 10 m i.e., below 30 MHz. We briefly present the achievable science cases, and discuss the system design for selected scenarios such as extra-galactic surveys. An extensive discussion is presented on various sub-systems of the potential satellite array, such as radio astronomical antenna design, the on-board signal processing, communication architectures and joint space-time estimation of the satellite network. In light of a scalable array and to avert single point of failure, we propose both centralized and distributed solutions for the ULW space-based array. We highlight the benefits of various deployment locations and summarize the technological challenges for future space-based radio arrays. 1 Introduction The success of Earth-based radio astronomy in the frequencies between 30 MHz and 3 GHz is jointly credited to Earth’s transparent ionosphere and the steady technological advancements during the past few decades. In recent times, radio astronomy has seen the advent of a large suite of radio telescopes, particularly towards the longer observational wavelengths, i.e., ≥ 3 m. These arrays include the Murchison Widefield Array (MWA) [ 47 ], LOw Frequency ARray (LOFAR) [ 74 ] and the Long Wavelength Array (LWA) [ 27 ] to name a few. However, Earth-based astronomical observations at these ultra-long wavelengths are severely restricted [ 37 ]. Due to ionospheric distortion, especially during the solar maximum period, when scintillation occurs, the celestial signals suffer from de-correlation among the elements of a ground based telescope array [ 39 ]. Currently, advanced calibration and mitigation techniques are employed in the LOFAR telescope array, which can be used to remove these distortions, provided the time scale of disturbances is much longer than the time needed for calibration [77]. Furthermore, at frequencies below 10 MHz the ionosphere is completely non-transparent which impede observations by ground-based instruments. In addition to ionospheric interference, man-made transmitter signals below 30 MHz also impede terrestrial based astronomical observations. This terrestrial interference was even observed as far as ∼400,000 km away from Earth by the RAE-2 lunar orbiter, which was limited by very poor spatial resolution at these wavelengths, e.g., 37◦ at 9.18 MHz [ 3 ]. Due to the above mentioned reasons, the very low frequency range of 0.3 − 30 MHz remains one of the last unexplored frontier in astronomy. An unequivocal solution to observe the radio sky at ULW with the desired resolution and sensitivity is to deploy a dedicated satellite array in outerspace. Such a space-based array must be deployed sufficiently far away from Earths’ ionosphere, to avoid terre (...truncated)


This is a preview of a remote PDF: https://link.springer.com/content/pdf/10.1007%2Fs10686-015-9486-6.pdf

Raj Thilak Rajan, Albert-Jan Boonstra, Mark Bentum, Marc Klein-Wolt, Frederik Belien, Michel Arts, Noah Saks, Alle-Jan van der Veen. Space-based aperture array for ultra-long wavelength radio astronomy, Experimental Astronomy, 2016, pp. 271-306, Volume 41, Issue 1-2, DOI: 10.1007/s10686-015-9486-6