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

Abstract

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.

Relative kinematics of an anchorless network

“…2) Absolute kinematics: In addition to the relative kinematics, (18b) can also be reformulated to estimate the absolute kinematics Y M of the network. Recall from (11b), that the relative kinematics of the M th order is Y M = Y M P under the assumption (9). Substituting this expression in (18b), we have…”

A roadmap towards a space-based radio telescope for ultra-low frequency radio astronomy

“…In addition, the radio quietness at the Lunar surface would greatly benefit such measurements (Scholten et al, 2006). Table 2 lists the high level system requirements of OLFAR derived from the science cases (see (Rajan et al, 2016)). The minimum distances between the satellites must be more than 10 km and due to interstellar scattering the maximum baseline is limited to 100 km, giving a resolution of 1 arcminute at 10 MHz.…”

“…One of the last unexplored domains of the electromagnetic spectrum unexplored by astronomers is the ultra-low frequency band. This frequency domain, which covers frequencies from 0.1 to 30 MHz, contains footprints of the early Universe (between 0.38 and 400 million years after the Big Bang) (Jester and Falcke, 2009;Rajan et al, 2016), magnetospheric radio emissions from planets in the Solar System and perhaps from exoplanets (Zarka, 1998(Zarka, , 2007Bentum, 2017Bentum, , 2018, and imprints of a broad variety of astrophysical processes (Jester and Falcke, 2009).…”

Prospects of probing the radio emission of lunar UHECRv events

“…In addition to Discrete Fourier Transform (DFT) or cross-correlation between input channels, the digitized data can be channelised in the frequency domain. This, for instance, can be done using a Polyphase Filter Bank(PFB) which has been suggested for a distributed space-based radio array (Rajan et al, 2016). Compared to DFT, PFB is more complex and roughly costs 1.5 times more but it produces a relatively flat response across the channels and prevents the leakage of the signals in the nearby channels (Chennamangalam, 2014).…”