In the last decades, there has been a steady adoption of digital online platforms as learning environments applied to all levels of education. This increasing adoption forces a transition in educational resources which has further been accelerated by the recent pandemic, leading to an almost complete online-only learning environment in some cases. The aim of this paper is to outline the methodology involved in setting up a framework for mapping course-specific data based on student activity to standard learning indicators, which will serve as an input to performance prediction algorithms. The process involves systematically surveying, capturing, and categorising the vast range of data available in digital learning platforms. The data are collected from two sample courses and distilled into five dimensions represented by the generic learning indicators: prior knowledge, preparation, participation, interaction, and performance. The data is weighted based on course development and teaching member’s perspectives to account for course-wise variations. The framework established will allow portability of prediction algorithms between courses and provide a means for meaningful and directed learner formative feedback. Two courses, both bachelor-level and worth 5 European Credits (ECs), that use several online learning platforms in their teaching tools have been chosen in this study to explore the nature and range of student interaction data available, accessible, and usable in a course. The first course is Electromagnetics II at Eindhoven University of Technology, and the second course is Electronics at Delft University of Technology. Both Universities are located in the Netherlands. This work is in the scope of a broader study to use such learning indicators with predictive algorithms to provide a prognosis on individual student performance. The findings in this paper will enable the realization of student performance prediction at a very early stage in the course.@en

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.

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Observing the universe in the Ultra-Long Wavelength (ULW) regime has been called the ‘last frontier in astronomy’—real imaging capabilities here are yet to be achieved. Obtaining an image of the sky in this frequency band can be done by employing a swarm of satellites that together act as an interferometer and collect the required imaging information pieces throughout the course of their operational life. Meeting the mission objective is challenging for such a swarm, since this imposes restrictions on the operational environment and the relative position and velocity vectors between the swarm elements. This work proposes an orbit solution in a Heliocentric Earth-Leading Orbit (HELO) for an autonomous CubeSat swarm with chemical thrusters. A distributed formation flying algorithm is used to aid the collection of the required imaging information pieces. Furthermore, the estimated total mission launch mass is reduced by optimising cost functions and finding favourable position and velocity at start of operational life, as well as by finding favourable thrust manoeuvre patterns. The results show that the mission objective—obtaining a 3D map of the Universe in ULW—can be achieved with 68 6U spacecraft (S/C). Moreover, the swarm can remain in a Radio Frequency Interference (RFI) quiet zone of >5 × 10⁶ km, whilst not drifting further than ~ 6.6 × 10⁶ km from Earth for an operational life of one year.

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The past two decades have witnessed a renewed interest in low frequency radio astronomy, with a particular focus on frequencies above 30 MHz e.g., LOFAR (LOw Frequency ARray) in the Netherlands and its European extension ILT, the International LOFAR Telescope. However, at frequencies below 30 MHz, Earth-based observations are limited due to a combination of severe ionospheric distortions, almost full reflection of radio waves below 10 MHz, solar eruptions and the radio frequency interference (RFI) of human-made signals. Moreover, there are interesting scientific processes which naturally occur at these low frequencies. A space or Lunar-based ultra-low-frequency (also referred to as ultra-long-wavelength, ULW) radio array would suffer significantly less from these limitations and hence would open up the last, virtually unexplored frequency domain in the electromagnetic spectrum. A roadmap has been initiated by astronomers and researchers in the Netherlands to explore the opportunity of building a swarm of satellites to observe at the frequency band below 30 MHz. This roadmap dubbed Orbiting Low Frequency Antennas for Radio Astronomy (OLFAR), a space-based ultra-low frequency radio telescope that will explore the Universe's so-called dark ages, map the interstellar medium, and study planetary and solar bursts in the solar system and search them in other planetary systems. Such a radio astronomy system will comprise of a swarm of hundreds to thousands of satellites, working together as a single aperture synthesis instrument deployed sufficiently far away from Earth to avoid terrestrial RFI. The OLFAR telescope is a novel and complex system, requiring yet to be proven engineering solutions. Therefore, a number of key technologies are still required to be developed and proven. The first step in this roadmap is the NCLE (Netherlands China Low Frequency Explorer) experiment, which was launched in May 2018 on the Chinese Chang'e 4 mission. The NCLE payload consists of a three monopole antenna system for low frequency observations, from which the first data stream is expected in the second half of 2019, which will provide important feedback for future science and technology opportunities. In this paper, the roadmap towards OLFAR, a brief overview of the science opportunities, and the technological and programmatic challenges of the mission are presented.

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This paper presents a software-defined testbed to perform hardware-in-The-loop test of miniaturized coherent transponders. Such a setup has been designed to minimize the access threshold for future users, heavily relying on available open source applications and commercial hardware, targeting future coherent transponders for interplanetary CubeSats. The paper presents the overall architecture of the testbed, a tradeoff to select the most suited development framework and the detailed design of the different blocks. Upcoming interplanetary CubeSat missions that would require a coherent transponder are also presented to highlight the need sof such a system. Software qualification, given the use of third-party software with multiple developers, was also addressed to guarantee performances can be consistent and reliable.

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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.

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The (computational) complexity involved by beamforming in moving constellations of (nano) satellites is investigated by means of illustrative numerical experiments. While the number of radiators in such three-dimensional (3D) array antennas is not large, evaluating their radiation patterns entails challenging computational intricacies in view of the satellites being in motion and each satellite performing general 3D rotations. As a result, the relevant array radiation patterns become time-dependent, the elementary radiation patterns being themselves time-dependent. The discussed experiments will illustrate the time evolution of the radiation pattern for given individual satellite orbits and rotation laws. At the same time, they will provide a basis for estimating the computational complexity involved by predicting the complete beamforming in future space-bound remote sensing missions using constellations of (nano) satellites.

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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.@en

The impact of the fluctuations in the locations of elementary radiators on the radiation properties of three-dimensional (3D) array antennas is studied. The principal radiation features (sidelobes level, beam squint) are examined based on illustrative examples. Some atypical behaviours, that are specific to 3D arrays, are highlighted. The effect of fluctuations is demonstrated via examples concerning non-uniform arrays. This study is important for designing beamforming strategies in case of constellations of (nano) satellites for space-bound remote sensing of the Earth and the Universe.@en

The Orbiting Low Frequency Antennas for Radio Astronomy (OLFAR) project aims to develop a space-based low frequency radio telescope that will explore the universe's so-called dark ages, map the interstellar medium, and discover planetary and solar bursts in other solar systems. The telescope, composed of a swarm of at least fifty satellites working as a single instrument, will be sent to a location far from Earth in order to avoid the high Radio Frequency Interference (RFI) found at frequencies below 30 MHz, originating from Earth. The OLFAR telescope is a novel and complex system, requiring not-yet proven technologies and systems, therefore, a number of key technologies are still to be developed and proven. Most of these can be tested on Earth, but four aspects in particular require in-space verification. Those are (1) the satellite's propulsion and attitude control systems, and (2) their interactions with the large science antennas, as well as the (3) payload system itself and finally (4) the in-space interferometry and 3D-imaging. Furthermore, the RFI environment in the intended target orbits is mostly unknown. Indeed, only three satellites missions have previously been launched into orbit shedding light on the RFI environment, but sufficiently detailed measurements allowing for the creation of a usable RFI model have never been performed. To carry out both the hardware qualification and RFI measurements, a few pathfinder missions are deemed in order. This paper describes these pathfinders in detail; outlining the scientific objective, the technologies being demonstrated as well as the missions' roadmap which revolves around a novel systems engineering approach. This approach resembles those used in certain fast-paced industries where development is heavily parallelised and products are launched as soon as opportunities arise. This will be combined with in-space upgrading of mission firmware to allow for high flexibility within the limited time and budget constraints of these pathfinders. Copyright@en

The resistance of a preferentially oriented thin film of Y2Ba4Cu8O16-δ has been measured as a function of temperature and fields up to 20 T oriented parallel to the c-axis. The normal state resistance could be well fitted by the Bloch-Grüneisen theory. Below Tc (≈ 80 K) the resistance transitions both in field and temperature could be separated in three regimes. A regime where the data can be analysed in terms of a model combining thermally activated flux flow and flux pinning due to dislocation lines. The upper critical field Bc2, obtained from this analysis, exhibits an upward curvature at low temperatures, possibly indicating a mixed s-d wave pairing. In the second regime just below Bc2 viscous forces are increasingly important for flux-line motion and the resistance is determined by the flux-flow resistance ρ{variant}f. At fields higher than Bc2 we find a large contribution of superconducting fluctuations to the conduction.@en