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66-499: LOFAR consists of a vast array of omnidirectional radio antennas using a modern concept, in which the signals from the separate antennas are not connected directly electrically to act as a single large antenna, as they are in most array antennas . Instead, the LOFAR dipole antennas (of two types) are distributed in stations, within which the antenna signals can be partly combined in analogue electronics, then digitised, then combined again across

132-466: A VHF receiver either in stand-alone mode or part of a bistatic radar system together with EISCAT transmitter in Tromsø . Data transport requirements are in the range of several gigabits per second per station and the processing power needed is several tens of TeraFLOPS . The data from LOFAR is stored in the LOFAR long-term archive. The archive is implemented as distributed storage, with data spread over

198-549: A "mini-array" of 19 crossed-dipole antennas, distributed in a circle with a diameter of approximately 400 m. The tiles are a hexagonal cluster with analogically phased antennas. The telescope can capture radio frequencies in the 10–85 MHz range, covering the LOFAR-Low Band (30–80 MHz) range as well. The NenuFAR array can work as a high-sensitivity LOFAR-compatible super-LBA station (LSS), operating together with rest of LOFAR to increase to array's global sensitivity by nearly

264-608: A 110 m long coaxial cable to the receiver unit (RCU). On April 26, 2005, an IBM Blue Gene/L supercomputer was installed at the University of Groningen 's math centre, for LOFAR's data processing . At that time it was the second most powerful supercomputer in Europe , after the MareNostrum in Barcelona . Since 2014 an even more powerful computing cluster (correlator) called COBALT performs

330-423: A beam. This type is called an aperture antenna . A parabolic dish is an example of this type of antenna. A second technique is to use multiple antennas which are fed from the same transmitter or receiver; this is called an array antenna, or antenna array. For a transmitting antenna the electromagnetic wave received at any point is the vector sum of the electromagnetic waves from each of the antenna elements. If

396-539: A deep survey for radio pulsars at low radio frequencies, and will attempt to detect giant radio bursts from rotating neutron stars in distant galaxies. LOFAR offers a unique possibility in particle physics for studying the origin of high-energy and ultra-high-energy cosmic rays (HECRs and UHECRs) at energies between 10–10 eV. Both the sites and processes for accelerating particles are unknown. Possible candidate sources of these HECRs are shocks in radio lobes of powerful radio galaxies, intergalactic shocks created during

462-487: A factor of two, and improve the array's imaging capabilities. It can also function as a second supercore to improve array availability. Due to its dedicated receiver, NenuFAR can also operate as a standalone instrument, known as NenuFAR/Standalone in this mode. Additionally, a set of LOFAR antennas is deployed at the KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array) near Kilpisjärvi , Finland . This installation functions as

528-402: A fixed radiation pattern, we may consider that the feed network is a part of the antenna array. Thus, the antenna array has a single port. Narrow beams can be formed, provided the phasing of each element of the array is appropriate. If, in addition, the amplitude of the excitation received by each element (during emission) is also well chosen, it is possible to synthesize a single-port array having

594-649: A new era in the monitoring of the radio sky. It will be possible to make sensitive radio maps of the entire sky visible from The Netherlands (about 60% of the entire sky) in only one night. Transient radio phenomena, only hinted at by previous narrow-field surveys, will be discovered, rapidly localised with unprecedented accuracy, and automatically compared to data from other facilities (e.g. gamma-ray, optical, and X-ray observatories). Such transient phenomena may be associated with exploding stars, black holes, flares on Sun-like stars, radio bursts from exoplanets or even SETI signals. In addition, this key science project will make

660-402: A radiation pattern that closely approximates a specified pattern. Many methods have been developed for array pattern synthesis. Additional issues to be considered are matching, radiation efficiency and bandwidth. The design of an electronically steerable antenna array is different, because the phasing of each element can be varied, and possibly also the relative amplitude for each element. Here,

726-469: A resolution comparable to the Hubble Space Telescope . The hunebed dolmen D30  [ nl ] used to have four capstones, however one is missing. Unless the others which are placed east-west, it is placed north-south. In 1918, it was explored by Albert Egges van Giffen . There were five floors underneath, and you could stand up straight in the first part of the grave underneath. D31

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792-619: A separate antenna element or group of elements. An antenna array can achieve higher gain ( directivity ), that is a narrower beam of radio waves, than could be achieved by a single element. In general, the larger the number of individual antenna elements used, the higher the gain and the narrower the beam. Some antenna arrays (such as military phased array radars) are composed of thousands of individual antennas. Arrays can be used to achieve higher gain, to give path diversity (also called MIMO ) which increases communication reliability, to cancel interference from specific directions, to steer

858-405: A source of disadvantages in both transmission and reception. In fact, in transmission, they can lead to radiation in unwanted directions, while, in reception, they can be a source of ambiguity since the desired signal entering the mainlobe region could be strongly disturbed by other signals (unwanted interfering signals) entering the regions of the various grating lobes. Therefore, in periodic arrays,

924-402: A wide angle. To create a directional antenna ( high gain antenna ), which radiates radio waves in a narrow beam, two general techniques can be used: One technique is to use reflection by large metal surfaces such as parabolic reflectors or horns , or refraction by dielectric lenses to change the direction of the radio waves, to focus the radio waves from a single low gain antenna into

990-453: Is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves . The individual antennas (called elements ) are usually connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. The radio waves radiated by each individual antenna combine and superpose , adding together ( interfering constructively ) to enhance

1056-426: Is assumed that radiators have the same orientation and the same polarization of the electric field. Based on this, the array factor can be written as follows F ( u ) = ∑ n = 1 N I n e j k x n u {\displaystyle F(u)=\sum _{n=1}^{N}I_{n}\,e^{jk\,x_{n}u}} where N {\displaystyle N}

1122-456: Is intended to receive independent excitations during emission, and to deliver more or less independent signals during reception. Here also, the subject matters of matching and efficiency are involved, especially in the case of an antenna array of a mobile device (see chapter 10 of ), since, in this case, the surroundings of the antenna array influence its behavior, and vary over time. Suitable matching metrics and efficiency metrics take into account

1188-524: Is small, and disentangling it from the much stronger foreground emission is challenging. One of the most important applications of LOFAR will be to carry out large-sky surveys. Such surveys are well suited to the characteristics of LOFAR and have been designated as one of the key projects that have driven LOFAR since its inception. Such deep LOFAR surveys of the accessible sky at several frequencies will provide unique catalogues of radio sources for investigating several fundamental areas of astrophysics, including

1254-607: Is the number of antenna elements, k {\displaystyle k} is the wavenumber, I n {\displaystyle I_{n}} and x n {\displaystyle x_{n}} (in meters) are the complex excitation coefficient and the position of the n-th radiator, respectively, u = sin ⁡ θ cos ⁡ ϕ {\displaystyle u=\sin \theta \cos \phi } , with θ {\displaystyle \theta } and ϕ {\displaystyle \phi } being

1320-471: Is thought that the 'Dark Ages', the period after recombination when the Universe turned neutral, lasted until around z=20. WMAP polarization results appear to suggest that there may have been extended, or even multiple phases of reionisation, the start possibly being around z~15–20 and ending at z~6. Using LOFAR, the redshift range from z=11.4 (115 MHz) to z=6 (200 MHz) can be probed. The expected signal

1386-631: Is thus an interferometric array, using about 20,000 small antennas concentrated in 52 stations since 2019. 38 of these stations are distributed across the Netherlands, built with regional and national funding. The six stations in Germany , three in Poland , and one each in France , Great Britain , Ireland , Latvia , and Sweden , with various national, regional, and local funding and ownership. Italy officially joined

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1452-419: Is usually another name for a Yagi–Uda antenna . A phased array usually means an electronically scanned array ; a driven array antenna in which each individual element is connected to the transmitter or receiver through a phase shifter controlled by a computer. The beam of radio waves can be steered electronically to point instantly in any direction over a wide angle, without moving the antennas. However

1518-672: The One-Mile Telescope or the Very Large Array ), arrays of one-dimensional antennas (e.g. the Molonglo Observatory Synthesis Telescope ) or two-dimensional arrays of omnidirectional antennas (e.g. Antony Hewish 's Interplanetary Scintillation Array ). LOFAR combines aspects of many of these earlier telescopes; in particular, it uses omnidirectional dipole antennas as elements of a phased array at individual stations, and combines those phased arrays using

1584-721: The Solar Dynamics Observatory (SDO) , and eventually the Advanced Technology Solar Telescope and the Solar Orbiter provide insights into this fundamental astrophysical process. In the early 1990s, the study of aperture array technology for radio astronomy was being actively studied by ASTRON – the Netherlands Institute for Radio Astronomy. At the same time, scientific interest in a low-frequency radio telescope began to emerge at ASTRON and at

1650-707: The Target data centre located in the Donald Smits Center for Information Technology at the University of Groningen , SURFsara  [ nl ] centre in Amsterdam, and the Forschungszentrum Jülich in Germany. The mission of LOFAR is to map the Universe at radio frequencies from ~10–240 MHz with greater resolution and greater sensitivity than previous surveys, such as the 7C and 8C surveys, and surveys by

1716-583: The Very Large Array (VLA) and Giant Meterwave Radio Telescope (GMRT) . LOFAR will be the most sensitive radio observatory at its low observing frequencies until the Square Kilometre Array (SKA) comes online in the late 2020s. Even then, the SKA will only observe at frequencies >50 MHz and LOFAR's angular resolution will remain far superior. The sensitivities and spatial resolutions attainable with LOFAR make possible several fundamental new studies of

1782-411: The aperture synthesis technique developed in the 1950s. Like the earlier Cambridge Low Frequency Synthesis Telescope (CLFST) low-frequency radio telescope, the design of LOFAR has concentrated on the use of large numbers of relatively cheap antennas without any moving parts, concentrated in stations, with the mapping performed using aperture synthesis software . The direction of observation ("beam") of

1848-489: The raised bog near Exloo was excavated around 1800. In 1850, it turned into an industry and excavation villages such as 1e Exloërmond and 2e Exloërmond were established. Exloo was home to 570 people in 1840. In 2010, LOFAR , a low frequency radio telescope, opened near Exloo. There are 6 stations in Exloo with a 18 stations within a 2 kilometre radius, and a further 28 in eight European countries. The set up will give LOFAR

1914-746: The Dutch Universities. A feasibility study was carried out and international partners sought during 1999. In 2000 the Netherlands LOFAR Steering Committee was set up by the ASTRON Board with representatives from all interested Dutch university departments and ASTRON. In November 2003 the Dutch Government allocated 52 million euro to fund the infrastructure of LOFAR under the Bsik programme. In accordance with Bsik guidelines, LOFAR

1980-604: The International LOFAR Telescope (ILT) in 2018; construction at the INAF observatory site in Medicina , near Bologna , is planned as soon as upgraded (so-called LOFAR2.0) hardware becomes available. Further stations in other European countries are in various stages of planning. The total effective collecting area is approximately 300,000 square meters, depending on frequency and antenna configuration. Until 2014, data processing

2046-514: The LOFAR array. To make radio surveys of the sky with adequate resolution, the antennas are arranged in clusters that are spread out over an area of more than 1000 km in diameter. The LOFAR stations in the Netherlands reach baselines of about 100 km. LOFAR currently receives data from 24 core stations (in Exloo ), 14 'remote' stations in The Netherlands, and 14 international stations. Each of

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2112-440: The LOFAR stations are digitised, transported to a central digital processor, and combined in software in order to map the sky. Therefore, LOFAR is a "software telescope". The cost of such telescopes is dominated by the cost of electronics and will therefore mostly follow Moore's law , becoming cheaper with time and allowing increasingly large telescopes to be built. Each antenna is fairly simple- but there are about 20,000 of them in

2178-503: The Universe as well as facilitating unique practical investigations of the Earth's environment. In the following list the term z is a dimensionless quantity indicating the redshift of the radio sources seen by LOFAR. One of the most exciting, but technically most challenging, applications of LOFAR will be the search for redshifted 21 cm line emission from the Epoch of Reionization (EoR). It

2244-427: The antenna array has multiple ports, so that the subject matters of matching and efficiency are more involved than in the single-port case. Moreover, matching and efficiency depend on the excitation, except when the interactions between the antennas can be ignored. An antenna array used for spatial diversity and/or spatial multiplexing (which are different types of MIMO radio communication) always has multiple ports. It

2310-463: The central cluster with 48 dipoles and other three clusters with 16 dipoles each. Each cluster is about 100 m in size. The clusters are distributed over an area of ~500 m in diameter. In November 2007 the first international LOFAR station ( DE601 ) next to the Effelsberg 100 m radio telescope became the first operational station. The first fully complete station, ( CS302 ) on the edge of the LOFAR core,

2376-454: The component antennas' axis relates to the radiation direction. There are also arrays (such as phased arrays ) which don't belong to either of these categories, in which the direction of radiation is at some other angle to the antenna axis. Array antennas can also be categorized by how the element antennas are arranged: Let us consider a linear array whose elements are arranged along the x-axis of an orthogonal Cartesian reference system. It

2442-535: The core and remote stations has 48 HBAs and 96 LBAs and a total of 48 digital Receiver Units (RCUs). International stations have 96 LBAs and 96 HBAs and a total of 96 digital Receiver Units (RCUs). The locations of the international LOFAR stations are: The NenuFAR telescope is co-located at the Nançay radio telescope . It is an extension of the Nançay LOFAR station (FR606), adding 96 low frequency tiles, each consisting of

2508-407: The correlation of signals from all individual stations. In August/September 2006 the first LOFAR station ( Core Station CS001 , aka. CS1 52°54′32″N 6°52′8″E  /  52.90889°N 6.86889°E  / 52.90889; 6.86889 ) was put in the field using pre-production hardware. A total of 96 dual-dipole antennas (the equivalent of a full LOFAR station) are grouped in four clusters,

2574-420: The currents are fed to the antennas with the proper phase , due to the phenomenon of interference the spherical waves from the individual antennas combine (superpose) in front of the array to create plane waves , a beam of radio waves traveling in a specific direction. In directions in which the waves from the individual antennas arrive in phase , the waves add together ( constructive interference ) to enhance

2640-400: The epoch of galaxy formation, so-called Hyper-novae, gamma-ray bursts , or decay products of super-massive particles from topological defects, left over from phase transitions in the early Universe. The primary observable is the intense radio pulse that is produced when a primary CR hits the atmosphere and produces an extensive air shower (EAS). An EAS is aligned along the direction of motion of

2706-494: The extent of the visible space also changes accordingly. Now, suppose that the excitation coefficients are positive real variables. In this case, always in the domain of u {\displaystyle u} , the array factor magnitude has a main lobe with maximum value at u = 0 {\displaystyle u=0} , called mainlobe , several secondary lobes lower than the mainlobe, called sidelobes and mainlobe replicas called grating-lobes . Grating lobes are

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2772-580: The field of radio astronomy , in which multiple radio telescopes consisting of large parabolic antennas are linked together into an antenna array, to achieve higher resolution. Using the technique called aperture synthesis such an array can have the resolution of an antenna with a diameter equal to the distance between the antennas. In the technique called Very Long Baseline Interferometry (VLBI) dishes on separate continents have been linked, creating "array antennas" thousands of miles in size. Most array antennas can be divided into two classes based on how

2838-466: The first to detect weak radio emission from such regions. LOFAR will also measure the Faraday effect , which is the rotation of polarization plane of low-frequency radio waves, and gives another tool to detect weak magnetic fields. The Sun is an intense radio source. The already strong thermal radiation of the 10 K hot solar corona is superimposed by intense radio bursts that are associated with phenomena of

2904-643: The formation of massive black holes , galaxies and clusters of galaxies. Because the LOFAR surveys will probe an unexplored parameter of the Universe, it is likely that they will discover new phenomena. In February 2021, astronomers released, for the first time, a very high-resolution image of 25,000 active supermassive black holes , covering four percent of the Northern celestial hemisphere , based on ultra-low radio wavelengths , as detected by LOFAR. The combination of low frequencies, omnidirectional antennae, high-speed data transport and computing means that LOFAR will open

2970-437: The full station. This step-wise approach provides great flexibility in setting and rapidly changing the directional sensitivity on the sky of an antenna station. The data from all stations are then transported over fiber to a central digital processor, and combined in software to emulate a conventional radio telescope dish with a resolving power corresponding to the greatest distance between the antenna stations across Europe. LOFAR

3036-538: The grating lobes are outside the interval [-1,1]. As seen above, when the spacing is constant between adjacent radiators, the array factor is characterized by the presence of grating lobes. In the literature, it has been amply demonstrated that to destroy the array factor's periodicity, the same array's geometry must also be made aperiodic. It is possible to act on the positions of the radiators so that these positions are not commensurable with each other. Several methods have been developed to synthesize arrays in which also

3102-428: The interval [ − 1 , 1 ] {\displaystyle [-1,1]} , which is associated with the values of θ {\displaystyle \theta } and ϕ {\displaystyle \phi } . In this case, the interval [-1,1] is called visible space . As shown further, if the definition of the variable u {\displaystyle u} changes,

3168-572: The positions represent further degrees of freedom (unknowns). There are both deterministic and probabilistic methodologies. Since the probabilistic theory of aperiodic arrays is a sufficiently systematised theory, with a strong general methodological basis, let us first concentrate on describing its peculiarities. Suppose that the radiators positions, { x n } n = 1 N {\displaystyle \{x_{n}\}_{n=1}^{N}} , are independent and identically distributed random variables whose support coincides with

3234-506: The power radiated in desired directions, and cancelling ( interfering destructively ) to reduce the power radiated in other directions. Similarly, when used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to

3300-416: The power radiated. In directions in which the individual waves arrive out of phase , with the peak of one wave coinciding with the valley of another, the waves cancel ( destructive interference ) reducing the power radiated in that direction. Similarly, when receiving, the oscillating currents received by the separate antennas from radio waves received from desired directions are in phase and when combined in

3366-499: The primary particle, and a substantial part of its component consists of electron-positron pairs which emit radio emission in the terrestrial magnetosphere (e.g., geo-synchrotron emission). LOFAR opens the window to the so far unexplored low-energy synchrotron radio waves, emitted by cosmic-ray electrons in weak magnetic fields. Very little is known about the origin and evolution of cosmic magnetic fields. The space around galaxies and between galaxies may all be magnetic, and LOFAR may be

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3432-416: The radio beam electronically to point in different directions, and for radio direction finding (RDF). The term antenna array most commonly means a driven array consisting of multiple identical driven elements all connected to the receiver or transmitter. A parasitic array consists of a single driven element connected to the feedline, and other elements which are not, called parasitic elements . It

3498-419: The receiver reinforce each other, while currents from radio waves received from other directions are out of phase and when combined in the receiver cancel each other. The radiation pattern of such an antenna consists of a strong beam in one direction, the main lobe , plus a series of weaker beams at different angles called sidelobes , usually representing residual radiation in unwanted directions. The larger

3564-434: The solar activity as the root of space weather. Furthermore, LOFAR's flexibility enables rapid responses to solar radio bursts with follow-up observations. Solar flares produce energetic electrons that not only lead to the emission of non-thermal solar radio radiation. The electrons also emit X-rays and heat the ambient plasma. So joint observation campaigns with other ground- and space-based instruments, e.g. RHESSI , Hinode ,

3630-502: The solar activity, like flares and coronal mass ejections (CMEs). Solar radio radiation in the LOFAR frequency range is emitted in the middle and upper corona. So LOFAR is an ideal instrument for studies of the launch of CMEs heading towards interplanetary space. LOFAR's imaging capabilities will yield information on whether such a CMEs might hit the Earth. This makes LOFAR is a valuable instrument for space weather studies. Solar observations with LOFAR will include routine monitoring of

3696-529: The spacing between adjacent radiators must not exceed a specific value to prevent the appearance of grating lobes (in the visible space) in the visible space ), the spacing between adjacent radiators must not exceed a specific value. For example, as seen previously, the first grating lobes for d = λ / 2 {\displaystyle d=\lambda /2} occur in u = ± 2 {\displaystyle u=\pm 2} . So, in this case, there are no problems since, in this way,

3762-487: The stations is chosen electronically by phase delays between the antennas. LOFAR can observe in several directions simultaneously, as long as the aggregated data rate remains under its cap. This in principle allows a multi-user operation. LOFAR makes observations in the 10 MHz to 240 MHz frequency range with two types of antennas: Low Band Antenna (LBA) and High Band Antenna (HBA), optimized for 10–80 MHz and 120–240 MHz respectively. The electric signals from

3828-549: The term "phased array" is sometimes used to mean an ordinary array antenna. From the Rayleigh criterion , the directivity of an antenna, the angular width of the beam of radio waves it emits, is proportional to the wavelength of the radio waves divided by the width of the antenna. Small antennas around one wavelength in size, such as quarter-wave monopoles and half-wave dipoles , don't have much directivity ( gain ); they are omnidirectional antennas which radiate radio waves over

3894-441: The wavelength, then the magnitude of the array factor has a period, in the domain of u {\displaystyle u} , equal to 2 {\displaystyle 2} . It is worth emphasising that u {\displaystyle u} is an auxiliary variable. In fact, from a physical point of view, the values of u {\displaystyle u} that are of interest for radiative purposes fall in

3960-457: The whole array aperture. Consequently, the array factor is a stochastic process, whose mean is as follows E [ F ( u ) ] = ∫ − L / 2 L / 2 f ( x ) e j k x u d x {\displaystyle E\left[F(u)\right]=\textstyle \int \limits _{-L/2}^{L/2}f(x)\,e^{jkxu}\,dx} In an antenna array providing

4026-408: The width of the antenna and the greater the number of component antenna elements, the narrower the main lobe, and the higher the gain which can be achieved, and the smaller the sidelobes will be. Arrays in which the antenna elements are fed in phase are broadside arrays; the main lobe is emitted perpendicular to the plane of the elements. The largest array antennas are radio interferometers used in

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4092-535: The worst possible excitations. Exloo Exloo ( Low German : Eksel) is a village in the province of Drenthe , Netherlands , part of the municipality of Borger-Odoorn . It lies about 12 km north of Emmen . The village was first mentioned in 1376 as "tot Exle", and means "forest of the oak trees". Exloo is an esdorp which developed in the Middle Ages probably from Odoorn . It has three essen (communal pastures), but no church. The peat in

4158-589: The zenith angle and azimuth angle, respectively. If the spacing between adjacent elements is constant, then it can be written that x n + 1 − x n = d {\displaystyle x_{n+1}-x_{n}=d} , and the array is said to be periodic. The array is periodic both spatially (physically) and in the variable u {\displaystyle u} . For example, if d = λ / 2 {\displaystyle d=\lambda /2} , with λ {\displaystyle \lambda } being

4224-764: Was delivered in May 2009, with a total of 40 Dutch stations scheduled for completion in 2013. By 2014, 38 stations in the Netherlands, five stations in Germany (Effelsberg, Tautenburg, Unterweilenbach, Bornim/Potsdam, and Jülich), and one each in the UK (Chilbolton), in France (Nançay) and in Sweden (Onsala) were operational. LOFAR was officially opened on 12 June 2010 by Queen Beatrix of the Netherlands. Regular observations started in December 2012. Array antenna An antenna array (or array antenna )

4290-422: Was funded as a multidisciplinary sensor array to facilitate research in geophysics , computer sciences and agriculture as well as astronomy . In December 2003 LOFAR's Initial Test Station (ITS) became operational. The ITS system consists of 60 inverse V-shaped dipoles; each dipole is connected to a low-noise amplifier (LNA), which provides enough amplification of the incoming signals to transport them over

4356-585: Was performed by a Blue Gene/P supercomputer situated in the Netherlands at the University of Groningen . Since 2014 LOFAR uses a GPU-based correlator and beamformer, COBALT, for that task. LOFAR is also a technology and science pathfinder for the Square Kilometre Array . LOFAR was conceived as an innovative effort to force a breakthrough in sensitivity for astronomical observations at radio-frequencies below 250 MHz. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g.

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