INVITED PAPER TheSquareKilometreArray This telescope, to be the largest in the world, will probe the evolution of black holes as well as the basic properties, birth and death of the Universe. By Peter E. Dewdney, Peter J. Hall, Richard T. Schilizzi, and T. Joseph L. W. Lazio ABSTRACT | The Square Kilometre Array (SKA) will be an I. INTRODUCTION ultrasensitive radio telescope, built to further the understand- Advances in astronomy over the past decades have brought ing of the most important phenomena in the Universe, the international community to the verge of charting a including some pertaining to the birth and eventual death of complete history of the Universe. In order to achieve this the Universe itself. Over the next few years, the SKA will make goal, the world community is pooling resources and ex- the transition from an early formative to a well-defined design. pertise to design and construct powerful observatories that This paper outlines how the scientific challenges are translated will probe the entire electromagnetic spectrum, from radio to into technical challenges, how the application of recent gamma-rays, and even beyond the electromagnetic spectrum, technology offers the potential of affordably meeting these studying gravitational waves, cosmic rays, and neutrinos. challenges, and how the choices of technology will ultimately The Square Kilometre Array (SKA) will be one of these be made. The SKA will be an array of coherently connected telescopes, a radio telescope with an aperture of up to a antennas spread over an area about 3000 km in extent, with an million square meters. The SKA was formulated from the 2 aggregate antenna collecting area of up to 106 m at centimeter very beginning as an international, astronomer-led (Bgrass and meter wavelengths. A key scientific requirement is the roots[) initiative. The International Union for Radio Science ability to carry out sensitive observations of the sky over large (URSI) established a working group in 1993 to study the B [ areas (surveys). The survey speed of the SKA will be enabled next-generation radio wavelength observatory. Since that by the application of the most up-to-date signal-processing time, the effort has grown to comprise 19 countries and technology available. The SKA science impact will be widely felt 55 institutes. As many as seven different initial technical in astroparticle physics and cosmology, fundamental physics, concepts for the SKA have been narrowed substantially. galactic and extragalactic astronomy, solar system science, and After a rigorous process, two sites, in the Karoo region of astrobiology. central South Africa and in the state of Western Australia, were identified as suitable for much of the SKA’s intended KEYWORDS | Aperture synthesis; digital correlator; digital data wavelength range (see below). Fifteen funding agencies transmission; digital signal processing; Fourier imaging; low- now regularly discuss SKA development and funding noise amplifier; radio astronomy; radio telescope options. The project timeline has the telescope operation- al below 10 GHz by 2022. At the highest level of generalization all telescopes are designed to tackle, in the wavebands at which they oper- Manuscript received November 5, 2008; revised February 10, 2009 and ate, the most important problems in astronomy, especially March 27, 2009. First published June 26, 2009; current version published July 15, 2009. The Square Kilometre Array Program Development Office is those for which technology can provide new opportunities hosted by the University of Manchester and supported by the European SKA for advancement. The U.S. Decadal Review Panel for Consortium, the U.S. SKA Technology Development Program, the Commonwealth Scientific and Industrial Research Organization in Australia, the National Research 2000–2010 expressed these goals succinctly in their Foundation in South Africa, the National Research Council, and the University of report, Astronomy and Astrophysics in the New Millennium Calgary, Canada. Basic research in radio astronomy at the Naval Research Laboratory is supported by 6.1 Base funding. [1], as follows. P. E. Dewdney and R. T. Schilizzi are with the Physics and Astronomy Department, • Determine the large-scale properties of the University of Manchester, Manchester, U.K. (e-mail: [email protected]; [email protected]; [email protected]). universe: the amount, distribution, and nature of P. J. Hall is with the Electrical and Computer Engineering Department, its matter and energy, its age, and the history of its Curtin University, Perth, Western Australia, Australia (e-mail: [email protected]). T. J. L. W. Lazio is with the Naval Research Laboratory, Washington, D.C. 20375 USA expansion. (e-mail: [email protected]). • Study the dawn of the modern universe, when the Digital Object Identifier: 10.1109/JPROC.2009.2021005 first stars and galaxies formed. 1482 Proceedings of the IEEE |Vol.97,No.8,August2009 0018-9219/$25.00 Ó2009 IEEE Authorized licensed use limited to: The University of Manchester. Downloaded on July 28, 2009 at 11:40 from IEEE Xplore. Restrictions apply. Dewdney et al.: The Square Kilometre Array • Understand the formation and evolution of black The enormous capabilities of radio telescopes come at a holes of all sizes. price: radio emission in astrophysical situations is weak, • Study the formation of stars and their planetary and very sensitive telescopes are needed to carry out ob- systems, and the birth and evolution of giant and servations of the phenomena described above, particularly terrestrial planets. in the early Universe. To achieve the main science goals • Understand how the astronomical environment related to early Universe studies, the SKA will have to be affects Earth. much more sensitive than the current generation of radio Similar goals are identified in similar reports from other telescopes in the centimeter and meter wave bands. These countries and regions, such as the recent European goals are fully described in the SKA science case [2]. What AstroNet process. follows is a section on what new science is anticipated and Radio-wave observations address these goals in a way how that translates into technical requirements; a section that is both unique and complementary to that provided in onpotentialdesignsolutions;asectiononimportanttech- other wavebands. Observations and technical develop- nical and cost challenges that must be met; and a section ments in radio astronomy have led to several Nobel prizes on how the project may proceed to make technology and in physics. These successes were largely serendipitous but design decisions. At this stage of the project, there is no flowed from novel applications of technology; they are also direct route to a simple solution. Many different aspects attributable to the penetrating power of centimeter and must be considered in parallel and even iteratively. meter waves to reveal phenomena and objects in the Universe that would otherwise be hidden, and to Nature’s propensity to generate radio waves almost everywhere. II. DESIGN REQUIREMENTS AND Moreover, telescopes in these bands can be designed with MOTIVATION FOR NEW TECHNOLOGIES extraordinary capabilitiesVlarge imaging fields of view, The SKA will utilize radio-telescope design techniques polarization measurement, high spatial and spectral reso- developed over the past four to five decades. The most lutions, and high sensitivity. Using recent and emerging sophisticated concept, aperture synthesis [3], is an appli- technology, all of these attributes may be achieved simul- cation of the Van Cittert–Zernike theorem [3, Ch. 14] and taneously. Although the precise range of wavelengths has amounts to spatial, spectral, and temporal sampling of the not yet been determined, the SKA will eventually produce incoming radio-radiation field so as to match the expected images and other data over wavelengths from 4.3 m structure of the field in those three domains. In addition, (70 MHz) to 1 cm (30 GHz). careful attention must be paid to rejecting extraneous, Of exceptional scientific significance in radio astron- man-made signals [radio-frequency interference (RFI)]. omy are: the ability to utilize optically thin spectral lines as The manifestation of these concepts as a ground-based tracers of many non stellar components of the universe, radio telescope requires an array of antennas and receivers especially the fundamentally important 21-cm spectral line covering a large area on the ground and configured so as of hydrogen, the most abundant element; the ability to to provide the required spatial sampling or telescope probe physics in extreme environments, such as by high- resolution. The signals from the antennas, including precision timing of the arrival times of pulses from radio appropriately represented amplitude and phase, are cross- pulsars; the exploitation of astrophysical masers found in correlated in pairs and integrated to reduce noise. These the spectral lines of molecules; the sensing of magnetic data can be used to reconstruct the original brightness fields in the Universe; and the tracing of almost ubiquitous distribution, spectra at each point in the sky, and some- interaction of high-energy electrons with magnetic fields times spectral and spatial variations with time. to produce synchrotron radiation in stars, galaxies, and For several decades, radio telescopes and telescope ar- clusters of galaxies. rays have been limited to apertures of about 104 m2 (with In order to make progress