The Square Kilometre Array (SKA)
The Square Kilometre Array (SKA) is a multi-purpose radio telescope, covering the frequency range from 50 MHz to 20 GHz that will play a major role in answering key questions in modern astrophysics and cosmology. It will be one of a small number of cornerstone observatories across the electromagnetic spectrum that will provide astrophysicists and cosmologists with a transformational view of the Universe.
The SKA will have collecting area approaching one million square metres and be capable of observing across a wide frequency range. Its construction is thus a major undertaking and will be implemented in phases to allow significant observations to be made before construction is completed. The international project has adopted the following terminology to describe this phased approach: Phase 1 is the initial deployment to begin in 2018 and integrating the MeerKAT and ASKAP precursor telescopes; Phase 2 will become fully operational in the mid-2020s, increase both the sensitivity and resolution by over an order of magnitude. For the latest information see the SKA Timeline on the SKA Telescope website.
The international community has developed a detailed and compelling science case for the SKA, as described in detail in New Astronomy Reviews, volume 48 (2004). The core of the science case is five Key Science Projects; each project represents an unanswered question in fundamental physics or astrophysics, is science either unique to the SKA or for which the SKA plays a key role, and is something which can excite the broader community.
The five Key Science Projects are:
- Probing the Dark Ages: investigating the formation of the first structures, as the Universe made the transition from largely neutral to its largely ionized state today.
- Galaxy Evolution, Cosmology and Dark Energy: probing the structure of the Universe and its fundamental constituent, galaxies, by carrying out all-sky surveys of continuum emission and of HI to a redshift z ~ 2. HI surveys can probe both cosmology (including dark energy) and the properties of galaxy assembly and evolution.
- The Origin and Evolution of Cosmic Magnetism: magnetic fields are an essential part of many astrophysical phenomena, but fundamental questions remain about their evolution, structure, and origin. The goal of this project is to trace magnetic field evolution and structure across cosmic time.
- Strong Field Tests of Gravity Using Pulsars and Black Holes: identifying a set of pulsars on which to conduct high precision timing measurements. The gravitational physics that can be extracted from these data can be used to probe the nature of space and time.
- The Cradle of Life: probing the full range of astrobiology, from the formation of prebiotic molecules in the interstellar medium to the emergence of technological civilisations on habitable planets.
The SKA has been conceived as a observational facility that will test fundamental physical laws and transform our current picture of the Universe. However, the scientific challenges outlined above are today's problems; will they still be the outstanding problems that will confront astronomers in the period 2020-2050 and beyond, when the SKA will be in its most productive years? Thus, the SKA community has adopted "Exploration of the Unknown" as a goal for the facility as part of a firmly founded expectation that the most exciting things to be discovered by the SKA are those that we have not yet conceived.
The SKA will be co-hosted at the South Africa’s Karoo region (as a core site) and Western Australia’s Murchison region (MRO). Different elements of the telescope will be deployed at the two different sites, with SKA-Low and SKA-Survey located at the MRO and SKA-Mid at the Karoo.
The SKA-Mid will comprise 15-m offset Gregorian dishes equipped with wide-band single pixel feeds and will be incorporated with the MeerKAT telescope, giving a total of 250 antennas. The array configuration will extend to a radius of 100km from a highly filled inner core of antennas. This implementation of the mid-band SKA represents a low risk approach to cover the 350 MHz to 15 GHz frequency range. The highly filled core is necessary for experiments such as the pulsar surveys, while the long baselines provide the angular resolution required for cosmology.
The SKA-Survey telescope will comprise the same dishes used in SKA-Mid, but equipped with Phased Array Feeds and incorporating the ASKAP telescope, giving a total of 90 antennas. Many of the main SKA science projects involve surveys of the sky made at frequencies below ~3 GHz. To implement these surveys within a reasonable time frame requires a high survey speed. By the use of a Phased Array Feed, each telescope is able to view a considerably greater area of sky than would be the case with a single feed system. This greatly reduces the time to undertake a survey and can provide a cost effective way of reaching the SKA design goals.
The SKA-Low telescope will comprise 250,000 individual element configured as an aperture array. In an aperture array a beam is formed and steered by combining all the received signals from a ‘station’ of 256 elements after appropriate time delays have been introduced to align the phases of the signals coming from a particular direction. By simultaneously using different sets of delays, this can be repeated many times to create many independent beams, yielding very large total Field of Views. The number of useful beams produced, or total Field of View, is essentially limited by signal processing, data communications and computing capacity. Aperture arrays can readily operate at low frequencies and can provide large effective areas. Arrays using substantial digital processing systems are inherently very flexible since the system can 'trade' Field of View and bandwidth and hence provide an instrument that can be matched to that required by the experiment.
The SKA project is now in the process of conducting the detailed design work for the telescope and various international consortia have been formed to carry out this technical work. The University of Manchester is a member of five of these consortia and in the case of SaDT is also the lead institute.
Signal and Data Transport Consortium (SaDT)
Signal and data transport is the backbone of the SKA telescope. The Signal and Data Transport (SADT) consortium is responsible for the design of three data transport networks. These are: a network to take astronomical data from the antennas to the correlator and then on to the High Performance Computer (HPC) facility; a network to distribute clock and synchronization signals from a central ensemble of very accurate clocks (probably hydrogen masers) out to each antenna; a network to allow telescope control and monitoring information to flow back and forth throughout the whole SKA system. Each of these has its own individual challenges and we will also be working to find an optimal solution for combination of the three networks taken as a whole.
The volume of telescope data to be transported is vast. Although the final data rate depends on features of the overall system still to be determined, current estimates put the data rate as approximately 80 Tb/s (terabits per second) from the antennas to the correlator (provided by CSP) and then another 80 Tb/s from the correlator to the HPC (provided by SDP). This is equivalent to 26,000 PB/month (petabytes per month); for comparison the World's total IP traffic in 2011 was 27,000 PB/month. In addition to the volume, we have to transport the data for considerable distances e.g. in Australia the correlator is approx 800km from the HPC centre in Perth.
The Synchronisation and Timing (SAT) provides frequency and clock signals from a central clock ensemble to all elements of the system to maintain phase information to the required accuracy for all receptors, and timing signals for data identification and time critical activities at the receptors, the CSP and SDP. To maintain phase coherence across the array requires short-term timing precisions of around 1pico-sec, while for the requirements for pulsar timing experiments require 10 nano-secs accuracy over 10 year periods.
The Telescope Manager network transmits and receives monitoring and control information throughout the system and itself comprised of three logical networks: Production Network, Engineering Network and Safety Network. Depending on detailed design there are likely to be a huge number of points requiring monitoring e.g. each of the 250,000 elements of the Low Frequency Aperture Array (LFAA) are currently planned to have 4 monitoring points giving 1,000,000 signals just for this element.
The University of Manchester is the lead organisation for the SaDT Consortium, with the group headed by Dr Keith Grainge. We have collaborators in AARNet (Australia), ASTRON (The Netherlands), CSIRO (Australia), DANTE (UK), IT (Portugal), JIVE (The Netherlands), NCRA-TIFR (India), NMMU (South Africa), the National Physical Labortary (UK), SANReN (South Africa), SKA Africa, Tata Consulting (India), Tsinghua University/ Peking University (China), University of Granada (Spain)
Central Signal Processing (CSP) and Science Data Processing (SDP)
Within the CSP and SDP consortia the University of Manchester leads the Non-Imaging Processing sub-tasks which are required for Pulsar searches and Pulsar timing experiments. This work is led by Dr Ben Stappers. The computational load for this task is daunting since a huge parameter space must be searched over in order to find pulsar candidates in the data stream.
Low Frequency Aperture Array (LFAA) and Mid Frequency Aperture Array (MFAA)
Within the LFAA and MFAA consortia the University of Manchester is leading work on a novel type of receiver element, an Octagonal Ring Antenna (ORA). This work is led by Prof Tony Brown in EEE. This technology is a planar array of antennas which can be easily fabricated at low cost while maintaining excellent performance.
The ORA's properties include:
- A simple planer structure based on low cost manufacturing techniques
- Low and stable cross polarization over broad bandwidth and wide scan angle
- Wide scan angle with a stable scan pattern
- Feed connection flexibility, can be fed with a differential and single-ended LNA
- Wide Band Array Antenna GB2469075A and international patent WO2010/112857A1
The Office for the SKA Organisation is responsible for coordinating the global activities of the SKA project. This includes engineering, science, site evaluation, operations and public outreach. It is located at the Jodrell Bank Observatory, hosted by the University of Manchester. The history runs from the first discussions in 1993, to the establishment of the project Office at the Jodrell Bank Centre for Astrophysics in 2008, and the SKA organisation in 2011.