The Square Kilometre Array (SKA)

The international community has developed a detailed and compelling science case for the SKA, as described in detail in the book "Advancing Astrophysics with the Square Kilometre Array", published in 2015. The JBCA group has been a major contributor to it, with 45 of the 135 papers having Manchester authors and 12 of those having a lead author from the JBCA. You can download a copy of the book from the SKA Telescope website.

The core of the science case comprises eight key topics; each topic represents an unanswered question in fundamental physics or astrophysics, and the SKA has the potential to make transformational discoveries in these fields

The eight topics are as follows:

• Epoch of Reionization and 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.
• Fundamental Physics with Pulsars: Identifying a set of pulsars on which to conduct high precision timing measurements. The gravitational physics that can be extracted from this data can be used to probe the nature of space and time.
• HI and Galaxy Evolution: Probing the properties of galaxies, their assembly and their evolution by carrying out all-sky surveys of continuum emission and of HI to a redshift z ~ 2.
• Cosmology: The SKA will deliver precision cosmology and probe the foundations of the standard model, including dark energy, and open the door to new discoveries on large-scale features of the Universe.
• 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.
• Astrobiology/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.
• Continuum Surveys: The SKA’s angular resolution and survey speed capability will mean that continuum surveys will be more detailed than ever before. These surveys will allow new studies into galaxy and galaxy cluster physics.
• Radio Transients: The SKA will be able to survey the sky at a rate faster than any survey telescope that has ever existed and so will be able to discover transient phenomena with time scales from below even one nanosecond through to several years. These will include Gamma Ray Burst, supernovae and Fast Radio Bursts but will also allow an "Exploration of the Unknown".

The SKA has been conceived as a general purpose observatory that will test fundamental physical laws and transform our current picture of the Universe, with a design lifetime of 50 years. The scientific themes outlined above are today's problems, but the history of radio astronomy suggests that new challenges arising from new discoveries will quickly arise and will require the SKA’s capabilities to explore thoroughly.

The SKA’s receptor arrays will be co-hosted in South Africa's Karoo region and Western Australia's Murchison region, with science processing facilities in Cape Town and Perth respectively.

SKA-Mid

SKA-Mid will consist of 133 15-m offset Gregorian dishes and 64 MeerKAT dishes equipped with multiple receivers that  span the frequency band 350MHz to 15GHz. The array configuration will extend to a radius of 100km providing long interferometric baselines from a high density inner core of dishes. The high density core is necessary for experiments such as pulsar surveys, while the long baselines provide the angular resolution required for cosmology.

SKA-Low

The SKA-Low telescope receptor array will comprise 131,072 individual antennas 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 log-periodic 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 View (FoV).

The number of useful beams produced, or total FoV, 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' FoV and frequency bandwidth and hence provide an instrument that can be matched to that required by the experiment.

In 2013 the detailed design work for the telescope was awarded by SKAO to various international consortia, formed to carry out this technical work. The University of Manchester was a member of five of these consortia and in the case of the Signal and Data Transport (SaDT) consortium was also the lead institute.

Signal and data transport is the backbone of the SKA telescope. The SADT consortium was responsible for the design of three data transport networks, each of which had its own individual challenges. 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  out to each of the 197 MID dished and to 36 LOW remote processing facilities;
• a non-science data network (NSDN) to allow distribution of the monitoring, control and auxiliary networking data throughout the whole SKA system

The volume of telescope data to be transported is vast. Across the two telescopes, a total of around 30 terabits per second of data will be transported from the array sites to processing facilities in Perth and Cape Town for the SKA-Low and SKA-Mid telescopes respectively.  This is 1 million times faster than a typical 2021 home broadband connection of 30Mbps. In addition to the volume, the data has to be transported over considerable distances eg in Australia the receptor array is approx 800km from the science processing facility  in Perth. Here high-performance supercomputers, with processing speeds of around 135 petaFLOPS, will produce the science data that will be sent to the SKA Regional Centres for astronomers around the world to analyse. A phenomenal 700 Petabytes of data will be sent each year.

The Synchronisation and Timing (SAT) Element creates  reference frequency and clock signals from a central clock ensemble and distributes them 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 correlator beamformer  and science data processing. To maintain phase coherence across the MID 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 non-science data network (NSDN) consists of an access layer, distribution layer and a core switching network, which has a footprint across the entire observatory site. It is primarily responsible for transporting monitoring and control information between the Telescope Manager and the monitoring and control interfaces of all sub-systems, but it also transports general data communication services such as Internet connectivity, desk top, voice and building management services. It is designed to have high availability, good security and connect all the SKA trusted offices, including locations off-site.

The University of Manchester was the lead organisation for the SaDT Consortium, with the group headed by Prof Keith Grainge. The other collaborators were: AARNet (Australia), CSIRO (Australia),GÉANT (EU), IT (Portugal), JIVE (The Netherlands), NCRA-TIFR (India), NMMU (South Africa), the National Physical Laboratory (UK), SANReN (South Africa), SKA SA (South Africa), Tata Consulting (India), Tsinghua University/ Peking University (China), University of Granada (Spain) and the University of Western Australia (Australia).

Within the Central Signal Processing (CSP) and Science Data Processing (SDP) consortia The University of Manchester led the Non-Imagining Processing (NIP) sub-tasks which are required for Pulsar searches and Pulsar timing experiments. This work was led by Prof Benjamin 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.

The SKA Observatory is an Inter-Governmental Organisation that was established by an international treaty signed on 12th March 2019 in Rome. Its Headquarters is located at the Jodrell Bank Observatory, hosted by The University of Manchester. Its history includes the first SKA concept discussions in 1993, to the establishment of the Project Office at the Jodrell Bank Centre for Astrophysics in 2008, and the formation of the SKA organisation in 2011.

The SKA design phase culminated in a series of element-level and system-level Critical Design Reviews, the designs of which were successfully approved at the end of 2019. The focus then changed to construction, with the first contracts to be issued in the second half of 2021. The experience built up by the UoM teams during the design phase places us well to continue to contribute during construction in the following areas, to be confirmed:

• Pulsar Search. The UoM experts who co-designed the Pulsar Search engine for the SKA will form one of two Scaled Agile Framework (SAFe) development teams, along with one team from the University of Oxford, who will produce the software and firmware to run on the heterogeneous CPU, FPGA and GPU real-time hardware.
• MCCS and SAT.LMC. The UoM will provide the Scrum Master and Product Owner for the SAFe team writing the software for the Monitoring, Control and Calibration System (MCCS) that drives the SKA-Low telescope array and the SAT monitoring and control software
• SAT. The experience built up by the UoM team who oversaw the design of the SKA Synchronisation and Timing (SAT) system will be retained to provide scientific, technology, engineering and management advisory services on SAT construction issues to the SKAO.
• Networks. The UoM is supporting the SKAO with ongoing network design including changes to the system level design and considering the implementation of new technologies such as coherent optical transceivers. The UoM will be well placed to advise the SKAO in networks procurement and the management of associated construction contracts
• Outreach. The vision of the Outreach team is of a wider society that is both aware of, and values the importance of, the UK’s involvement in the SKA project and its future benefit to both our scientific knowledge and the UK economy.

The two SKA telescopes will produce around 700 Pbytes of Observatory Data Products (ODPs) per year. These data will need to be distributed out from the South African and Australian sites to SKA Regional Centres (SRCs) located in the member countries. Here further analysis of the data will be performed, resulting in Advanced Data Products (ADPs), which will then be interpreted by astronomers.

Planning for the SRCs is now underway and UoM will contribute to the design of the data transport, the archiving, the analysis pipelines and the user support necessary to provide astronomers access to SKA data during both the commissioning and the full operations phase.