The Square Kilometre Array (SKA) Central Signal Processor (CSP) is a real-time backend system that processes incoming astronomical signals to produce visibilities and detects and profiles pulsars. The CSP is composed of the Local Monitoring and Control (LMC), the Correlator and Beam-Former (CBF), and the Pulsar Search and Timing (PSS, PST) engines. Each subsystem is developed by a different team in the SKA control software domain following the Scaled Agile Framework (SAFe) to guarantee coherence in the development. The definition of an engineering User Interface (UI) for the CSP is challenging due to the variety of skills that are required to identify the most relevant design concepts and potential roadblocks to an effective representation and the fact that several teams are involved. For this reason, we chose to leverage a collaborative design approach that can easily fit SKA’s biweekly sprint cadence while involving experts from different fields in a “think outside the box” process. Sketches and wireframes undergo multiple refinement sessions that lead to the realization of an engineering dashboard representing the current state of CSP implementation. User testing sessions constitute the means by which the success of the proposed UI is measured. Additional positive effects are alignment across different teams on the current capabilities of the system and its future development, as well as a way for continuously adapting the UI to the system’s evolution. In this paper, we describe the challenges we faced while coordinating the design across multiple teams, show how the process was implemented to fit the short agile iterations and overall SAFe framework and present the results of the work.
KEYWORDS: Optical correlators, Switches, Field programmable gate arrays, Commercial off the shelf technology, Phased arrays, Signal processing, Zoom lenses, Control systems, Antennas, Pulsars
The Square Kilometre Array Low is a next generation radio telescope, consisting of 512 antenna stations spread over 65 km, to be built in Western Australia. The correlator and beamformer (CBF) design is central to the telescope signal processing. CBF receives 6 Tera-bits-per-second (Tbps) of station data continuously and processes it in real time with a compute load of 2 Peta-operations-per-second (Pops). The correlator calculates up to 22 million cross products between all pairs of stations, whereas the beamformers (BFs) coherently sum station data to form more than 500 beams. The output of the correlator is up to 7 Tbps, and the BF 2 Tbps. The design philosophy, called “Atomic COTS,” is based on commercial off-the-shelf (COTS) hardware. Data routing is implemented in network switches programmed using the Programming Protocol-Independent Packet Processors (P4) language and the signal processing occurs in COTS field-programmable gate array (FPGA) cards. The P4 language allows routing to be determined from the metadata in the Ethernet packets from the stations. That is, metadata describing the contents of the packet determines the routing. Each FPGA card inputs a fraction of the overall bandwidth for all stations and then implements the processing needed to generate complete science data products. Generation of complete science products in a single FPGA is named here as Atomic processing. A Tango distributed control system configures the multitude of processing modes as well as maintaining the overall health of the CBF system hardware. The resulting 6 Tbps in and 9 Tbps out, 2 Pops Atomic COTS network attached accelerator occupies five racks and consumes 60 kW.
The Square Kilometre Array (SKA) organisation is building a low frequency (50-350 MHz) aperture array to be located in remote Western Australia. The array consists of 512-stations, each consisting of 256-dual polarisation log-periodic antennas. The stations are distributed over a distance of 80km, with the greatest density of stations located in the central core. The input bandwidth is processed in a two stage polyphase filterbank, with the first stage channeliser producing 384 x 781 kHz narrow-band channels. Each station beamforms the antennas together to form a single dual polarisation beam with a bandwidth of 300 MHz (additional beams can also be traded for bandwidth). The second stage polyphase filterbank is located in a system called the Correlator and BeamFormer (CBF) which is the topic of this paper. In the CBF the station signals are first aligned in time. Thereafter the signals are simultaneously correlated and beamformed.
Various unique concepts are studied for the Square Kilometer Array expected to be operational between 2010 and 2015. We compare telescopes in operation today with the SKA requirements and among these concepts, emphasize the active adaptive aperture antenna. This electronic concept for aperture synthesis has intrinsic capabilities that may widen avenues of new astronomical research with SKA even further and is topic of active R & D within NFRA. Within the program, technology demonstrators are built with increasing complexity. As desirable side effect of the ongoing studies for SKA, new albeit smaller telescopes are coming into operation earlier. Of these, a new generation low frequency array to be operational in 2005 may result with close ties to the exploration of new technologies in our R & D program together with firm scientific push from the U.S. and Holland. Experience gained with this telescope are relevant for SKA's mainstream development and in particular the electronic array. Results of these and other research aspects of the R & D program are presented.
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