Astrophysical radio transients are excellent probes of extreme physical processes originating fro... more Astrophysical radio transients are excellent probes of extreme physical processes originating from compact sources within our Galaxy and beyond. Radio frequency signals emitted from these objects provide a means to study the intervening medium through which they travel. Next generation radio telescopes are designed to explore the vast unexplored parameter space of high time resolution astronomy, but require High Performance Computing (HPC) solutions to process the enormous volumes of data that are produced by these telescopes. We have developed a combined software/hardware solution (code named ARTEMIS) for real-time searches for millisecond radio transients, which uses GPU technology to remove interstellar dispersion and detect millisecond radio bursts from astronomical sources in real-time. Here we present an introduction to ARTEMIS. We give a brief overview of the software pipeline, then focus specifically on the intricacies of performing incoherent de-dispersion. We present results from two brute-force algorithms. The first is a GPU based algorithm, designed to exploit the L1 cache of the NVIDIA Fermi GPU. Our second algorithm is CPU based and exploits the new AVX units in Intel Sandy Bridge CPUs.
Monthly Notices of the Royal Astronomical Society, Jul 14, 2015
Fast radio bursts (FRBs) are millisecond radio signals that exhibit dispersion larger than what t... more Fast radio bursts (FRBs) are millisecond radio signals that exhibit dispersion larger than what the Galactic electron density can account for. We have conducted a 1446 h survey for FRBs at 145 MHz, covering a total of 4193 deg 2 on the sky. We used the UK station of the low frequency array (LOFAR) radio telescope-the Rawlings Array-accompanied for a majority of the time by the LOFAR station at Nançay, observing the same fields at the same frequency. Our real-time search backend, Advanced Radio Transient Event Monitor and Identification System-ARTEMIS, utilizes graphics processing units to search for pulses with dispersion measures up to 320 cm −3 pc. Previous derived FRB rates from surveys around 1.4 GHz, and favoured FRB interpretations, motivated this survey, despite all previous detections occurring at higher dispersion measures. We detected no new FRBs above a signal-to-noise threshold of 10, leading to the most stringent upper limit yet on the FRB event rate at these frequencies: 29 sky −1 d −1 for five ms-duration pulses above 62 Jy. The non-detection could be due to scatter-broadening, limitations on the volume and time searched, or the shape of FRB flux density spectra. Assuming the latter and that FRBs are standard candles, the non-detection is compatible with the published FRB sky rate, if their spectra follow a power law with frequency (∝ ν α), with α +0.1, demonstrating a marked difference from pulsar spectra. Our results suggest that surveys at higher frequencies, including the low frequency component of the Square Kilometre Array, will have better chances to detect, estimate rates and understand the origin and properties of FRBs.
In this paper we show how phase errors between different analogue signal channels of a phased arr... more In this paper we show how phase errors between different analogue signal channels of a phased array affects the quality of the output phased array beam. Specifically we look at the reduction in forward gain of the array beam and the array beam pointing offset as a function of phase errors and array size. We use a combination of simulations made using the aperture array simulator OSKAR, developed at the University of Oxford, in conjunction with measured results taken from 2-PAD; a functioning, astronomical, dual-polarisation, digital beamforming, 4 × 4 element, aperture array prototype (Greenwood 2007) for the Square Kilometre Array (Taylor 2007) developed by a consortium of UK universities. We show that phase errors between different signal channels is specifically an important issue for broadband arrays like 2-PAD, and comment on the relative benefit of digital beamforming versus analogue beamforming engines.
Simulations of SKA1-low were performed to estimate the noise level in images produced by the tele... more Simulations of SKA1-low were performed to estimate the noise level in images produced by the telescope over a frequency range 50-600 MHz, which extends the 50-350 MHz range of the current baseline design. The root-mean-square (RMS) deviation between images produced by an ideal, error-free SKA1-low and those produced by SKA1-low with varying levels of uncorrelated gain and phase errors was simulated. The residual in-field and sidelobe noise levels were assessed. It was found that the RMS deviations decreased as the frequency increased. The residual sidelobe noise decreased by a factor of ∼ 5 from 50 to 100 MHz, and continued to decrease at higher frequencies, attributable to wider strong sidelobes and brighter sources at lower frequencies. The thermal noise limit is found to range between ∼ 10 − 0.3 µJy and is reached after ∼ 100 − 100 000 hrs integration, depending on observation frequency, with the shortest integration time required at ∼ 100 MHz. I. INTRODUCTION The SKA low frequency aperture array, SKA1-low, is proposed to cover the frequency range 50-350 MHz. At the lower end of the frequency band, images are created with relatively
Building the World's Largest Radio Telescope: The Square Kilometre Array Science Data Processor
The Square Kilometre Array (SKA) will be the largest radio telescope constructed to date and the ... more The Square Kilometre Array (SKA) will be the largest radio telescope constructed to date and the largest Big Data project in the known Universe. The first phase of the project will generate 160 terabytes every second. This amounts to 5 zettabytes (5 million petabytes) of data that will be generated by the facility each year - a data rate equivalent to 5 times the estimated global internet traffic in 2015. These data need to be reduced and then continuously ingested by the SKA Science Data Processor (SDP). Within the SDP Consortium, we are contributing to various roles in the development of the telescope including building a lightweight end-to-end prototype of the major components of the SDP system - a project we call the SDP Integration Prototype (SIP). The aim is to build a mini, fully-operational SDP, for which we have been developing realistic SKA-like science pipelines that can handle these unprecedented data volumes.
Monthly Notices of the Royal Astronomical Society, Nov 3, 2016
The Square Kilometre Array's Low Frequency instrument (SKA-LOW) will be the most sensitive apertu... more The Square Kilometre Array's Low Frequency instrument (SKA-LOW) will be the most sensitive aperture array ever used for radio astronomy, and will operate in the under-sampled regime for most of the frequency band where grating-lobes pose particular challenges. To achieve the expected level of sensitivity for SKA-LOW, it is particularly important to understand how interfering sources in both near and far side-lobes of the station beam affect the imaging performance. We discuss options for station designs, and adopting a random element layout, we assess its effectiveness by investigating how sources far from the main lobe of the station beam degrade images of the target field. These sources have the effect of introducing a noise-like corruption to images, which we call the Far Side-lobe Source Noise (FSSN). Using OSKAR, a GPU-accelerated software simulator, we carried out end-to-end simulations using an all-sky model and telescope configuration representative of the SKA-LOW instrument. The FSSN is a function of both the station beam and the interferometric point spread function, and decreases with increasing observation time until the coverage of the aperture plane no longer improves. Using apodisation to reduce the level of near-in side-lobes of the station beam had a noticeable improvement on the level of FSSN at low frequencies. Our results indicate that the effects of picking up sources in the side-lobes are worse at low frequencies, where the array is less sparse.
The removal of the Galactic and extragalactic foregrounds remains a major challenge for those wis... more The removal of the Galactic and extragalactic foregrounds remains a major challenge for those wishing to make a detection of the Epoch of Reionization 21-cm signal. Multiple methods of modelling these foregrounds with varying levels of assumption have been trialled and shown promising recoveries on simulated data. Recently however there has been increased discussion of using the expected shape of the foregrounds in Fourier space to define an EoR window free of foreground contamination. By carrying out analysis within this window only, one can avoid the foregrounds and any statistical bias they might introduce by instead removing these foregrounds. In this paper we discuss the advantages and disadvantages of both foreground removal and foreground avoidance. We create a series of simulations with noise levels in line with both current and future experiments and compare the recovered statistical cosmological signal from foreground avoidance and a simplified, frequency independent foreground removal model. We find that while, for current generation experiments, foreground avoidance enables a better recovery at k perp > 0.6Mpc −1 , foreground removal is able to recover significantly more signal at small k los for both current and future experiments. We also relax the assumption that the foregrounds are smooth by introducing a Gaussian random factor along the line-of-sight and then also spatially. We find that both methods perform well for foreground models with line-of-sight and spatial variations around 0.1% however at levels larger than this foreground removal shows a greater signal recovery.
Practical aperture synthesis imaging algorithms work by iterating between estimating the sky brig... more Practical aperture synthesis imaging algorithms work by iterating between estimating the sky brightness distribution and a comparison of a prediction based on this estimate with the measured data ("visibilities"). Accuracy in the latter step is crucial but is made difficult by irregular and non-planar sampling of data by the telescope. In this work we present a GPU implementation of 3d de-gridding which accurately deals with these two difficulties and is designed for distributed operation. We address the load balancing issues caused by large variation in visibilities that need to be computed. Using CUDA and NVidia GPUs we measure performance up to 1.2 billion visibilities per second.
All the 512 station layouts can be found in the Antenna engineering data pack / Array folder atta... more All the 512 station layouts can be found in the Antenna engineering data pack / Array folder attached to this document set.
The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and th... more The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of five zettabytes of data per year. A critical task for the SKA will be its ability to process data for science, which will need to be conducted by science pipelines. Together with polarization data from the LOFAR Multifrequency Snapshot Sky Survey (MSSS), we have been developing a realistic SKA-like science pipeline that can handle the large data volumes generated by LOFAR at 150 MHz. The pipeline uses task-based parallelism to image, detect sources and perform Faraday tomography across the entire LOFAR sky. The project thereby provides a unique opportunity to contribute to the technological development of the SKA telescope, while simultaneously enabling cutting-edge scientific results. In this paper, we provide an update on current efforts to develop a science pipeline that can enable tight constraints on the magnetised large-scale structure of the Universe.
Practical aperture synthesis imaging algorithms work by iterating between estimating the sky brig... more Practical aperture synthesis imaging algorithms work by iterating between estimating the sky brightness distribution and a comparison of a prediction based on this estimate with the measured data ("visibilities"). Accuracy in the latter step is crucial but is made difficult by irregular and non-planar sampling of data by the telescope. In this work we present a GPU implementation of 3d de-gridding which accurately deals with these two difficulties and is designed for distributed operation. We address the load balancing issues caused by large variation in visibilities that need to be computed. Using CUDA and NVidia GPUs we measure performance up to 1.2 billion visibilities per second.
arXiv: Instrumentation and Methods for Astrophysics, 2020
This report was submitted as part of the SKA Low Frequency Aperture Array Critical Design Review ... more This report was submitted as part of the SKA Low Frequency Aperture Array Critical Design Review describing the design of the SKA1-LOW station that took place between 2013 and 2018. The SKA1 LOW field station is inscribed in a circular area having an effective station diameter (centre to centre) of 38 meters and has 256 SKALA4 elements. This document describes the electromagnetic design of the field station. In particular it describes the layout design and the electromagnetic modelling and characteristics of the station. This document describes the effects associated with the layout and array such as mutual coupling effects, side lobe pattern and beam shape (eg. smoothness, calibration models) and presents the state of the art of our ability to measure the array performance and validate the simulation work. The current LFAA field node requirements, derived from the SKA L1 requirements, have evolved over the last years since the LFAA PDR and the System Baseline Design. The SKA1 LOW f...
The design stage of the first phase of the Square Kilometre Array (SKA) telescopes has been recen... more The design stage of the first phase of the Square Kilometre Array (SKA) telescopes has been recently completed by the SKA Organisation (SKAO). This marks the initial step towards an ambitious vision of designing, constructing, and operating telescopes with an equivalent collecting area of one square kilometre. At the completion of this design stage the project has entered a bridging period to manage work until construction funds are available. During this period many of the software teams from the design stage have been engaged in prototyping lean-agile processes, structures, and practices. By the end of the period the goal is to have pivoted from a document based, stage-gated set of processes arranged around design consortia to a code based, value-flow driven, lean-agile set of processes unified around the Scaled Agile Framework. Two years since the start of this transformation this paper reflects on these processes. We highlight some practices that have been found helpful, as well...
Proceedings of Wide Field Astronomy & Technology for the Square Kilometre Array — PoS(SKADS 2009), 2011
In this paper we show how phase errors between different analogue signal channels of a phased arr... more In this paper we show how phase errors between different analogue signal channels of a phased array affects the quality of the output phased array beam. Specifically we look at the reduction in forward gain of the array beam and the array beam pointing offset as a function of phase errors and array size. We use a combination of simulations made using the aperture array simulator OSKAR, developed at the University of Oxford, in conjunction with measured results taken from 2-PAD; a functioning, astronomical, dual-polarisation, digital beamforming, 4 × 4 element, aperture array prototype (Greenwood 2007) for the Square Kilometre Array (Taylor 2007) developed by a consortium of UK universities. We show that phase errors between different signal channels is specifically an important issue for broadband arrays like 2-PAD, and comment on the relative benefit of digital beamforming versus analogue beamforming engines.
2018 IEEE 14th International Conference on e-Science (e-Science), 2018
The Square Kilometre Array (SKA) will be the largest radio telescope constructed to date and the ... more The Square Kilometre Array (SKA) will be the largest radio telescope constructed to date and the largest Big Data project in the known Universe. The first phase of the project will generate 160 terabytes every second. This amounts to 5 zettabytes (5 million petabytes) of data that will be generated by the facility each year-a data rate equivalent to 5 times the estimated global internet traffic in 2015. These data need to be reduced and then continuously ingested by the SKA Science Data Processor (SDP). Within the SDP Consortium, we are contributing to various roles in the development of the telescope including building a lightweight end-to-end prototype of the major components of the SDP system-a project we call the SDP Integration Prototype (SIP). The aim is to build a mini, fully-operational SDP, for which we have been developing realistic SKA-like science pipelines that can handle these unprecedented data volumes.
Proceedings of Advancing Astrophysics with the Square Kilometre Array — PoS(AASKA14), 2015
The exceptional sensitivity of the SKA will allow observations of the Cosmic Dawn and Epoch of Re... more The exceptional sensitivity of the SKA will allow observations of the Cosmic Dawn and Epoch of Reionization (CD/EoR) in unprecedented detail, both spectrally and spatially. This wealth of information is buried under Galactic and extragalactic foregrounds, which must be removed accurately and precisely in order to reveal the cosmological signal. This problem has been addressed already for the previous generation of radio telescopes, but the application to SKA is different in many aspects. We summarise the contributions to the field of foreground removal in the context of high redshift and high sensitivity 21-cm measurements. We use a state-of-the-art simulation of the SKA Phase 1 observations complete with cosmological signal, foregrounds and frequencydependent instrumental effects to test both parametric and non-parametric foreground removal methods. We compare the recovered cosmological signal using several different statistics and explore one of the most exciting possibilities with the SKA-imaging of the ionized bubbles. We find that with current methods it is possible to remove the foregrounds with great accuracy and to get impressive power spectra and images of the cosmological signal. The frequency-dependent PSF of the instrument complicates this recovery, so we resort to splitting the observation bandwidth into smaller segments, each of a common resolution. If the foregrounds are allowed a random variation from the smooth power law along the line of sight, methods exploiting the smoothness of foregrounds or a parametrization of their behaviour are challenged much more than non-parametric ones. However, we show that correction techniques can be implemented to restore the performances of parametric approaches, as long as the first-order approximation of a power law stands.
Monthly Notices of the Royal Astronomical Society, 2016
The Square Kilometre Array's Low Frequency instrument (SKA-LOW) will operate in the undersampled ... more The Square Kilometre Array's Low Frequency instrument (SKA-LOW) will operate in the undersampled regime for most of the frequency band where grating lobes pose particular challenges. To achieve the expected level of sensitivity for SKA-LOW, it is particularly important to understand how interfering sources in both near and far side-lobes of the station beam affect the imaging performance. In this study, we discuss options for station designs, and adopting a random element layout, we assess its effectiveness by investigating how sources far from the main lobe of the station beam degrade images of the target field. These sources have the effect of introducing a noise-like corruption to images, which is called the far sidelobe confusion noise (FSCN). Using OSKAR, a software simulator accelerated using graphics processing units, we carried out end-to-end simulations using an all-sky model and telescope configuration representative of the SKA-LOW instrument. The FSCN is a function of both the station beam and the interferometric point spread function, and decreases with increasing observation time until the coverage of the aperture plane no longer improves. Using apodization to reduce the level of near-in sidelobes of the station beam had a notable improvement on the level of the FSCN at low frequencies. Our results indicate that the effects of picking up sources in the sidelobes are worse at low frequencies, where the array is less sparse.
Monthly Notices of the Royal Astronomical Society, 2016
The removal of the Galactic and extragalactic foregrounds remains a major challenge for those wis... more The removal of the Galactic and extragalactic foregrounds remains a major challenge for those wishing to make a detection of the Epoch of Reionization (EoR) 21 cm signal. Multiple methods of modelling these foregrounds with varying levels of assumption have been trialled and shown promising recoveries on simulated data. Recently however there has been increased discussion of using the expected shape of the foregrounds in Fourier space to define an EoR window free of foreground contamination. By carrying out analysis within this window only, one can avoid the foregrounds and any statistical bias they might introduce by instead removing these foregrounds. In this paper, we discuss the advantages and disadvantages of both foreground removal and foreground avoidance. We create a series of simulations with noise levels in line with both current and future experiments and compare the recovered statistical cosmological signal from foreground avoidance and a simplified, frequency independent foreground removal model. We find that for current generation experiments, while foreground avoidance enables a better recovery at k perp > 0.6 Mpc −1 , foreground removal is able to recover significantly more signal at small k los for both current and future experiments. We also relax the assumption that the foregrounds are smooth. For line-of-sight variations only, foreground removal is able to make a good signal recovery even at 1 per cent while foreground avoidance is compromised significantly. We find that both methods perform well for foreground models with line-of-sight and spatial variations around 0.1 per cent however at levels larger than this both methods are compromised.
Monthly Notices of the Royal Astronomical Society, 2015
Fast radio bursts (FRBs) are millisecond radio signals that exhibit dispersion larger than what t... more Fast radio bursts (FRBs) are millisecond radio signals that exhibit dispersion larger than what the Galactic electron density can account for. We have conducted a 1446 h survey for FRBs at 145 MHz, covering a total of 4193 deg 2 on the sky. We used the UK station of the low frequency array (LOFAR) radio telescope-the Rawlings Array-accompanied for a majority of the time by the LOFAR station at Nançay, observing the same fields at the same frequency. Our real-time search backend, Advanced Radio Transient Event Monitor and Identification System-ARTEMIS, utilizes graphics processing units to search for pulses with dispersion measures up to 320 cm −3 pc. Previous derived FRB rates from surveys around 1.4 GHz, and favoured FRB interpretations, motivated this survey, despite all previous detections occurring at higher dispersion measures. We detected no new FRBs above a signal-to-noise threshold of 10, leading to the most stringent upper limit yet on the FRB event rate at these frequencies: 29 sky −1 d −1 for five ms-duration pulses above 62 Jy. The non-detection could be due to scatter-broadening, limitations on the volume and time searched, or the shape of FRB flux density spectra. Assuming the latter and that FRBs are standard candles, the non-detection is compatible with the published FRB sky rate, if their spectra follow a power law with frequency (∝ ν α), with α +0.1, demonstrating a marked difference from pulsar spectra. Our results suggest that surveys at higher frequencies, including the low frequency component of the Square Kilometre Array, will have better chances to detect, estimate rates and understand the origin and properties of FRBs.
Astrophysical radio transients are excellent probes of extreme physical processes originating fro... more Astrophysical radio transients are excellent probes of extreme physical processes originating from compact sources within our Galaxy and beyond. Radio frequency signals emitted from these objects provide a means to study the intervening medium through which they travel. Next generation radio telescopes are designed to explore the vast unexplored parameter space of high time resolution astronomy, but require High Performance Computing (HPC) solutions to process the enormous volumes of data that are produced by these telescopes. We have developed a combined software/hardware solution (code named ARTEMIS) for real-time searches for millisecond radio transients, which uses GPU technology to remove interstellar dispersion and detect millisecond radio bursts from astronomical sources in real-time. Here we present an introduction to ARTEMIS. We give a brief overview of the software pipeline, then focus specifically on the intricacies of performing incoherent de-dispersion. We present results from two brute-force algorithms. The first is a GPU based algorithm, designed to exploit the L1 cache of the NVIDIA Fermi GPU. Our second algorithm is CPU based and exploits the new AVX units in Intel Sandy Bridge CPUs.
Monthly Notices of the Royal Astronomical Society, Jul 14, 2015
Fast radio bursts (FRBs) are millisecond radio signals that exhibit dispersion larger than what t... more Fast radio bursts (FRBs) are millisecond radio signals that exhibit dispersion larger than what the Galactic electron density can account for. We have conducted a 1446 h survey for FRBs at 145 MHz, covering a total of 4193 deg 2 on the sky. We used the UK station of the low frequency array (LOFAR) radio telescope-the Rawlings Array-accompanied for a majority of the time by the LOFAR station at Nançay, observing the same fields at the same frequency. Our real-time search backend, Advanced Radio Transient Event Monitor and Identification System-ARTEMIS, utilizes graphics processing units to search for pulses with dispersion measures up to 320 cm −3 pc. Previous derived FRB rates from surveys around 1.4 GHz, and favoured FRB interpretations, motivated this survey, despite all previous detections occurring at higher dispersion measures. We detected no new FRBs above a signal-to-noise threshold of 10, leading to the most stringent upper limit yet on the FRB event rate at these frequencies: 29 sky −1 d −1 for five ms-duration pulses above 62 Jy. The non-detection could be due to scatter-broadening, limitations on the volume and time searched, or the shape of FRB flux density spectra. Assuming the latter and that FRBs are standard candles, the non-detection is compatible with the published FRB sky rate, if their spectra follow a power law with frequency (∝ ν α), with α +0.1, demonstrating a marked difference from pulsar spectra. Our results suggest that surveys at higher frequencies, including the low frequency component of the Square Kilometre Array, will have better chances to detect, estimate rates and understand the origin and properties of FRBs.
In this paper we show how phase errors between different analogue signal channels of a phased arr... more In this paper we show how phase errors between different analogue signal channels of a phased array affects the quality of the output phased array beam. Specifically we look at the reduction in forward gain of the array beam and the array beam pointing offset as a function of phase errors and array size. We use a combination of simulations made using the aperture array simulator OSKAR, developed at the University of Oxford, in conjunction with measured results taken from 2-PAD; a functioning, astronomical, dual-polarisation, digital beamforming, 4 × 4 element, aperture array prototype (Greenwood 2007) for the Square Kilometre Array (Taylor 2007) developed by a consortium of UK universities. We show that phase errors between different signal channels is specifically an important issue for broadband arrays like 2-PAD, and comment on the relative benefit of digital beamforming versus analogue beamforming engines.
Simulations of SKA1-low were performed to estimate the noise level in images produced by the tele... more Simulations of SKA1-low were performed to estimate the noise level in images produced by the telescope over a frequency range 50-600 MHz, which extends the 50-350 MHz range of the current baseline design. The root-mean-square (RMS) deviation between images produced by an ideal, error-free SKA1-low and those produced by SKA1-low with varying levels of uncorrelated gain and phase errors was simulated. The residual in-field and sidelobe noise levels were assessed. It was found that the RMS deviations decreased as the frequency increased. The residual sidelobe noise decreased by a factor of ∼ 5 from 50 to 100 MHz, and continued to decrease at higher frequencies, attributable to wider strong sidelobes and brighter sources at lower frequencies. The thermal noise limit is found to range between ∼ 10 − 0.3 µJy and is reached after ∼ 100 − 100 000 hrs integration, depending on observation frequency, with the shortest integration time required at ∼ 100 MHz. I. INTRODUCTION The SKA low frequency aperture array, SKA1-low, is proposed to cover the frequency range 50-350 MHz. At the lower end of the frequency band, images are created with relatively
Building the World's Largest Radio Telescope: The Square Kilometre Array Science Data Processor
The Square Kilometre Array (SKA) will be the largest radio telescope constructed to date and the ... more The Square Kilometre Array (SKA) will be the largest radio telescope constructed to date and the largest Big Data project in the known Universe. The first phase of the project will generate 160 terabytes every second. This amounts to 5 zettabytes (5 million petabytes) of data that will be generated by the facility each year - a data rate equivalent to 5 times the estimated global internet traffic in 2015. These data need to be reduced and then continuously ingested by the SKA Science Data Processor (SDP). Within the SDP Consortium, we are contributing to various roles in the development of the telescope including building a lightweight end-to-end prototype of the major components of the SDP system - a project we call the SDP Integration Prototype (SIP). The aim is to build a mini, fully-operational SDP, for which we have been developing realistic SKA-like science pipelines that can handle these unprecedented data volumes.
Monthly Notices of the Royal Astronomical Society, Nov 3, 2016
The Square Kilometre Array's Low Frequency instrument (SKA-LOW) will be the most sensitive apertu... more The Square Kilometre Array's Low Frequency instrument (SKA-LOW) will be the most sensitive aperture array ever used for radio astronomy, and will operate in the under-sampled regime for most of the frequency band where grating-lobes pose particular challenges. To achieve the expected level of sensitivity for SKA-LOW, it is particularly important to understand how interfering sources in both near and far side-lobes of the station beam affect the imaging performance. We discuss options for station designs, and adopting a random element layout, we assess its effectiveness by investigating how sources far from the main lobe of the station beam degrade images of the target field. These sources have the effect of introducing a noise-like corruption to images, which we call the Far Side-lobe Source Noise (FSSN). Using OSKAR, a GPU-accelerated software simulator, we carried out end-to-end simulations using an all-sky model and telescope configuration representative of the SKA-LOW instrument. The FSSN is a function of both the station beam and the interferometric point spread function, and decreases with increasing observation time until the coverage of the aperture plane no longer improves. Using apodisation to reduce the level of near-in side-lobes of the station beam had a noticeable improvement on the level of FSSN at low frequencies. Our results indicate that the effects of picking up sources in the side-lobes are worse at low frequencies, where the array is less sparse.
The removal of the Galactic and extragalactic foregrounds remains a major challenge for those wis... more The removal of the Galactic and extragalactic foregrounds remains a major challenge for those wishing to make a detection of the Epoch of Reionization 21-cm signal. Multiple methods of modelling these foregrounds with varying levels of assumption have been trialled and shown promising recoveries on simulated data. Recently however there has been increased discussion of using the expected shape of the foregrounds in Fourier space to define an EoR window free of foreground contamination. By carrying out analysis within this window only, one can avoid the foregrounds and any statistical bias they might introduce by instead removing these foregrounds. In this paper we discuss the advantages and disadvantages of both foreground removal and foreground avoidance. We create a series of simulations with noise levels in line with both current and future experiments and compare the recovered statistical cosmological signal from foreground avoidance and a simplified, frequency independent foreground removal model. We find that while, for current generation experiments, foreground avoidance enables a better recovery at k perp > 0.6Mpc −1 , foreground removal is able to recover significantly more signal at small k los for both current and future experiments. We also relax the assumption that the foregrounds are smooth by introducing a Gaussian random factor along the line-of-sight and then also spatially. We find that both methods perform well for foreground models with line-of-sight and spatial variations around 0.1% however at levels larger than this foreground removal shows a greater signal recovery.
Practical aperture synthesis imaging algorithms work by iterating between estimating the sky brig... more Practical aperture synthesis imaging algorithms work by iterating between estimating the sky brightness distribution and a comparison of a prediction based on this estimate with the measured data ("visibilities"). Accuracy in the latter step is crucial but is made difficult by irregular and non-planar sampling of data by the telescope. In this work we present a GPU implementation of 3d de-gridding which accurately deals with these two difficulties and is designed for distributed operation. We address the load balancing issues caused by large variation in visibilities that need to be computed. Using CUDA and NVidia GPUs we measure performance up to 1.2 billion visibilities per second.
All the 512 station layouts can be found in the Antenna engineering data pack / Array folder atta... more All the 512 station layouts can be found in the Antenna engineering data pack / Array folder attached to this document set.
The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and th... more The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of five zettabytes of data per year. A critical task for the SKA will be its ability to process data for science, which will need to be conducted by science pipelines. Together with polarization data from the LOFAR Multifrequency Snapshot Sky Survey (MSSS), we have been developing a realistic SKA-like science pipeline that can handle the large data volumes generated by LOFAR at 150 MHz. The pipeline uses task-based parallelism to image, detect sources and perform Faraday tomography across the entire LOFAR sky. The project thereby provides a unique opportunity to contribute to the technological development of the SKA telescope, while simultaneously enabling cutting-edge scientific results. In this paper, we provide an update on current efforts to develop a science pipeline that can enable tight constraints on the magnetised large-scale structure of the Universe.
Practical aperture synthesis imaging algorithms work by iterating between estimating the sky brig... more Practical aperture synthesis imaging algorithms work by iterating between estimating the sky brightness distribution and a comparison of a prediction based on this estimate with the measured data ("visibilities"). Accuracy in the latter step is crucial but is made difficult by irregular and non-planar sampling of data by the telescope. In this work we present a GPU implementation of 3d de-gridding which accurately deals with these two difficulties and is designed for distributed operation. We address the load balancing issues caused by large variation in visibilities that need to be computed. Using CUDA and NVidia GPUs we measure performance up to 1.2 billion visibilities per second.
arXiv: Instrumentation and Methods for Astrophysics, 2020
This report was submitted as part of the SKA Low Frequency Aperture Array Critical Design Review ... more This report was submitted as part of the SKA Low Frequency Aperture Array Critical Design Review describing the design of the SKA1-LOW station that took place between 2013 and 2018. The SKA1 LOW field station is inscribed in a circular area having an effective station diameter (centre to centre) of 38 meters and has 256 SKALA4 elements. This document describes the electromagnetic design of the field station. In particular it describes the layout design and the electromagnetic modelling and characteristics of the station. This document describes the effects associated with the layout and array such as mutual coupling effects, side lobe pattern and beam shape (eg. smoothness, calibration models) and presents the state of the art of our ability to measure the array performance and validate the simulation work. The current LFAA field node requirements, derived from the SKA L1 requirements, have evolved over the last years since the LFAA PDR and the System Baseline Design. The SKA1 LOW f...
The design stage of the first phase of the Square Kilometre Array (SKA) telescopes has been recen... more The design stage of the first phase of the Square Kilometre Array (SKA) telescopes has been recently completed by the SKA Organisation (SKAO). This marks the initial step towards an ambitious vision of designing, constructing, and operating telescopes with an equivalent collecting area of one square kilometre. At the completion of this design stage the project has entered a bridging period to manage work until construction funds are available. During this period many of the software teams from the design stage have been engaged in prototyping lean-agile processes, structures, and practices. By the end of the period the goal is to have pivoted from a document based, stage-gated set of processes arranged around design consortia to a code based, value-flow driven, lean-agile set of processes unified around the Scaled Agile Framework. Two years since the start of this transformation this paper reflects on these processes. We highlight some practices that have been found helpful, as well...
Proceedings of Wide Field Astronomy & Technology for the Square Kilometre Array — PoS(SKADS 2009), 2011
In this paper we show how phase errors between different analogue signal channels of a phased arr... more In this paper we show how phase errors between different analogue signal channels of a phased array affects the quality of the output phased array beam. Specifically we look at the reduction in forward gain of the array beam and the array beam pointing offset as a function of phase errors and array size. We use a combination of simulations made using the aperture array simulator OSKAR, developed at the University of Oxford, in conjunction with measured results taken from 2-PAD; a functioning, astronomical, dual-polarisation, digital beamforming, 4 × 4 element, aperture array prototype (Greenwood 2007) for the Square Kilometre Array (Taylor 2007) developed by a consortium of UK universities. We show that phase errors between different signal channels is specifically an important issue for broadband arrays like 2-PAD, and comment on the relative benefit of digital beamforming versus analogue beamforming engines.
2018 IEEE 14th International Conference on e-Science (e-Science), 2018
The Square Kilometre Array (SKA) will be the largest radio telescope constructed to date and the ... more The Square Kilometre Array (SKA) will be the largest radio telescope constructed to date and the largest Big Data project in the known Universe. The first phase of the project will generate 160 terabytes every second. This amounts to 5 zettabytes (5 million petabytes) of data that will be generated by the facility each year-a data rate equivalent to 5 times the estimated global internet traffic in 2015. These data need to be reduced and then continuously ingested by the SKA Science Data Processor (SDP). Within the SDP Consortium, we are contributing to various roles in the development of the telescope including building a lightweight end-to-end prototype of the major components of the SDP system-a project we call the SDP Integration Prototype (SIP). The aim is to build a mini, fully-operational SDP, for which we have been developing realistic SKA-like science pipelines that can handle these unprecedented data volumes.
Proceedings of Advancing Astrophysics with the Square Kilometre Array — PoS(AASKA14), 2015
The exceptional sensitivity of the SKA will allow observations of the Cosmic Dawn and Epoch of Re... more The exceptional sensitivity of the SKA will allow observations of the Cosmic Dawn and Epoch of Reionization (CD/EoR) in unprecedented detail, both spectrally and spatially. This wealth of information is buried under Galactic and extragalactic foregrounds, which must be removed accurately and precisely in order to reveal the cosmological signal. This problem has been addressed already for the previous generation of radio telescopes, but the application to SKA is different in many aspects. We summarise the contributions to the field of foreground removal in the context of high redshift and high sensitivity 21-cm measurements. We use a state-of-the-art simulation of the SKA Phase 1 observations complete with cosmological signal, foregrounds and frequencydependent instrumental effects to test both parametric and non-parametric foreground removal methods. We compare the recovered cosmological signal using several different statistics and explore one of the most exciting possibilities with the SKA-imaging of the ionized bubbles. We find that with current methods it is possible to remove the foregrounds with great accuracy and to get impressive power spectra and images of the cosmological signal. The frequency-dependent PSF of the instrument complicates this recovery, so we resort to splitting the observation bandwidth into smaller segments, each of a common resolution. If the foregrounds are allowed a random variation from the smooth power law along the line of sight, methods exploiting the smoothness of foregrounds or a parametrization of their behaviour are challenged much more than non-parametric ones. However, we show that correction techniques can be implemented to restore the performances of parametric approaches, as long as the first-order approximation of a power law stands.
Monthly Notices of the Royal Astronomical Society, 2016
The Square Kilometre Array's Low Frequency instrument (SKA-LOW) will operate in the undersampled ... more The Square Kilometre Array's Low Frequency instrument (SKA-LOW) will operate in the undersampled regime for most of the frequency band where grating lobes pose particular challenges. To achieve the expected level of sensitivity for SKA-LOW, it is particularly important to understand how interfering sources in both near and far side-lobes of the station beam affect the imaging performance. In this study, we discuss options for station designs, and adopting a random element layout, we assess its effectiveness by investigating how sources far from the main lobe of the station beam degrade images of the target field. These sources have the effect of introducing a noise-like corruption to images, which is called the far sidelobe confusion noise (FSCN). Using OSKAR, a software simulator accelerated using graphics processing units, we carried out end-to-end simulations using an all-sky model and telescope configuration representative of the SKA-LOW instrument. The FSCN is a function of both the station beam and the interferometric point spread function, and decreases with increasing observation time until the coverage of the aperture plane no longer improves. Using apodization to reduce the level of near-in sidelobes of the station beam had a notable improvement on the level of the FSCN at low frequencies. Our results indicate that the effects of picking up sources in the sidelobes are worse at low frequencies, where the array is less sparse.
Monthly Notices of the Royal Astronomical Society, 2016
The removal of the Galactic and extragalactic foregrounds remains a major challenge for those wis... more The removal of the Galactic and extragalactic foregrounds remains a major challenge for those wishing to make a detection of the Epoch of Reionization (EoR) 21 cm signal. Multiple methods of modelling these foregrounds with varying levels of assumption have been trialled and shown promising recoveries on simulated data. Recently however there has been increased discussion of using the expected shape of the foregrounds in Fourier space to define an EoR window free of foreground contamination. By carrying out analysis within this window only, one can avoid the foregrounds and any statistical bias they might introduce by instead removing these foregrounds. In this paper, we discuss the advantages and disadvantages of both foreground removal and foreground avoidance. We create a series of simulations with noise levels in line with both current and future experiments and compare the recovered statistical cosmological signal from foreground avoidance and a simplified, frequency independent foreground removal model. We find that for current generation experiments, while foreground avoidance enables a better recovery at k perp > 0.6 Mpc −1 , foreground removal is able to recover significantly more signal at small k los for both current and future experiments. We also relax the assumption that the foregrounds are smooth. For line-of-sight variations only, foreground removal is able to make a good signal recovery even at 1 per cent while foreground avoidance is compromised significantly. We find that both methods perform well for foreground models with line-of-sight and spatial variations around 0.1 per cent however at levels larger than this both methods are compromised.
Monthly Notices of the Royal Astronomical Society, 2015
Fast radio bursts (FRBs) are millisecond radio signals that exhibit dispersion larger than what t... more Fast radio bursts (FRBs) are millisecond radio signals that exhibit dispersion larger than what the Galactic electron density can account for. We have conducted a 1446 h survey for FRBs at 145 MHz, covering a total of 4193 deg 2 on the sky. We used the UK station of the low frequency array (LOFAR) radio telescope-the Rawlings Array-accompanied for a majority of the time by the LOFAR station at Nançay, observing the same fields at the same frequency. Our real-time search backend, Advanced Radio Transient Event Monitor and Identification System-ARTEMIS, utilizes graphics processing units to search for pulses with dispersion measures up to 320 cm −3 pc. Previous derived FRB rates from surveys around 1.4 GHz, and favoured FRB interpretations, motivated this survey, despite all previous detections occurring at higher dispersion measures. We detected no new FRBs above a signal-to-noise threshold of 10, leading to the most stringent upper limit yet on the FRB event rate at these frequencies: 29 sky −1 d −1 for five ms-duration pulses above 62 Jy. The non-detection could be due to scatter-broadening, limitations on the volume and time searched, or the shape of FRB flux density spectra. Assuming the latter and that FRBs are standard candles, the non-detection is compatible with the published FRB sky rate, if their spectra follow a power law with frequency (∝ ν α), with α +0.1, demonstrating a marked difference from pulsar spectra. Our results suggest that surveys at higher frequencies, including the low frequency component of the Square Kilometre Array, will have better chances to detect, estimate rates and understand the origin and properties of FRBs.
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Papers by Ben Mort