Hi Hari, Many thanks for your good points here and the links to the papers.
Using FFT, directly instead of going around the correlation route, visibility function+FFT certainly minimises the computation when the density of antennas becomes higher than that of a minimally redundant array. When the array is fully filled then clearly an FFT is the way to go. The FFT approach is then more like radar processing of received signals where the norm is an almost fully filled array of antennas, to get around the aliasing problem, which is more severe for radar systems. I have considered this approach for far-field (range > 2d^2/lambda) imaging, the benefits being greater sensitivity and particularly faster responses, which would be key for commercial applications. However, for portal security screening this is not just near-field, but extreme near field, right up almost to the inductive region of the antenna. In the extreme near field region, Fourier transform relationship between the image and the aperture sampled electric fields space breaks down, so the problem gets more complication. This is not just a problem for the FFT route, but also for the correlation approach. Using correlations from near-field array does actually work, but you have to create a 3-Dvisibility function, and then the Fourier transform relationship between the 3-D visibility function visibility and the 3-D image breaks down all except for a small volume around the phase centre of the 3-D image. A solution is to split the image up into a mosaic of physical smaller 3-D images. The mosaic approach is more of a brute force approach but I’m sure there are more subtle and computationally efficient means to do the near-field imaging, when I get time I make some effort in this direction. Best wishes, Neil From: Hariharan Krishnan <[email protected]> Sent: 15 August 2020 15:51 To: [email protected] Cc: Neil Salmon <[email protected]>; [email protected] Subject: Re: [casper] references to recent cross-correlator technology developments Hi Neil, We have a GPU-based direct imaging correlator (EPIC - E-Field Parallel Imaging Correlator) implemented and tested on one of the LWA stations at Sevilleta. In EPIC we directly grid the fourier transformed voltages from the individual antennas and form real-time dirty maps without having to estimate the visibilities. EPIC can essentially produce images at high time resolution on the order of a few ms. Currently we are optimizing the GPU-part of EPIC to increase the operating bandwidth per node for a commensal transient imaging backend at LWA-SV. You can refer to the following publications for more details on EPIC https://academic.oup.com/mnras/article-abstract/486/4/5052/5484888 https://academic.oup.com/mnras/article/467/1/715/2917985 Regards, Hari On Mon, Jul 20, 2020 at 7:12 AM Neil Salmon <[email protected]<mailto:[email protected]>> wrote: Hi Danny, Yes I can appreciate the difference here with respect to integration times. Furthermore, as our arrays tend to be more fully filled, some form of FT beam-former might be more efficient than a correlator. However, things do get more complicated in the near-field security screening scenarios where the FT relationship between physical space and spatial frequency space breaks down. Cheers, Neil From: Danny Price <[email protected]<mailto:[email protected]>> Sent: 20 July 2020 14:44 To: Neil Salmon <[email protected]<mailto:[email protected]>>; [email protected]<mailto:[email protected]> Subject: RE: [casper] references to recent cross-correlator technology developments Hi Neil, The correlation is indeed done in real time using stream processing frameworks for most interferometer telescopes. Conversion from (very sparse) visibilities to images is generally done offline (this can be very time consuming!). There are a few real-time imaging systems: the EPIC correlator that Jack mentioned, and the realfast system on the VLA (https://science.nrao.edu/facilities/vla/observing/realfast) are good examples. Cheers, Danny On 20 July 2020 at 9:55:06 pm, Neil Salmon ([email protected]<mailto:[email protected]>) wrote: Hi Danny, Thank you for these references. For security screening systems the name of the game is real-time, ie an image in less than 1 second. However, I see a great many references to GPU based correlators. I was used to seeing these devices as off-line correlators, as in software correlators. Are the GPUs being used by the radio astronomy community as real-time correlators, or as software correlators? Many thanks, Neil From: Danny Price <[email protected]<mailto:[email protected]>> Sent: 20 July 2020 12:21 To: [email protected]<mailto:[email protected]> Subject: Re: [casper] references to recent cross-correlator technology developments Hi Neil, To add to Jack's post, allow me to plug some overview articles that may be of interest. The first, https://arxiv.org/abs/1702.00442, was for an introduction for a special issue of JAI on DSP in radio astronomy in 2016. Table 1 summarises some of the larger correlators: the references therein may be of use. Jack (et al)'s CASPER article in said JAI special issue is also a font of references: https://arxiv.org/abs/1611.01826. The full special issue article listing is up here: https://www.worldscientific.com/toc/jai/05/04. More recently, here's my book chapter on real-time stream processing in radio astronomy, https://arxiv.org/abs/1912.09041, which delves a bit deeper into technical details for common approaches. In terms of cutting edge, there are various groups working with the Xilinx RFSoC components for next-gen systems -- you will no doubt have seen some traffic on this list. The ASKAP telescope group have plans to use an Alveo Xilinx U280 accelerator card for high time resolution imaging + dedispersion, which is an alternative to the GPU correlator. GPU correlators are still the most widespread for O(100) antennas. There's some discussion on GPU correlator performance in J. Kocz et al 2014 (https://arxiv.org/abs/1401.8288); for O(100) inputs a GPU correlator will likely be memory bandwidth bound. Cheers, Danny On 18 July 2020 at 7:54:49 pm, Neil Salmon ([email protected]<mailto:[email protected]>) wrote: I need references on recent developments in cross-correlator technology for an IEEE paper on the subject of aperture synthesis imaging in the area of security screening of people for concealed weapons. Typical requirements for this application are cross-correlators that can process in real-time signals from hundreds of receiver channels with around 1 GHz of RF bandwidth. As none of this technology is commercially available off-the-shelf I’m dependent on the radio astronomy community to get the latest information of correlator development. This might be just technical knowhow on the building of correlators, or communities who would be willing to supply for a fee correlators to a security screening technology development company. Could anyone provide me with any references of papers on recent correlator development that I could include in this paper? 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