high-level ska signal processing description - The Square Kilometre ...

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WP2‐040.030.010‐TD‐001 Revision : 1 in the Galactic plane where the density is large. Increasing the FoV to 200 deg^2 or more, also makes timing the pulsars off the plane vastly more efficient. For timing pulsars in the Galactic plane, it is highly beneficial that the upper frequency range of the Aperture Arrays is about 1 GHz and that more than 50% of the total collecting SKA area can be phased up. Using the strategy of Smits et al., a maximum FoV of 250 deg 2 for AA and 20 deg 2 for dishes (with PFA), as well as assuming that sufficient beams can be produced to pixelize these FoVs, it would take up to 6 days to obtain a single timing point for 14 000 pulsars to be discovered in the surveys. Obtaining one high‐precision timing point it will take up to 3 days with dishes and only 14 hours for timing with AA. In SKA Phase 1 the smaller field of view will influence how many pulsars will be in each field of view. There will of course also be less pulsars but not in the same proportion. Simulations for SKA phase 1 show for a FoV of 2.1 deg 2 , we will get a maximum of 15 pulsars in the FoV. For a FoV of 30 deg 2 , there will be 200 pulsars at most. So in order to be as efficient as possible we would need of the order of 50 beams for the timing, as these will be needed for the AAs, and will greatly improve the efficiency of the timing. 11.6 Forming the Beams: In order to achieve the maximum sensitivity the station/dish beams from each of the dishes/stations in as much of the array as possible need to be added coherently. To form the coherent sum requires that the phase relationship between the signals for each beam from each dish station be known precisely. It is important to note that phase calibration also requires that there is a known phase relationship between the time and frequency references at each of the stations. To obtain phase corrections will require the regular observation of calibration point sources and a decomposition analysis. These phases will then need to be applied to the data from each of the stations along with the geometric corrections to point each of the tied‐array beams in the correct direction. The output product for all beams should be complex channelized data so that it is possible to perform coherent de‐dispersion on all beams. While this is strictly necessary only for the high precision timing objects, it simplifies the pipeline, should be technically achievable for a sufficient number of beams and will also improve timing precision at lower frequencies. Moreover it will have application to other areas of pulsar science such as single pulse and polarization studies. 11.7 Time Resolution and Frequency Resolution. We assume a maximum time resolution of 0.2 us for the final data product. The ability to fully record the complex channelized data at high time resolution over the entire available bandwidths for a reduced number of beams will also be required. 11.8 Data rates: The data rate can be defined in terms of the Nyquist sampling of the baseband data as there will be no averaging of the data before it is transported back to the main computing centre even if there is channelisation. Assuming that the data is sampled at 4 bits, and that we need only transport data of about 50 beams (we obtain a data then we have a rate of 30 Gbytes/s. The exact number of required 2011‐03‐29 Page 58 of 59
WP2‐040.030.010‐TD‐001 Revision : 1 beams can be traded against observing time, as it will simply take longer to go through the list. However, that is only possible up to a point where other observations or sufficient sampling is not possible anymore. We consider 50 beams at the lowest possible limit but recommend more to reduce the impact on other science areas. We note that it is at the beamforming stage where it will be crucial that information about the quality of the station data, either through station calibration, or checks done once the data arrive a central processing facility, to determine that the data from all the station are providing data of sufficient quality. The exact algorithm for determining this quality and how to adjust data when stations drop in or out will need to be developed especially if changes are occurring on relatively short timescales, that is shorter than a typical observation duration. 11.9 Processing the Beams When all of the beams (one per pulsar to be timed) have been formed it is necessary to perform a number of steps which are common to all of them. These include to coherently de‐dispersing the data (thereby also forming all Stokes parameters), interference rejection, polarisation calibration and folding. 11.9.1 (Coherent) De‐dispersion: To correct for the dispersive delay due to the interstellar medium requires that the frequency dependent phase shift of the signal applied by the ISM is unwrapped again in a process known as coherent de‐dispersion (e.g. Lorimer & Kramer 2005). This process requires complex channelized data for each beam. In contrast to search observations, the dispersion measure for pulsars to be timed is known and the result of the discovery process. Improvements to the dispersion measure precision will be obtained in an off‐line analysis process. This process includes the Fourier‐Transform of the complex data into the frequency domain, this process may not be necessary if the data are already delivered as complex channelized data. The data is then multiplied with the inverse ISM filter function plus tapering and the re‐transformation into the time domain. 2011‐03‐29 Page 59 of 59
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HIGH‐LEVEL SKA SIGNAL PROCESSING
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