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The Haystack SKA/LOFAR Performance Simulator
Feb 13, 2004
Ramesh Bhat
MIT/Haystack
Cast of Characters
Shep DoelemanColin LonsdaleRoger CappalloRamesh Bhat
Joanne AttridgeDivya Oberoi
With contributions from
Gary Bust (ARL/U. Texas)Tanja Bode (REU, Cornell)Laurel Ruhlen (REU, MIT)
Goals of Haystack Simulator• Create datasets with SKA/LOFAR properties
– Large scope (baseline range, station numbers, …)– Generality of array and observation specification – Time-variable station beams– Atmospheric/ionospheric structure across FoV– Densely populated sky– Sources in station sidelobes
• Search design parameter space efficiently– Sophisticated, script-driven automation– Figures of merit for performance assessment
• Testing Calibration and Post-processing• Science Applications (e.g. EOR)
Functional Flow Diagram
array skies obs_specproc_spec
configgenerator
Listsimages
Script driven Simulator Module
sites
(u,v) FITS
MIRIADPSF StatsImagingFidelity
Calibration“ground truth”data
ionosphere
Architecture
Kernel of visibility generation:
Transform arbitrary sky to perfect “reference” uv grid
For each baseline
For each time
For each frequency channelCopy relevant piece of uv gridConvolve with station beam and atmosphere/ionosphere functionsPrecision numerical integration over correlation cellAdd noise, simple RFI …
Write output uv data file
Simulator Status• Includes
– FITS Image import– Variable station beams– Thermal Noise (Rx)– Arbitrary array configuration.– Arbitrary station config.– Gaussian sources– Arbitrary time/freq obs.– True parallelization in time.– Functional parallelization
otherwise– Exportable to FITS– Script driven to support
automated parameter searches
– 4-D Ionosphere with line integration.
– Site mask incorporation
– Sky noise due to Galactic Background.
• Will Include– Polarization
– Realistic skies
– Source Spectral Index
– Out-of-beam source contributions (CasA in sidelobes, etc…)
– Extension to 3-D FFTs for wide field imaging.
– RFI (limited)
• 20 nodes
– 2.4 GHz P4, $900 each
– 1 Gbyte of RAM
– 60 Gbyte of disk
• Gigabit ethernet switch
– 24-port
– $2000
• UPS and misc
• Excellent price/performance
– 50-80 Gflops
– 3 Tbytes of disk
– <$25,000
Simulator Beowulf
Image Import and Simulation
Before After
Effect of Variable Station Beams
Effects of Ionosphere (Virgo A at 74 MHz)
R. Perley (2003)
Ionospheric Effects • Line integration through realistic 4-D ionospheric model (from G. Bust)
• Vertical profile, TIDs, Kolmogorov Spectrum of inhomogeneities.
• Gaussian depletions/enhancements (transient, drifting, stationary…)
• IDL-based visualization software for 2-D or 3-D (animation) data.
Altitude (10km)
Latitude (26km)
ElectronDensity
Ionospheric effects
• 4-dimensional ionosphere generation
• 2-D phase screen by line integration
for each station/time
• (u,v) plane convolution
independently for each data
point/channel
Ionospheric Movie: Refraction,Defocusing
Demonstrate that PS can reproduce characteristics ofreal ionosphere: Virgo A.
Gain experience with Ionospheric generation codeand verify simulator code.
Original data images: 2048as
Movie images: 600x600as.
Elapsed time 1000 seconds
1 TID with wavelength largecompared to VLA A array.
Code verified with‘perfect wedge’.
Ionospheric Effects on Point Source
Motion of brightest component Flux of brightest component
Baseline Phases on VLA armsNorth Arm SE Arm
Ionosphere above the VLA
N_e as a functionof Altitude above the VLA.
Sky Noise Contribution
408 MHz All Sky MapUse spectral index (2.55) to scale in frequencyConvolved with receptor beam pattern.
LOFAR skies: deep field at 330MHz
Simulated Sky at 74MHzExtrapolation to 74MHzfrom VLA P-band image.
2000x2000 arcsec1Jy to 0.1mJy
24 Hour integration
Configuration Studies
• Different types of configurations: log spirals, symmetric/asymmetric, 1, 3
and 5 arms, random perturbations of station
• Scripts and codes that generate families of a given type, for a given parameter range
• Hard constraints from design, and other considerations, cable length, cable routing, etc
Configuration Studies - in progress
• Parameter space is vast:• Configurations (log spirals, random,…)
• Number of stations (variable sensitivity …)
• No of elements/station, Staion layout
• Bandwidth and integration time
• Sky properties and observing geometry
• Frequency, polarization, spectrum, …
• Weighting schemes, tapering
• Ranges of corrupting influences.
• Strategy: parameterize configurations and explore limited ranges, identify trends
• Input from ALMA, ATA, SMA studies.
Proposed USSKA Configuration
Inner ~100 km array
Figures of Merit:
• PSF statistics: RMS, size, min, max, deviation. Computed for declination, integration, bw.
• Cable Length (Prim’s Algorithm).• Sensitivity Loss due to: weighting, fixed taper.• PSF statistics for Inner compact array/core.• Image fidelity for a few benchmark images.• Robustness: impact of random station loss.• Calibratability: requires calibration software.
Searching in Parameter Space
Configuration Optimization
Figures of Merit:PSF RMS vs RadiusPSF Beam SizeCable Length
Configuration Optimization
Figures of Merit weighted And Combined into Optmization function
Effects of Integration Time and BW
x = instantaneouso = ½ hour
x = 0.25%o = 10% = 20%
Compact Core Configurations
• Outer configuration won’t continue to Core.
• Scale free distribution breaks down in Core• Lower limit of receptors/station relaxed.
• Calibration and cabling issues.• ASM and EOR require excellent Core PSF
Genetic Algorithm Optimization
-Optimizes using (u,v) coverage and cable length figures of merit.-Uses ‘mutations’ to avoid local minima-Excellent beam size and RMS characteristics-Scale free constraints to be included.
Configuration Editor
Site Constraint ImpactUsed idealized configurations andmeasured PSF metricsbefore and after editingto accommodate siteconstraints.
PSF Metrics:Beam SizeBeam EllipticityRMS at various radiiExtrema near centerSkewness vs. radius
Snapshot, 30min,2 hour integrations.
Configuration Editor
Effects of Configuration Constraints
• Real-world constraints (mountains, cities, existing fibers …)• Optimization of configuration is complicated problem
Summary
• Robust simulation package exists– Very general, supports full range of SKA designs
– Supports wide range of model sky properties
– Precision visibility generation, wrapped in support utilities
– Includes key sources of error for SKA/LOFAR
– Suitable for comprehensive analyses of array performance
• Does not simulate all observing modes– Imaging mode only at this stage
• Work in progress– Continued code development (3DFFT, out-of-beam, etc)
– Support for outside use (simulations/science)