LOFAR/SKA Simulator

Shep DoelemanColin LonsdaleRoger CappalloRamesh BhatDivya Oberoi

Joanne Attridge

Simulator at PDR

• What simulator does (will do) and general architecture.

• Illustrate simulator capabilities.• In depth look at configuration studies and strategy

(constraints and figures of merit).• Strategy for Calibration studies.

Simulator Functions• Create Synthetic Data Sets

– Realistic effects: Ionosphere, Variable beams, Skies– High level of LOFAR design specification

• Used to:– Test Calibration:

• Direct access to structures solved for• Controlled environment for isolation of effects

– Explore Design Parameter Space:• Automation to allow efficient searches• Identify trends in performance

– Test New Postprocessing Algorithms

• To be used by LOFAR community– Science Applications

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 LOFAR skies– Source Spectral Index– Out-of-beam source

contributions (CasA in sidelobes, etc…)

– Extension to 3-D FFTs for wide field imaging.

– RFI (limited)

Conceptual Flow Diagram

• Goals:• Flexibility• High Accuracy• Modular• Exportable output• In House – many needs not in other packages.

Functional Flow Diagram

array skies obs_spec sitesproc_spec

configgenerator LOsim

Script driven Simulator Module

(u,v) FITS

MIRIADPSF StatsImagingFidelity

Calibrationstructures forcomparison

Image Import and SimulationBefore After

Effect of Variable Station Beams

Ionospheric Effects • Line integration through realistic 4-D ionospheric model: Gary Bust• Vertical profile, TIDs, Kolmogorov Spectrum of inhomogeneities.• Time scale = 10 seconds, Linear scale = 100m (possible).• FT of 2-D phase screen for each station convolved with (u,v) plane.• Will generate few ‘ionospheres’ – large files.

Altitude (10km)

Latitude (26km)

ElectronDensity

Sky Noise Contribution

408 MHz All Sky MapUse spectral index (2.55) to scale in frequencyConvolved with receptor beam pattern.

• 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

– 1.5 Tbytes of disk

– <$25,000

Simulator Beowulf

Configuration Studies - in progress

• Parameter space is vast:• Configurations (star, spiral, random, expo shell…)• Number of stations (variable sensitivity …)• Weighting schemes• Bandwidth and integration time• Sky properties and observing geometry• Frequency, polarization, spectrum, …• Station design and receptor beamshape• Ranges of corrupting influences.

• Strategy: parameterize configurations and explore limited ranges other parameters.

• Input from ALMA, ATA, SMA experience.

Hard Constraints:

• Approximate scale free radial distribution: 25% within 2km diam., 50% within 12km, 75% within 75km.

• Beyond 2km diam.: – 60-160 stations comprising >50 receptors.– Receptors for each LOFAR band will be co-located.

• In compact Core:– 60-400 sub-stations– Receptors in each band do not have to be co-located

Figures of Merit:

• PSF statistics: RMS, size, min, max, deviation. Computed for declination, integration, bw.

• Sensitivity Loss due to: weighting, fixed taper.• Cable Length (Prim’s Algorithm)• PSF statistics for Core only (ASM, EOR)• Image fidelity for 2 benchmark images• Robustness: impact of random 10% station loss.• Calibratability: requires calibration software.

Beam RMS of 3-arm Log Spirals

Configuration Optimization

Figures of Merit:

PSF RMS vs RadiusPSF Beam SizeCable length

Configuration Optimization

Figures of Merit Weightedand Combined into Optimization 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

Configuration Editor

Asymmetric Configurations

Probably necessary for all sites.

FOMs reduced from optimal configs.

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.

Input to LOFAR Calibration Tests • Simulator capability developing in parallel with

Calibration software (weekly TIM).• Phase I: Tests of ~5 peeling sources• Phase II: Tests of ~10 peeling and ~100 faint

sources.• Phase III: Inclusion of Ionosphere and comparison

of simulated 2-D phase screens to solver output.• Phase IV: Inclusion of variable station beams.• As needed: inclusion of bright, out-of-beam

sources.

Sample SKA Config

Progress and Results:

• Code required for pivotal Calibration tests is complete.

• Configuration testing is underway (~5000 tested so far) with calculation of formal Figures of Merit automated.

• Several ‘promising’ configurations identified and being used to assess site flexibility.

• Strategy for continued work defined:– Configuration evaluation along FOM axes.– Generation of Calibration data sets– Continued coding of modules based on priority.