ska aa-low station configurations and trade-off analysis
DESCRIPTION
SKA AA-Low Station Configurations and Trade-off Analysis. Nima Razavi-Ghods , Ahmed El- Makadema AAVP 2011, ASTRON, Dwingeloo 12-16 Dec 2011. Requirements for AA-low configuration design Possible geometries and their limitations Controlling trade-offs - PowerPoint PPT PresentationTRANSCRIPT
SKA AA-Low Station Configurations and SKA AA-Low Station Configurations and Trade-off AnalysisTrade-off Analysis
Nima Razavi-Ghods, Ahmed El-Makadema
AAVP 2011, ASTRON, Dwingeloo 12-16 Dec 2011
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OverviewOverview
Requirements for AA-low configuration design
Possible geometries and their limitations
Controlling trade-offs
Typical configuration design (based on DRM specs)
AA-low: single array versus dual-band array
Xarray: Code to evaluate AA design parameters
Conclusions and Future work
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Configuration Design SpaceConfiguration Design Space
Sensitivity◦Depends on diameter, number of elements,
and their configurationBeam-width (Calibration)
◦We can increase this by either reducing station size or using a tapering function but at the cost of A/T
Side lobes (Noise suppression) ◦We can reduce this by tapering and irregular
configurations like GRSFilling Factor (one used figure of merit)
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A/T RequirementsA/T Requirements
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A/T RequirementsA/T Requirements
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A/T RequirementsA/T Requirements
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A/T RequirementsA/T Requirements
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Possible Geometries for AA-LowPossible Geometries for AA-Low
Possible Geometries for AA-LowPossible Geometries for AA-Low10
AAeffeff/T/Tsyssys for a typical observation for a typical observation11
Pattern vs. Array SizePattern vs. Array Size12
Coherent and Incoherent RegimesCoherent and Incoherent Regimes13
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255 m 255 m
447 m
Random Random Gaussian Taper (2.5)
Chebyshev Taper (100 dB)Chebyshev Taper (70dB)
390 m
345 m
Spatial Spatial TaperingTapering
Controllable Beam-width = K/D
Filling factor Filling factor
Defined either as the ratio of Aeff to Aphys or the number of antennas in the array divided by the number of elements required to Nyquist sample the wavefront at each frequency point.
Is it as vital as we all think?Beamwidth can be controlled by D and
taperingA/T is maintained by N, D, and configurationSide-lobe adds to noise when FF<1 but can
be controlled too.
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Example SKA1 Station Distribution Example SKA1 Station Distribution Using Single or Dual Array Solution Using Single or Dual Array Solution
Single Band70-450MHzRandom (dense packed)N = 2440 elementsD = 90mAvg. Spacing = 1.43mElement BW = ±35Trec = 0.1*Tsky + 40Rad. Efficiency = 93%
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Dual Band70-180MHz, 200-450MHzRandomN1 = 1540, N2 = 2440D1 = 80m, D2 = 50mBW1 = BW2 = ±35Rad. Efficiency = 93% (~63% extra elements) (~50% if lower gain)
Aim for A/T of 1000 m2/k @ 45 Scan(100 to 450 MHz)
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Side-lobe Control Side-lobe Control
The system noise temperature increases by sources out side the main beam.
Side-lobe level requirement can be driven from station sensitivity and the maximum noise source flux to be suppressed down to the thermal noise level.
The minimum peak side lobe level of an un-tapered station is -17 dB (uniform circular aperture). However, a lower side lobe level can be achieved with a tapered station.
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Xarray Tool: MATLAB GUIXarray Tool: MATLAB GUIsites.google.com/site/xarraytool/sites.google.com/site/xarraytool/
ConclusionsConclusions
AA configuration design space should be based on optimising A/T, FOV and mean SLL.
Typical increase of A/T can be defined by N and D but some configurations can result in a very rapidly changing A/T.
Beam-width is defined by K/D, where K can be changed by use of tapering but should be done cautiously.
Mean SLL can be controlled by tapering to achieve better than typical -17dB. Smaller arrays can be beneficial in this regard as well as for larger FOV.
Single vs. Dual argument should be thought about carefully with more realistic assumptions.
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Future Configurations workFuture Configurations work
We MUST use OSKAR 2 to test station configurations from an interferometric aspect.
Initial first–level station design can be made in Xarray which includes a sky and receiver model.
Design Can be further checked and validated with MoM-MBF code developed at UCL, Belgium which work along-side Commercial software packages such as CST and HFSS.
Develop optimisation tools which can be analytical (collaboration with UCL).
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Thank You.Thank You.
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