SKA AA-Low Station Configurations and Trade-off Analysis

1. SKA AA-Low Station Configurations and Trade-off Analysis NimaRazavi-Ghods, Ahmed El-Makadema AAVP 2011, ASTRON, Dwingeloo 12-16 Dec 2011

2. 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 Overview

3. Sensitivity • Depends on diameter, number of elements, and their configuration • Beam-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 GRS • Filling Factor (one used figure of merit) Configuration Design Space

4. A/T Requirements

5. A/T Requirements

6. A/T Requirements

7. A/T Requirements

9. Possible Geometries for AA-Low

10. Possible Geometries for AA-Low

11. Aeff/Tsys for a typical observation

12. Pattern vs. Array Size

13. Coherent and Incoherent Regimes

14. 345 m 255 m 255 m Gaussian Taper (2.5) Random Random 447 m 390 m Controllable Beam-width = K/D Spatial Tapering Chebyshev Taper (70dB) Chebyshev Taper (100 dB)

15. Defined either as the ratio of Aeff to Aphysor 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 tapering • A/T is maintained by N, D, and configuration • Side-lobe adds to noise when FF<1 but can be controlled too. Filling factor 

16. Single Band 70-450MHz • Random (dense packed) • N = 2440 elements • D = 90m • Avg. Spacing = 1.43m • Element BW = ±35 • Trec = 0.1*Tsky + 40 • Rad. Efficiency = 93% • Dual Band • 70-180MHz, 200-450MHz • Random • N1 = 1540, N2 = 2440 • D1 = 80m, D2 = 50m • BW1 = BW2 = ±35 • Rad. Efficiency = 93% • (~63% extra elements) • (~50% if lower gain) Aim for A/T of 1000 m2/k@ 45 Scan (100 to 450 MHz) Example SKA1 Station Distribution Using Single or Dual Array Solution

19. 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. Side-lobe Control

20. Xarray Tool: MATLAB GUIsites.google.com/site/xarraytool/

21. 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. Conclusions

22. 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). Future Configurations work

23. Thank You.