SKA 1 AA-low system design considerations

1. SKA1 AA-low system design considerations Andrew Faulkner

2. SKA1 AA-low is a major System: >10x LOFAR sensitivity >1 km2 collecting area >6:1 frequency range 70 – 450MHz ~50 deg2 Field of View Very quiet site Required in SKA Memo 125

3. But........ It is also part of a bigger vision: SKA2 Some implications....

4. SKA1 AA-low System requirements Priority • Stand alone instrument meeting SKA1 science goals • Deployable 2016 onwards • Specification will mesh with SKA2 system design e.g. • Frequency range ̶ Data rates • Calibration capability ̶ Correlation requirements etc... • Scaleable architecture for 2018+ commonality with AA-mid • Architecture will support upgraded processing devices • Phase 2 expansion ̶ Retro-fit Phase 1 (if good pay-back) SKA1 performance must not be compromised?

5. Element requirements

6. One or two elements?

7. LWA element: mechanical example Electronics at top – well away from floods etc. Simple “skeleton” elements (delivered flat) Clamp type rotational adjustment Single pole fixing – just sunk into ground Buried cables Cheap mesh groundplane Easy and quick deployment

8. Integrated AA-low element possibility • Elements onlyconnected by glass....

9. Where to digitise? Can be done – safe option Likely to require distributed digitisation around station. (Cable cost and range) Requires good, stable analogue design More advanced – more stable Requires: low power, quiet digitisers. Low cost short range optical drivers. Likely, requires custom chips No option to upgrade digitisers.

10. Potential AA-low station: Safe solution: • Single element Analog to clustered digitisation Centrally powered elements

11. Station processor Requirements: • High bandwidth in • High bandwidth out • Largely cross connected • Scaleable at various levels • Programmable beamforming

12. Station size • For a given total Aeff the collector cost is roughly constant • Increasing overhead with many smaller stations • Possibly increasing station processor complexity with large arrays • Total data rate to correlator for a fixed survey speed remains constant whatever number of stations BUT • Correlator and central processor demands become more challenging Station size largely determined by central processing costs

13. Core Design Core for SKA1AA-low becomes virtually fully filled. More so for SKA2. Core “stations” are not separated – there is a “sea” of elements Design options/considerations to be made: Implies interconnected “station processing”, especially for SKA2 • Non-circular “stations” easier? e.g. Square or hexagonal? • Maximising the sensitivity from each element: • overlapping “stations”? • smaller “stations” (how small) with more correlation? • Apodising element density within areas of the core: • Benefit? Save money? Correlation goes up as n2, but incoming data rate is constant

14. AA-low: SKA1 to SKA2 Fixed Aspects • Frequency range • Intrinsic to the design • Element & array electromagnetic design • Front end functionality • Analogue implementation • Unlikely to change • Interfaces • Could bypass a level of interconnect Possible Variables • Station size & configuration • Core layout (getting bigger) • Move element positions.....? • Trenching etc. • Processing technology level • Essential for economical deployment • Upgraded calibration methods • Needs flexible processing system

15. SKA2 System A reminder for AA-mid! A tremendous step forward in SKA2 There must be ongoing development: which will also benefit AA-low

16. Top design decisions...... Urgent (by CoDR April 2011) • Confirm specification in Memo 125: • Frequency range, sensitivity .... Immediate(by CDR, end 2011): • Single or dual element? • Demonstrable element in small array Medium Term (by AAVS1, end 2013) • Station size and configuration • Core configuration • Station system design • digitisation, processing location, data flow