1. AAVP Workshop December 2010 SKA 1 AA-low design

2. AAVP Workshop December 2010 SKA 1 AA-low design SKA 1 AA-low is a major System: >10x LOFAR sensitivity >1 km 2 collecting area >6:1 frequency range 70 – 450MHz ~50 deg 2 Field of View Very quiet site SKA 1 AA-low is a major System: >10x LOFAR sensitivity >1 km 2 collecting area >6:1 frequency range 70 – 450MHz ~50 deg 2 Field of View Very quiet site Required in SKA Memo 125

3. AAVP Workshop December 2010 SKA 1 AA-low design But........ It is also part of a bigger vision: SKA 2 Some implications....

4. AAVP Workshop December 2010 SKA 1 AA-low design Priority SKA 1 AA-low System requirements Stand alone instrument meeting SKA 1 science goals Deployable 2016 onwards Specification will mesh with SKA 2 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) SKA 1 performance must not be compromised?

5. AAVP Workshop December 2010 SKA 1 AA-low design Element requirements FeatureRequirementsComments ElectromagneticsFrequency range, beampattern, part of an array It must work properly! Quickly deployable Simple10,000 per array..... ManufacturableEasy, repeatable, cheapEssential for cost and reliability TransportableIdeally “flat pack”The volume (m 3 ) is huge ReliableFailure rate must be very lowMTBF of 100,000 hrs → ~1 element/array/day “Low” costFrom aboveOf course... Low RFINegligible radiation Low Power~100-200mWEssential for solar UpgradingDeployed elements cannot be upgraded Additions functionally equivalent Ideally avoid any modifications

6. AAVP Workshop December 2010 SKA 1 AA-low design One or two elements? One elementTwo elements CostMinimisedHigher Doubles # RF chains, ground planes and interconnects Element performanceSome compromise across band Less compromise across band Station designForces highly sparse design at top-end of band Reduces sparcity while maintaining sensitivity PowerMinimises powerIncreases power Filling factorGood filling factor at low f Poor filling factor unless multiple cores MatchingChallengingEasier

7. AAVP Workshop December 2010 SKA 1 AA-low design LWA element: mechanical example Simple “skeleton” elements (delivered flat) Cheap mesh groundplane Single pole fixing – just sunk into ground Electronics at top – well away from floods etc. Clamp type rotational adjustment Easy and quick deployment Buried cables

8. AAVP Workshop December 2010 SKA 1 AA-low design Integrated AA-low element possibility Elements only connected by glass....

9. AAVP Workshop December 2010 SKA 1 AA-low design Where to digitise? At elementAt processor RFIMaximisedMinimised Phase stabilityLNA & Filters + Clock distribution LNA + Filters + Second stage Gain + Cables Data transportDigital possibly over fibre RF over copper PowerAt element or over copper Over RF cable or at element Lightning protectionCan be good if link is fibre Can be challenging if link is copper BandpassVery goodEqualization after transport Cross talkMinimised between elements and polarizations Dependent on screening and design of RF boards 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. AAVP Workshop December 2010 SKA 1 AA-low design Potential AA-low station: Safe solution: Single element Analog to clustered digitisation Centrally powered elements

11. AAVP Workshop December 2010 SKA 1 AA-low design Station processor Requirements: High bandwidth in High bandwidth out Largely cross connected Scaleable at various levels Programmable beamforming

12. AAVP Workshop December 2010 SKA 1 AA-low design Station size For a given total A eff 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. AAVP Workshop December 2010 SKA 1 AA-low design Core Design Core for SKA 1 AA-low becomes virtually fully filled. More so for SKA 2. Core “stations” are not separated – there is a “sea” of elements Design options/considerations to be made: 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? Implies interconnected “station processing”, especially for SKA 2 Correlation goes up as n 2, but incoming data rate is constant

14. AAVP Workshop December 2010 SKA 1 AA-low design AA-low: SKA 1 to SKA 2 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. AAVP Workshop December 2010 SKA 1 AA-low design SKA 2 System A reminder for AA-mid! A tremendous step forward in SKA 2 There must be ongoing development: which will also benefit AA-low

16. AAVP Workshop December 2010 SKA 1 AA-low design 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 AAVS 1, end 2013) Station size and configuration Core configuration Station system design –digitisation, processing location, data flow