1. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Design issues and implementation challenges Paul Alexander and Peter Hall

2. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Aim and scope Concentrate on the design issues for SKA 1  SKA 1 AA-low is a (transformational) world leading instrument  Essential to design for SKA 1  Consider how to transition to SKA 2 Identify issues which are independent of detailed design  Then consider issues which drive detailed design Aim is to pose questions that we can aim to make progress on during the course of this meeting  Some questions should be answered General point:  Transition from a research programme to an instrument project means we need to retire questions with an accountable path of how and why the decision was reached.

3. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 SKA-low Excellent learning platforms in pathfinders –LOFAR, MWA,... –Science, engineering, project management, operational lessons Why optimize SKA-low? –Evolving science case Possible new specification optimization –Pathfinders not scalable to SKA-1 e.g. LOFAR x10 > SKA-1 budget –Rapid technology changes Verify or change long-standing assumptions –Cost optimization funds new capabilities More independent FoVs, increased time domain processing,... –Actual SKA site conditions impact SKA-low design significantly

4. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 SKA-low design SKA-low is part of bigger SKA system –Specifications flow from (updated) SKA Design Reference Mission –Performance/cost analysis must be done in SKA design environment –Cost must reflect “total cost of ownership” –SKA environment must capture key AA-lo issues SKA operational model is critical to costing, e.g. –Simultaneity of SKA-low & SKA-mid operations Data transmission, signal + post-processing –Data archiving –Site infrastructure constraints and costs Including energy availability and cost –Support model, and lifetime costs (“maintenance”)

5. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Top level issues Insensitive to detailed design

6. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Basic specifications A low-frequency sparse aperture array with A/T sys of up to 2000 m 2 /K  At what frequency is this optimised (100MHz?) ? Operating at frequencies between 70 and 450 MHz  At what range of frequencies is this optimised  How tight are the constraints both scientifically and technically? Array will be centrally condensed but some of the collecting area will be in stations located out to a maximum baseline length of 100 km from the core  What fraction of the collector is on longer baselines?  How large is the core?

7. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Possible trade-offs (cost constrained design) Built area vs FoV –More area, or more accessible and/or processed FoV? Accessible bandwidth vs sensitivity –Fewer compromises in a narrower band array Accessible bandwidth vs polarization capability, polarimetry performance Processed FoV, bandwidth vs other parameters –Optimum investment in data transmission, DSP, computing –Investment level as a function of time U-V coverage vs other parameters –More stations are costly (e.g. infrastructure, correlation) –Station numbers and size related to calibration strategy (esp. ionospheric)

8. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Frequency range 6.5:1 What frequency range must the array elements be designed/optimised for? Approach 1: Observatory Aim for best “average” or “uniform” response across the frequency range Approach 2: Observatory, but prioritising EoR Design antenna for good performance in EoR frequency range What is the EoR frequency range 70 – 200 MHz? What about foregrounds? Approach 3: EoR instrument with observatory function Optimise design for EoR frequency range Approach 4: Identify the technical difficulties and relax frequency range 100-450 MHz is only 4.5:1

9. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Sky coverage ALMA 45 degree scan 30 degree scan Critical design driver for element Observatory requirement – large sky coverage  lower gain antenna larger scan angle of 45 degrees. What is largest scan angle we would like? Dedicated EoR experiment perhaps require smaller scan angle  higher gain antenna possible Circumpolar limit GC SKA 1

10. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Aside SKA 1 specification is for an amazing instrument ~ 1 order of magnitude in sensitivity ~ 2-3 orders of magnitude in survey speed

11. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Sensitivity requirement Design specification: 2000 m 2 /K fT sky (K)A eff (km 2 ) 100 MHz9882.1 150 MHz3500.70 We will be building approximately a square kilometre of collecting area What sensitivity do we require across the band ? Very dependent on the frequency at which the array becomes sparse Major impact on element design

12. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Tailoring the AA system 100 10 1 100 1000 Frequency (MHz) Sky Brightness Temperature (K) A eff A eff /T sys Fully sampled AA-hi Sparse AA-lo T sky Becoming sparse A eff / T sys (m 2 / K) AA frequency overlap Dish operation f AA f max 10000

13. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 SKA 1 sensitivity model 2000 m 2 /K at 100 MHz T rec = 60 K AA sparse above 150 MHz 2000 m 2 /K at 100 MHz T rec = 60 K AA sparse above 150 MHz

14. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 T sys across the band Matching fT sky (K) 100 MHz988 150 MHz350 180 MHz221 210 MHz150 240 MHz106 400 MHz29 T rec important even at 200 MHz Dominant at upper end of band True low-noise LNAs still important Challenges: “Matching” across the band to ensure T rec dominated at upper end and T sky at lower

15. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 SKA 1 survey speed 2000 m 2 /K at 100 MHz T rec = 60 K AA sparse above 150 MHz N B gives 100 sq degrees across band 2000 m 2 /K at 100 MHz T rec = 60 K AA sparse above 150 MHz N B gives 100 sq degrees across band

16. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Survey speed What survey speed do we require at fixed A eff /T sys ? Direct implication for cost of correlator and post-correlator processing See next section for possible trade off Upgrade path Increasing survey speed is perhaps easiest designed in upgrade path for AA-low

17. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Data rate Data rate and survey speed intimately linked Review basic design equations Re-write in terms of FoV and total collecting area B D N s Stations

18. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 SKA 1 data rates and configuration AA Line experiment 50 AA-low stations 100 sq degrees, 10000 channels over 380 MHz bandwidth  3.3 GS/s Issues What data rate can we process? Trade UV coverage ( N s ) for FoV and hence survey speed (  ) Line vs continuum requirements What is the longest baseline What temperature sensitivity do we need and on what scales  Defines filling factor in the core

19. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 SKA 1 configuration Ideally – do not design in these trade-offs Need to consider evolution of processing capability in designing configuration  Or even repositionable antenna positions?!

20. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Station and element design One or two elements? Many aspects to this – see later How sparse can the station be? Side lobes even for a random configuration when very sparse Complicates imaging, and increases T sys Station size? Increasing D  reduced UV coverage, reduced processing load, less complicated ionospheric model, move DSP from correlator to station B/F

21. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Station design regulartriangularsparse thinnedcircularrandom Embedded element pattern Random minimum  / 2 Random minimum 2 Nima and Eloy

22. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Configuration, station design and SKA 2 Is SKA 1 a subset of SKA 2 Should we compromise the design (and hence science return) of SKA 1 to ease implementation of SKA 2 ? Optimum SKA 1 AA-low core may have f ~ 0.5 D core ~ 1km.  SKA2 AA-low core is larger with f ~ 1  Almost certainly need to reposition elements on SKA 1  SKA 2 Do not compromise design of SKA 1  maximise science return for SKA 1 & accept additional cost in SKA 2

23. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 SKA information and data system Imaging processor Visibility processors Science product archive Local science reduction Science proposal Data product distribution Data routing Collectors Grid science reduction and visualisation Cloud store Correlator Data excision Monitor and Control system M&C database Global and local sky model Calibration loop Observation definition Hierarchical station beam former

24. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Processing – how much and where? For a given sensitivity and survey speed we can decide where and how to do the processing  Beam forming vs correlation  survey speed vs imaging fidelity? Physical location of processing  Physically distribute processing only if it leads to a reduction in data rate

25. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Processing – how much and where? For a given sensitivity and survey speed we can decide where and how to do the processing  Beam forming vs correlation  survey speed vs imaging fidelity? Physical location of processing  Physically distribute processing only if it leads to a reduction in data rate – e.g. Station beamformer

26. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Specific Design and Implementation Issues

27. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Element and communications Can we cover band with a single element?  Where are the compromises?  Can we afford two elements? Where do we digitise  Link, power consumption, lightning protection... What is the communication link?  Cost, calibratability and lightning protection How is the element powered?  Cost, sustainabilty, manufacturability and deployability What is the element assembly and how are they deployed?  Cost, sustainabilty, manufacturability and deployability

28. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Station B/F and correlator Station B/F  What is, and can we meet the power budget with an all digital design?  Do we deploy ASICs in the SKA 1 design? If so what are the timescales for development cycle.  Note cost of Station B/F dominated by number of elements not how they are deployed (e.g. Station size)  Internal station correlation for calibration? Correlator  Is a software correlator possible or desirable for SKA 1 or commissioning?

29. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Post-correlator processing What is our system concept for SKA 1 processing? o Is the post correlator processing a single Peta-scale machine or machine designed for our data flow?  Our problem is highly parallel in places and we could deploy a “UV-processor”  Need to be sure of processing model to go down this route, but can deliver more Flops cheaply  Single-pass algorithms will reduce cost  do we want to restrict ourselves in this way?

30. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Cost control ItemAdvantageChallenge One elementSingle core, one RF chain Adequate performance across band; sparcity at high f Larger station sizeReduce cost of correlator, infrastructure and post- processor Loss of UV coverage ASICs deployed in DSPPower reduction saves on operating budget Time to deployment, commissioning harder? Loss of flexibility Custom processing path Maximise Flops for costLoses flexibility

31. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 SKA-low implementation challenges Low capital cost –N x 100,000 active antennas  integrated, reproducible –Strong incentive to incorporate Design for Manufacture early in development cycle Low operating cost –Easily dominates capital cost over life of SKA –Reliability and maintainability are crucial Probably dominant aspect of designing “outdoor” portion SKA-low –Robust system is essential Damage limitation strategies (lightning etc), intelligent and resilient processing Low deployment cost (next slide) Data processing and archiving prominent in SKA Observatory plan EMC –SKA-low is especially vulnerable to poor EMC practices, or poor site management with respect to RFI

32. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010 Deployment challenge 300,000 elements (or tiles) deployed over 2 years –1 element/tile every minute! Connectivity and commissioning need to keep pace with deployment Parallel, industrialized deployment needed –… and during pre-construction Substantial site specific and environmental issues “Design for deployment” essential –Results in highly modular, maintainable design

33. Paul Alexander, Peter Hall Design Issues and Implementation ChallengesAAVP 2010