SKA Technology A Quick Overview

1
SKA Technology A Quick Overview

• Peter Hall
• SKA International Project Engineer
• www.skatelescope.org
• Dwingeloo, June 22 2005

2
Outline

• SKA concept
• Technology drivers
• Antennas and wide fields
• An example hybrid SKA solution
• Demonstrators and pathfinders

3
New Technology Radioastronomy
Single Field Instruments
Growth in sensitivity has allowed discovery
rate to match or exceed other branches
of astronomy
(Line is the envelope of many Livingstone Curves)
decade per decade slope
Wide fields buy speed survey sensitivity Multi-f
ields add great flexibility
4
SKA Concept
(About 150 stations)
1015 ops/sec
0.1 Tb/s
Wide fields push up data volumes and rates
(About 2000 antennas)
5
SKA Whats New?

• Sensitivity 100 x biggest existing synthesis
  instruments
• Goal of multi-field (multi-beam) instrument, at
  least at lower frequencies
• Re-use area 2 lt M lt 8 looks feasible
• Operational and science advantage
• Wide field-of-view wide-bandwidth
• Very large data volumes and rates
• FOV 1 deg2 at 1.4 GHz 200 deg2 at 0.7 GHz
• gt 106 dynamic range
• Interference mitigation built into system design

Courtesy M. Kramer, JBO
6
SKA Challenges

• Technology
• Project Management

• Wideband, efficient antennas
• Fast, long-distance, data transport
• High performance DSP computing hardware
• New data processing and visualization techniques
• Evolving science goals
• High levels of technical risk
• International politics
• Possible funding phase slips
• Ambitious delivery timescale
• Industry liaison
• Pre-competitive alliances procurement project
  delivery

Performance Cost
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Cost Reduction Strategies

• Push towards a software telescope
• Exploit convergence of radio and computing
  technologies, replacing hardware with firmware or
  software
• But antennas still account for 30 of array
  total cost
• Exploit emerging technologies
• Irony a number of these contribute to RF
  spectrum congestion and make radio astronomy more
  difficult
• Learn from industry
• Mass production is new to astronomy
• Learn from fast-track demonstrator and
  prototyping projects
• Start simple - plan evolution of SKA capability
• E.g. ultimate signal processing capacity will not
  exist in 2015

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SKA Technology Drivers

• Frequency range
• Field-of-view
• Number of independent fields-of-view
• Balance between survey and targetted instrument
• See EWG whitepaper reviews demonstrator
  evaluations
• www.skatelescope.org

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SKA Antennas

• Range of possible solutions
• Aperture phased arrays
• Flux concentrators (dishes)
• Need at least two antenna types to meet current
  spec
• Cost effective high-frequency solutions dont
  provide enough area at low frequencies
• Want good efficiency at high frequency AND
  multi-fielding (or at least wide field-of-view)
  at low frequency
• The hybrid approach
• SKA concepts have different antennas BUT much
  post-antenna system similarity

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Phased Arrays SKA

• Originally
• Phased FPAs for very large concentrators (dish,
  cylinder) to get reasonable FOV (1 deg2 at 1.4
  GHz)
• Small N concepts
• Aperture arrays with very small RF-phased
  elements (patches)
• Large N concept
• Now
• All of the above
• Wide-field cylinder (gt tens of deg2)
• Small dish (12m) FPA to get wide FOV below 2
  GHz
• (tens of deg2)
• Digital AA concept feasible?
• Phased arrays are (almost) ubiquitous in the SKA
• Central to (almost) all wide-field concepts

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Story So Far

• Concept whitepapers and EWG/SWG reviews
• Rounds 1 and 2
• Demonstrator EWG reviews and ranking
• Including initial risk (performance economic)
  assessment
• New concept combining versatile wide-field
  concentrator with FPA may be attractive
• Concentrator small dish
• Captures some (cost?) benefits of dishes with
  some wide FOV advantages of phased arrays
• In post-antenna SKA system design, wide fields
  can be exchanged for excess bandwidth
• A wide-field LF instrument can fit within
  envelopes defined by the more conventional
  high-frequency concepts
• Recognize compelling case for aperture array
  sub-300 MHz

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Phased Focal Plane Arrays

• Distinguished from multi-feed systems by
• Elements combined in a beamformer
• Element spacing chosen to fully-sample the focal
  field information

• For radio astronomy
• Bandwidth gt21
• Low noise

Overlappingfar fieldbeams
Amplitudeand phase weighting
Focal plane array
Conceptual beamformer architecture
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Plain Persons Viewof FOV Expansion
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Field and Encircled Power Plots
Sz (dB)
Sz (dB)
Power is integrated over circles of increasing
radius from the focus and the corresponding
efficiency is plotted as a function of focal
plane radius
Normalized Encircled Power
Normalized Encircled Power
FPA Radius (?)
FPA Radius (?)
Courtesy Doug Hayman
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Wide Fields

• Wide FOV comes from having many receiving chains
  (rx, sig transport, DSP, )
• 1st order approximation on receiver numbers comes
  from just working out dish dimensions, and number
  of dishes
• FOV is constant for dish FPA scales as l2
  for sparse aperture arrays
• Only aperture arrays (and Luneburg lenses) allow
  independent multi-fielding
• Wide fields ? more information ? more data
  transport processing
• BUT access to a wide FOVdoes not imply all of FOV
  is transported processed
• Important science choice affecting e.g. SKA
  station and long-haul data transport design

Courtesy Doug Hayman
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Wide Fields Technology Challenges

• Low-noise receivers (lots)
• Very low-cost optical fibre transmission
• DSP
• Post-processing
• Distribution, archive

Courtesy Suzy Jackson
Line between DSP and general purpose computers
will be blurred
17
A Hybrid SKA?
gt 2 GHz
Courtesy ASKACC
Via SD/FPA?
Courtesy S. Weinreb, Caltech
lt 2 GHz
Courtesy ASTRON
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One Hybrid SKA

• Frequency range 0.1 to 3 GHz
• Budget remains at 1B /
• Need to design a survey instrument from Day-1
• Biases some resource allocation in design
• Acknowledge the insight of Jaap Bregman
• See forthcoming EXPA papers

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Thumbnail of Instrument

• A sky-noise limited aperture array covering 0.1
  0.3 GHz
• 33 tiles, each, 1.8 m square per aperture (12 m
  dish equiv.)
• Each tile 2 x 2 bow-tie elements spaced at 0.9 m
• 2900/cos(?) deg2 FOV at 0.17 GHz scales with ?2
• 33 beams per FOV multiple FOVs possible
• Const Aeff to 0.2 GHz (dense array)
• Above 0.2 GHz Aeff scales with ?2 (sparse array)
• A small dish/FPA array covering 0.3 3 (??) GHz
• 4000 x 12 m dishes F/d 0.5
• 8 x 8 FPAs (Vivaldi notch elements)
• 3 bands 0.3-0.7 GHz, 0.7-1.6 GHz, 1.6-3.6 GHz
• Aeff/Tsys per beam 9000
• Aphys 452 000 m2 Aeff 272 000 m2 Tsys 30
  K
• Acknowledged issues of FPA co-location or
  switching (translation)

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Thumbnail (2)
0.3 3 GHz
0.1 0.3 GHz
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SD/FPA Visualization
Visualization by Scitech
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Performance Snapshot

• For 0.1 0.3 GHz array
• Aeff 1 km2 at lt 0.17 GHz
• 7 sr sky survey in 1.5 days with 5 hr integration
  per field (reaches thermal noise sensitivity,
  assumes full U,V coverage in 5 hrs)
• For 0.3 3 GHz array
• Aeff/Tsys per beam 9000 (cf 20 000 current SKA
  target
• 25 fractional bandwidth target met or exceeded
• 0.7 GHz survey 2 x 1018 units (cf 1.5 x 1019
  target)
• 1.5 GHz survey 8 x 1017 units (cf 3 x 1017
  target)
• Survey LF sensitivity reduced because of FOV and
  A/T shortfall
• Maybe gain factor of 2 with less conservative BW
  assumptions
• FOV approx frequency independent within each band
• 130 deg2 at 0.7 GHz
• 25 deg2 at 1.5 GHz
• 5 deg2 at 3 GHz

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SD/FPA Remarks

• SD approach is not new
• Allen Telescope Array(6 m dishes)
• See also e.g. Braun (1996) for 6 m, lo freq ,
  spherical ref. proposal
• 6 m dish ? 300 MHz lower limit
• Is SD/FPA more viable than (very)SD at low freq?
• Presumably but depends on calibratability and
  amount of correlation required in SD/FPA
• Demonstrators are key to SKA technology selection
• Expect play-off between aperture array and SD/FPA
  for lt 2 GHz SKA
• Can putative cost benefits of SD/FPA be realized?
• Does the SD/FPA win over just having more
  (smaller) dishes?
• Can maturity of AA be sufficiently demonstrated?
• What are the science trade-offs for each approach?

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Large SKA Pathfinders
Multiple fields-of-view
(350 x 6.1m)
Multiple beams in FOV
ATA
LOFAR

• DSAN (JPL) EVLA (USA) ALMA eEVN (Eu)

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A Few SKA Innovations
Allen Telescope Array
New Dielectric
Interference Mitigation
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Closing Thoughts

• SKA engineering timescales are aggressive
• Credible demonstrators by 2009? Our major
  challenge!
• Demonstrators underway will advance radio science
  astronomy enormously
• Wide-field multi-field requirements are primary
  antenna technology drivers
• Phased arrays are pivotal
• Multi-fielding science drivers addressed only
  superficially so far by astronomers
• Cylindrical reflectors are potentially good at
  all-sky surveys
• Flexibility limitations need urgent consideration
  by SKA science community
• Probably the make or break issue for CR concept
• Calibration issues are significant for SKA
• Wide field calibration is harder, at least with
  current tools
• No-one yet has an astronomically-productive
  phased array demonstrator!
• Neither aperture, 2-D FPA, nor line feed
• Extrapolation from Parkes multi-beam and similar
  augurs well for science