LOFARLOFARThe Low Frequency ArrayThe Low Frequency Array

Shep DoelemanLOFAR Group

What is LOFAR?What is LOFAR?

• Major new array for 10-240 MHz range

• 400 km across, fixed dipole receptors

• Fully digital, all-sky coverage, extreme agility

• Planned initial operation in 2006

• Three-way collaboration

– ASTRON, in Dwingeloo, Netherlands

– Naval Research Lab (Remote Sensing Div.), Washington

DC

– MIT/Haystack

When and Where?When and Where?

• What is the timeline?– Target for initial operations – 2006

– Target for full operation – 2008

– Instrument lifetime – decades, with upgrades/refinements

• Preliminary Design Review, June 3-5– All systems except antennas and receiver

– Delta-PDR scheduled for early September

• Site selection– Three candidate sites: Netherlands, SW USA, W. Australia

– Announcement of preferred site expected by September

– Decision needed to permit focused design

• Subsystem Critical Design Reviews– Second half of 2004

– System-level CDR in late 2004

SKA/LOFAR Common ChallengesSKA/LOFAR Common Challenges• Large Bitrates over long distances.

– 10’s Tb/s over 1000’s km vs. 100’s Gb/s over 100’s km

• x100 collecting areas from previous generation. • Use of phased arrays as stations.• Complex siting issues: large geographical area.• Wide science base: scale free configurations.• Large frequency range: will require multiple

elements. • Calibration challenges: ionosphere, station beam

– New direction dependent calibration formalism required

• RFI mitigation, nulling.• Requires powerful central processor for analysis

of full FOV.• New Scheduling Paradigm: multiple beams,

subnetting, remote operation.

LOFAR:

Overall layout 400 km diameterRemote operations centers

One LOFAR station, ~150 meters

Array layout

~130 antennas generate 250 Gbit/sec

Filtering and beamforming reduces this to ~2 Gbit/sec

Outer 3/4 of stations create ~150 Gbits/sec aggregate

Central core, 2km, ~3300 antennasAggregate data rate ~ 6 Tbit/sec

Redshifted HI from the Epoch of Reionization

High-z starbursts

Galaxy clusters and the IGM

Cosmic ray distribution, and airshower radio bursts

Steep spectrum and fossil radio galaxies

Supernova remnants and ISM energy budget

Interstellar recombination lines

Nearby pulsars, ghost nebulae

Extrasolar gas giant planetary radio emission

Stellar flares

Interstellar medium propagation effects

Transients, GRB and LIGO event counterparts, buffering

Solar radio studies

CME detection, mapping by IPS, scattering

Extremely high resolution ionospheric tomography

Passive Ionospheric Radar

Tozzi et al. (2000)

Courtesy: B. Gaensler

Courtesy: B. Jackson

Lane et al. (2001)

Scientific VersatilityScientific Versatility

Low frequency antennas: 10-90MHzLow frequency antennas: 10-90MHz

• Inverted-V shaped dipole– Electrically short design

• Possible because sky noise dominates

• Broadband

– Simple, cheap, robust– Comes in 2 sizes, LBL and LBH

1 sq. km. equiv. at 17MHz

PrototypingPrototyping

• THETA (10 single polarized LBH elements)– Sky noise dominated in 40-

80 MHz band

High Freq. Antenna Array:120-240MHzHigh Freq. Antenna Array:120-240MHz

• Nominal 4x4 crossed dipole array• Electronic analog beamforming/steering

– PC board switched delay lines (used before at 74 MHz)

• Low cost is main challenge

Single 4x4 unit

Multiple units may be butted together for economy and performance

LOFAR CalibrationLOFAR Calibration• Maximize ratio of knowns/unknowns

– Make lots of independent measurements

– Know as much as possible about sky a priori

– Know the instrument (e.g. response to environment)

– Solve for smooth functions wherever possible

– Gather a priori knowledge of ionosphere (e.g. GPS)

• Bootstrapping approach to selfcal

– Develop solutions for strong sources & subtract

– Interpolate and improve coherence for weaker sources

– Develop solutions for weaker sources & subtract

– Fully position dependent across the Field of View

“Peeling” – prototyping in progress

All Sky MonitorAll Sky Monitor

• Run 100% of time in the background• Make ~1000x1000 pixel map of sky every 0.5 sec

– Full cross-correlation of 3200 antennas– 5.4 million baselines– Full field of view

• Integrate on wide variety of timescales• Search for transient events, generate triggers

• React to triggers– Re-point one or more LOFAR beams– Freeze and download the data buffer

Huge discovery potential

WAN ImplementationWAN Implementation

• Viable and affordable technologies identified• Requirement vary across array

– Implementation matched to specific needs– Inexpensive, very high bandwidth for short-haul

• Strong emphasis on mass-market components

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1 6 SM F fib ersD C F

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tPADPAD PAD

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Central ProcessorCentral Processor• ~1000 nodes in a 3D switching fabric• Bandwidth 2 Gbit/sec point-to-point

Central Processing ComparisonsCentral Processing Comparisons

• In imaging mode, LOFAR requires ~40Tflops.• LOFAR CEP very flexible – calibration dominated.• Current LOFAR CEP cost: $16M.

• SKA (Large N – Small D) requires ~ 10 Peta Flops• Difference of 10-12 years of Moore’s Law

• But, LOFAR All Sky Monitor will achieve ~100Tflops and cost only $1M

• So, targeted computation can be done at lower cost.

SummarySummary

• LOFAR is a complete ‘first step’ towards SKA.

• May in some ways be more challenging than SKA.

• Will offer lessons on useful timescales.