e-VLBI: A Brief Overview

Alan R. WhitneyMIT Haystack Observatory

Traditional VLBIThe Very-Long Baseline Interferometry (VLBI) Technique(with traditional data recording)

The Global VLBI Array(up to ~20 stations can be used simultaneously)

Chautauqua 2001

Quasars, hotspots, polarization

VLBI astronomy example

ASTRONOMY• Highest resolution technique available to

astronomers – tens of microarcseconds• Allows detailed studies of the most distant

objects

GEODESY• Highest precision (few mm) technique

available for global tectonic measurements• Highest spatial and time resolution of

Earth’s motion in space for the study of Earth’s interior

•Earth-rotation measurements important for military/civilian navigation•Fundamental calibration for GPS constellation within Celestial Ref Frame

VLBI Science

Plate-tectonic motions from VLBI measurements

Scientific Advantages of e-VLBI

• Bandwidth growth potential for higher sensitivity– VLBI sensitivity (SNR) proportional to square root of Bandwidth

resulting in a large increase in number of observable objects(only alternative is bigger antennas – hugely expensive)

– e-VLBI bandwidth potential growth far exceeds recording capability(practical recordable data rate limited to ~1 Gbps)

• Rapid processing turnaround– Astronomy

• Ability to study transient phenomena with feedback to steer observations– Geodesy

• Higher-precision measurements for geophysical investigations• Better Earth-orientation predictions, particularly UT1, important for

military and civilian navigation

Practical Advantages of ‘e-VLBI’

• Increased Reliability– remove recording equipment out of field– remote performance monitor & control capability in near real-time

• Lower Cost– Automated Operation Possible

• eliminates manual handling and shipping of storage media– Near Real-time Processing

• forestalls growth of storage-capacity requirements with bandwidth growth – Elimination of recording-media pool (millions of $’s!)

• Avoid unexpected media-shipping interruptions and losses

Elements of e-VLBI Development

• Phase 1: Develop eVLBI-compatible data system– Mark 5 system development at MIT Haystack Observatory being supported by

NRAO, NASA, USNO plus four international partners– Prototypes now deployed in U.S. and Europe

• Phase 2: Demonstrate 1 Gbps e-VLBI using Bossnet(w/ DARPA and NASA support)

– ~700km link between Haystack Observatory and NASA/GSFC– First e-VLBI experiment achieved ~788Mbps transfer rate

• Phase 3: Establish adaptive network protocol(newly awarded NSF grant to Haystack Observatory; collaboration with MIT Lab for Computer Science and MIT Lincoln Laboratory);

– New IP-based protocol tailored to operate in shared-network ‘background’ to efficiently using available bandwidth

– Connect with telescopes worldwide (U.S., Europe, Japan)

Mark 5 VLBI Disk-Based Data System (Phase 1)

• 1 Gbps continuous recording/playback to/from set of 8 inexpensive (ATA) disks• Developed at MIT Haystack Observatory with multi-institutional support• Mostly COTS components• Two removable ‘8-pack’ disk modules in single 5U chassis• With currently available 200GB disks – capacity of single ‘8-pack’ 1.6TB; expected to increase to 2.5TB by early 2003 at cost of ~$1/GB• GigE connection for real-time and quasi-real-time e-VLBI operations• Inexpensive: <$20K• ~20 Mark 5 systems now installed at stations and correlators

Haystack

Westford

NASA/GSFCUSNO

(correlator)

(correlator)

Bossnet 1 Gbps e-VLBI demonstration experiment(Phase 2)

Initial experiment

Future

WS/Mark 5

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Figure 1: e-VLBI Path - Haystack to ISI-E

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MITCampus

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Scheduling onBossnet calendar

Schedule to becoordinated with LL

Schedule to becoordinated with LL

Dedicated -no scheduling

Dedicated -no scheduling

Details of path from Haystack to ISI-E – work in progress!

Trans-ponder

ISI-E (Tom)

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Figure 2: e-VLBI Path - ISI-E to GSFC/GGAO

JuniperM40

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GSFC/Bldg 28 (Pat) GGAO/Bldg 202 (Pat) GGAO Antenna Trailer (Bill)

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Preferred e-VLBI path,but may not be in place.Pat working with Jerry and Dan.

Other users ( no control)

No jumbo frames

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Plan to update toOC-48 ~Jan 02

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On order MAC on orderdue Jan-Feb 02

Dell Linux currently at Haystack willalso be available here

1600 ft of fiberto be installedJan 02

Other users(no control)

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Yellowdog Linux;need details

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Eventually replacedwith Mark 5 unit

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Scheduling onBossnet calendar

No scheduling;sufficient capacityfor e-VLBI

Scheduling necessaryif this path used

Will try to scheduleeither K2 or K3exclusively fore-VLBI experiment

Dedicated - noscheduling necessary

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Details of path from ISI-E to GSFC Antenna – work in progress

Performance test results – Haystack/GGAO

Average sustained rate >900 Mbps over 10 hours

Westford-GGAO e-VLBI results

• First near-real-time e-VLBI experiment conducted on 6 Oct 02– Recorded data at 1152 Mbps on Westford-GGAO baseline– GGAO disk-to-disk transfer at average 788 Mbps transfer rate– Immediate correlation on Haystack Mark 4 correlator– Nominal fringes observed

• Direct data transfer experiment conducted on 24 Oct 02– Direct transfer of GGAO data to disk at Haystack at 256 Mbps– Immediate correlation with Westford data– Nominal fringes observed

• Next step – full real-time e-vlbi– Mark 5 system is capable of transmitting in real-time– But, still need additional work on correlator software to synchronize

correlator operation to real-time– Hope to conduct first experiment in early 2003

• Conclusion– e-VLBI at near Gbps speeds over ordinary shared networks is possible

but still difficult

Westford-to-Kashima e-VLBI experiment

• Westford/Kashima experiment conducted on 15 Oct 02– Data recorded on K5 at Kashima and Mark 5 at Westford at 256 Mbps– Files exchanged over Abilene/GEMnet networks

• Nominal speed expected to be ~20 Mbps, but achieved <2 Mbps for unknown reasons - investigating

– File formats software translated– Correlation on Mark 4 correlator at Haystack and PC Software correlator

at Kashima– Nominal fringes obtained– Further experiments are anticipated

Plans for UT1 Intensive e-VLBI• Daily ~1 hour VLBI sessions between Kokee Park, Hawaii and

Wettzell, Germany are used for UT1 measurements• Data are time sensitive since they are used for predicting UT1• Currently requires ~4 day turnaround shipping media• These measurements are an ideal candidate for routine e-VLBI

– Short daily session collect <100 GB of data– Even 100 Mbps will allow transfer in a few hours

• Work now in progress to make necessary connections– Network being organized from Kokee Park to USNO;

connection speed OC-3– Data from Mark 5 system in Wettzell will be carried to Univ. of

Regensberg, about 1 hour drive; connection speed OC-3– Negotiations ongoing for extension of MAX network to USNO with GigE

connection

New IP Protocols for e-VLBI (Phase 3)

• Based on observed usage statistics of networks such as Abilene, it is clear there is much unused capacity

• New protocols are being developed to utilize networks in ‘background’ mode for applications such as e-VLBI– Take advantage of special characteristics of e-VLBI data– Will ‘scavenge’ and use ‘secondary’ bandwidth– Will give priority to ‘normal’ users– Requires a new ‘end-point adaptive strategy’

• Work being carried out by MIT Haystack Observatory in collaboration with MIT Laboratory for Computer Science and MIT Lincoln Laboratory– 3-year program; will demonstrate e-VLBI connections both nationally and

internationally

Typical bit-rate statistics on Abilene network

Conclusion: Average network usage is only a few % of capacity

Usage >20Mbps less than 1% of the time

100 500 Mbps

0.1

1.0

0.01

0.001

Typical distribution of heavy traffic on Abilene

Conclusion: Heavy usage of network tends to occur in bursts of <2 minutes

secs200 400 1000

1.0

0.9

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0.7

<10% of ‘bulk’ transfers exceed ~100 secs

Impact of e-VLBI Program

• Opens new doors for astronomical and geophysical research.

• Represents an excellent match between modern Information Technology and a real science need.

• Motivates the development of a new shared-network protocol that will benefit other similar applications.

• Drives an innovative IT research application and fosters a strong international science collaboration.