e-VLBI: Creating a Global Radio Telescopevia High-Speed Networks

Alan R. Whitney

MIT Haystack Observatory

SLAC Data Management Workshop

17 March 2004

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 (resolve dimples on golf ball at 3,000 miles)• 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•Earth-rotation measurements important for military/civilian navigation•Fundamental calibration for GPS constellation within Celestial Ref Frame•Study of Earth’s interior

VLBI Science

Plate-tectonic motions from VLBI measurements

Mark 4 Tape-Based 1-Gbps VLBI Data System (some still in use)

• expensive system (~$200K/transport)

• expensive special media (~$2/GB)

• unreliable

• slow random access to data

Mark 5 VLBI Disk-Based VLBI Data System(rapidly replacing tape systems)

• Developed in collaboration with Conduant Corp (Longmont, CO) • 1 Gbps continuous recording/playback to/from set of 8 inexpensive (ATA) disks

• Optimized for uninterrupted real-time recording and playback

• Two removable ‘8-pack’ disk modules in single low-cost 5U chassis• With currently available 250GB disks, capacity of single ‘8-pack’ is 2.0TB• Expect ~8TB/’8-pack’ by ~2005

• ~80 Mark 5 systems now installed at stations and correlators around the world• ~500 ‘8-pack’ modules currently in service (4000 disks); increasing rapidly

16-station VLBI correlator at JIVE in The Netherlands

(couple of similar installations in U.S.)

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 exceeds disk-recording capability(practical continuous recordable data rate limited to a few Gbps)

• Rapid processing turnaround– Astronomy

• Ability to study transient phenomena with feedback to steer observations

– Geodesy• Quick feedback for measurements of Earth orientation in space, particularly UT1,

which is important for high-precision military and civilian navigation

• Also several practical advantages– Eliminate media costs – Automated operation– Remote performance monitoring

e-VLBI Data Rates and Volume –just for 10-station U.S.-based VLBA

Short-term needs (for next 2-3 years)– Continuous 1 Gbps/station ~10 TB/station/day ~3 PB/station/year

– 10 U.S. stations (VLBA) ~100 TB/day ~30 PB/year

Medium-term projection (~4-6 years)– Continuous 10 Gbps/station ~30 PB/station/year

– 10 U.S. stations (VLBA) ~300 PB/year

Longer-term projection (~7-10 years)– Continuous 100 Gbps/station ~300 PB/station/year

– 10 U.S. stations (VLBA) ~3 EB/year

Adding global stations will add significant additional requirements!

Special characteristics of e-VLBI data

• Tolerant to random short-term data losses up to few percent of total– Can use ‘less-than-best-effort’ service (i.e. non-interference with higher-

priority applications)

• Temporary buffering at both station and correlator (up to a few hours if necessary) may be employed to overcome slow or overloaded networks

• Raw data are discarded after correlation processing– Data volume is reduced by factor 103-106 after correlation processing

(must be archived)

Haystack

Westford

NASA/GSFCUSNO

(correlator)

(correlator)

Bossnet 1 Gbps e-VLBI demonstration experiment

(October 2002)

Initial experiment

Future

Current e-VLBI activities

• U.S./Japan experiments conducted on ~monthly basis– Files exchanged over Abilene/GEMnet networks

– Data rates to 900 Mbps

– Typical transfer size - 500 GB; will ramp up to several TB

• Hawaii/Germany daily experiments– Daily earth-orientation measurements

– Typical transfer size – 50 GB

• Several international experiments up to 1 Gbps/station are planned for 2004

• Data-transport protocols that take advantage of these special e-VLBI characteristics are now being developed at MIT with support from NSF

Biggest problem

• ‘Last-mile’ connectivity to telescopes– Most telescopes are deliberately placed in remote areas

Intensive e-VLBI initiatives are underway in Europe and Japan –U.S. is currently lagging

Summary

• Disks are filling VLBI needs in short term, but are limited for future requirements

• There is rapid international movement to e-VLBI to meet a real science need

• ‘Last-mile’ problem poses biggest current obstacle; progress being made

• Unique nature of e-VLBI data presents opportunities to make efficient use of high-speed networks on ‘less-than-best-effort’ basis

• e-VLBI drives an innovative IT research application with inherently strong international collaboration and cooperation