e-VLBI: Creating a Global Radio Telescope via High-Speed Networks

1. e-VLBI: Creating a Global Radio Telescopevia High-Speed Networks Alan R. Whitney MIT Haystack Observatory SLAC Data Management Workshop 17 March 2004

2. Traditional VLBI The Very-Long Baseline Interferometry (VLBI) Technique(with traditional data recording) The Global VLBI Array(up to ~20 stations can be used simultaneously)

3. VLBI Science • 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 Plate-tectonic motions from VLBI measurements • 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 astronomy example

4. 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

5. 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

6. 16-station VLBI correlator at JIVE in The Netherlands(couple of similar installations in U.S.)

7. 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

8. 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!

9. 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)

10. Bossnet 1 Gbps e-VLBI demonstration experiment(October 2002) Future Initial experiment

11. 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

12. 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

13. 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