e-VLBI: Creating a Global Radio Telescope
via High-Speed Networks
Alan R. Whitney
MIT Haystack Observatory
SLAC Data Management Workshop
17 March 2004
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)
VLBI Science
Quasars, hotspots, polarization
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
Chautauqua 2001
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
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 higherpriority 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)
Bossnet 1 Gbps e-VLBI
demonstration experiment
(October 2002)
Westford
Haystack
(correlator)
Future
Initial experiment
USNO
(correlator)
NASA/GSFC
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