e-VLBI at the NRAO
R. Craig Walker
NRAO, Socorro NM
Connecting the Global VLBI Array in the
New Era of High-Speed Networks
Haystack, April 2002
INTRODUCTION
VLBA Upgrades (Desirable, but no definite plans or budgets):
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Recording system upgrade
Connect current systems and correlator at ~1 Gbps
Long term integration with EVLA at up to 96 Gbps
Real time data link projects at NRAO:
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Pie Town Link: In active use. Analog, 200 MHz.
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ALMA and EVLA: Funded projects
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~20 km. 96 Gbps. Similar technology for both.
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Observatory owned and installed fibers. No regeneration.
Technology not directly applicable to VLBI.
NMA: ~8 new antennas for EVLA. To be proposed.
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eVLBI April 2002
~300 km. 96 Gbps.
Phone company fibers.
Technology partially applicable to VLBI.
Craig Walker
The
VLBA
Ten 25m Antennas,
20 Station Correlator
327 MHz - 86 GHz
National Radio
Astronomy Observatory
A Facility of the
National Science
Foundation
WHY CONSIDER REAL
TIME FOR THE VLBA?
• Recording system forces large operations cost
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Separate correlator operations group
Correlator job preparation and “sniffing”
Shipping and tape library
Maintenance of many systems (about 50 now)
Tape changing at sites
• Real time gives more robust operation of instrument
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Real time feedback on performance
No lost or damaged tapes
No marginal playback
Recordings have always been the weak link in VLBI
More interferometer testing (pointing etc)
• A switch to real time is operations rather than science driven
– But may help geodesy (rapid EOP) and observations of transients
• But give up multi-pass processing unless data are buffered
eVLBI April 2002
Craig Walker
POSSIBLE INITIAL VLBA
REAL TIME SYSTEM
• Direct replacement for recording system
– Or use recording system for short term buffer
• Use commercial fiber networks and data formats
• Use current correlator
– 512 Mbps max per station, 20 stations (1 or 2 OC12 lines)
– 1 Gbps for 10 stations
• Partial step - move some recorders to correlator
• Expect mixed recording and real time operation for
some time
eVLBI April 2002
Craig Walker
REAL TIME VLBA
CONSIDERATIONS
• Network backbones can handle even EVLA bandwidth
– Some doing 1.6 Tbps per fiber (many wavelengths)
– But costs would be prohibitive now. Going down
• Last mile problems for VLBA not too bad
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PT, NL, BR, VLA have fibers at site
KP, MK have fibers on the mountain
HN, FD, SC, OV, LA have fiber within 3-10 km
Others like GBT and Arecibo might be a problem
• May need to deal with many companies
• Need plan to deal with global observations
– Need international standards
– Need access to intercontinental lines
eVLBI April 2002
Craig Walker
REAL TIME VLBA
QUESTIONS
• Cost and access are likely to be the problems, not
technical feasibility.
• What is best model for acquiring access to the fiber?
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Leased bandwidth
Leased fiber
“Customer owned fiber”
Other
• When will the cost be affordable?
– Standard leased bandwidth costs would be way to high now
– Special deals may be possible
• VLBI has relaxed requirements for error rates and down time
• Only need wide band in one direction
eVLBI April 2002
Craig Walker
EVLA & ALMA
DATA
TRANSMISSION
• EVLA and ALMA will use the same system
• Fibers to be installed by the observatory.
• Data rate is 96 Gbps
– 16 GHz bandwidth, 32 Gsamp/s, 3 bit/sample
• OC192 (10 Gbps) transmitters and receivers
• Will use 12 channels
• Special formatters and data format
– No error correction
• Single channel prototype working in lab
• Not appropriate for commercial networks
eVLBI April 2002
Craig Walker
NEW MEXICO
ARRAY
• Extend the VLA resolution by a factor of 10.
– Part of the EVLA completion phase
– This phase of the EVLA project is not yet funded
• About 8 new antennas with baselines 35-350
km.
– Will also use the VLBA LA and PT antennas.
• Will use telephone company fibers
– Small rural companies – several
– May lease just fiber. NRAO to provide
regenerators etc. Cost appears to be reasonable.
• Will need 16 Sonet OC192 channels for 96
Gbps plus overhead.
– Will not need the high availability and low error
rates required by most internet users
• Technology may be appropriate for VLBI
eVLBI April 2002
Craig Walker
VLBA/EVLA INTEGRATION
• NMA fills the gap in uv spacings between VLA and
VLBA
• Some VLBA/EVLA integration is required because 2
VLBA stations will be part of NMA (LA and PT)
– But integration is not yet in the completion phase budget
• EVLA will have 500 times the VLBA current sustained
bandwidth (16 GHz vs 32 MHz)!
– Reasonable integration requires VLBA bandwidth
upgrade.
– More bandwidth would enhance VLBA/NMA science
• Observe microJy sources
• Improved phase referencing with weaker and closer calibrators
• Study thermal sources with high resolution
eVLBI April 2002
Craig Walker
Resolution vs. Frequency
VLBA/EVLA INTEGRATION:
CORRELATOR
• Being built by HIA group in Penticton, Canada
• Basic capabilities:
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32 Stations in EVLA first phase, 40 or 48 later with NMA
16 GHz bandwidth, 3 bit samples
96 Gbps from each antenna
16384 spectral channels at maximum bandwidth
• Can handle VLBI baselines
– Could interface recorders, but prefer real time
– Can trade stations and bandwidth
– Uses VLBI standard bandwidths (N×1 MHz)
• EVLA correlator could serve VLA/NMA/VLBA
– Becomes one large instrument
eVLBI April 2002
Craig Walker
EVLA/VLBA INTEGRATION:
STATION ENHANCEMENTS
• A very wide band data transmission system will
require upgrades in other areas
• EVLA/NMA will have complete frequency coverage
over 1-50 GHz and will use 8 GHz in each polarization
• VLBA receivers and IFs more limited, but better than
current VLA
• At 1 GHz bandwidth, could sample current VLBA IFs
and use current receivers
• For more bandwidth, must upgrade LO/IF systems and
some receivers
eVLBI April 2002
Craig Walker
SUMMARY
• Real time links would be a boon for VLBA operations.
– Could use up to 1 Gbps with current correlator etc.
• NRAO is developing very wide band links for distances
up to a few hundred km for NMA.
• The NMA will require much of the same technology
needed for wideband eVLBI.
• Eventually the EVLA and VLBA must be integrated.
– Links of 16-96 Gbps needed.
– Must be compatible with rest of global VLBI
• The big question: How do we do it at an affordable cost in
a reasonable time scale?
eVLBI April 2002
Craig Walker