Atacama Large Millimeter/submillimeter ArrayExpanded Very Large Array
Robert C. Byrd Green Bank TelescopeVery Long Baseline Array
VLBA Operational StatusOngoing and Possible Future Upgrades
Walter BriskenNRAO, Socorro
Current capabilities of the VLBA
• 45 baselines from 10 identical 25m antennas• Peak / sustainable record rate is 512 Mbps
– 64 MHz/polarization with 2-bit samples• Operates at select bands between 330 MHz and 90 GHz
– Dual band 2.2 / 8.4 GHz supported• 1-hour continuum sensitivity at 8.4 GHz = 54 Jy RMS• Angular resolution at 8.4 GHz is 1x2 mas• Absolute astrometry to a precision of 100 as• Relative astrometry to a precision of 10 as• Flexible correlation with DiFX
– Can correlate at ~700 Mbps x 10 antennas for typical projects
– 16 playback Mark5 units available– Transient processor
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VLBA Sensitivity Upgrade Project(http://www.vlba.nrao.edu/memos/sensi/)
• Goal: increase peak bandwidth to 4 Gbps• Began as a project in 2007• Three major components of the upgrade are:
– New correlator• DiFX software correlator was ultimately chosen
– New data recorder• Mark5C, developed by Haystack/Conduant/NRAO
– New antenna back-end electronics• ROACH FPGA-based Digital Back End (RDBE) in
development• Project funded (including media) to complete 2 Gbps capability
– Funding from NRAO, NSF MRI grant and Conacyt (via UNAM)– Initial availability to astronomers anticipated this summer
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Mark5C recorder• Evolution of Mark5A/B recorder• Records packets transmitted over standard 10 Gb Ethernet• Single module operation demonstrated at 2 Gbps• Dual module operation at 4 Gbps plausible
– Has strong operational downsides– Software for this mode incomplete; mode not yet
testable– Alternatives for > 2 Gbps are being explored
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ROACH Digital Back-End (RDBE)• Collaborative project
– Hardware: CASPER, KAT, NRAO– Firmware/software: Haystack, NRAO
• One rack-mount chassis contains:– Analog step attenuator– Frequency synthesizer– Two samplers operating at 1 Gsps– 1 Roach board (Xilinx Virtex SX95 FPGA, PPC, QDR memory, …)
• Will replace existing baseband converters, samplers and formatters
• First firmware version, a polyphase filterbank, is nearing completion– 16x 32 MHz channels with coarse tuning
• The digital down-converter version will be much more flexible– Up to 8 (maybe 16 ultimately) fully configurable channels
• Two RDBEs will be installed at each antenna
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VLBA Sensitivity Upgrade Status
• First fringes were seen in August 2010– One month after first RDBE fringes achieved by
Haystack• 6 antennas demonstrated at 2 Gbps earlier this month• The stability and feature completeness is improving• 2 Gbps observing capability has been advertised in the Jan
7, 2011 VLBA Call for Proposals • Upgrades to the DiFX cluster are on order to increase
throughput to ~2 Gbps for 10 antennas• Early science to begin late March, 2011• First production science on full array expected at end of
June, 2011
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Wideband C-band Receiver Project• Goal: upgrade the existing 4.6-5.1 GHz receivers
– Main driver: access to the 6.7 GHz methanol maser line for measuring Galactic structure and kinematics (c.f. Reid & Menten)
• Will make use of EVLA technology to extend range from 4.0 to 7.9 GHz
• Down-converter will allow two separate 512 MHz IF pairs to be attached to this receiver
• Major components of this upgrade include– Retrofitted receiver with new Low Noise Amplifiers– New feed– New down-converter– Upgraded (EVLA-based) monitor and control system
• First prototype to be installed in summer 2011• Project to be completed in summer 2012
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Wideband C-band Receiver Performance
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Fiber Links to Pie Town and Mauna Kea
• USNO is funding installation and operation of fiber links to PT and MK to the USNO correlator in Washington
• Purpose is rapid eTransfer (non-real-time) of daily UT1-UTC data– Links will be too slow (1 Gbps shared link) to replace
data recorders• Baseline plan includes one USNO Mark5C unit per station
– Alternatives that will be mutually beneficial are being discussed
• David Boboltz to say more…
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Other Possible VLBA UpgradesI. New capabilities
• Ka-band receivers (costed at $1.6M)• Installation of additional dichroics for dual-band operation
– K/C– Ka/X
• Develop/install an ultra-wideband receiver• New sub-GHz feeds+receivers
– NRAO & NRL are exploring options for EVLA– Is there VLBI demand for this?
• Pie Town-EVLA link– Enhanced version of previous capability
• Wide-band existing receivers– 22 GHz (K-band) for high-z water maser work
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Other Possible VLBA UpgradesII. Sensitivity enhancements
• Upgrade the data transmission system beyond 4 Gbps– Astro 2010 white paper set goal of 32 Gbps (4 GHz/pol)
• Implement burst mode• Upgrade the Low Noise Amplifiers of the 43 GHz (Q-band)
receivers• Complete holography to improve the dish surface for 86
GHz (W-band)• Install a 86 GHz receiver on Hancock• Increase number of stations!
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Other Possible VLBA UpgradesIII. New calibration equipment
• Complete installation of dual-frequency GPS receivers– 5 sites remaining
• Develop & install water vapor radiometers– By default this will await such developments on the
EVLA• Installation of 6 m to 12 m diameter phase reference
antennas at each VLBA site– Increases on-source time– Multiple antennas would provide further capabilities
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Other Possible VLBA UpgradesIV. Infrastructure improvements
• Widen the analog intermediate frequencies (IFs)– Major overhaul to the LO/IF system– Required to transcend the 8 Gbps limit (4 Gbps limit for
most bands)– Not cheap, but possibly synergistic with SKA development
• Complete the migration to the EVLA monitor and control system
• Upgrade the antenna control unit• Acquire a spare hydrogen maser• Array-wide eVLBI
– Not expected to be affordable for most kinds of work– Needed only for a very limited set of applications
• Further increase media pool
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Back-up slides to follow…
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VLBA Observing time• Total Hours Used: 2008 2009 2010
– Hours observed: 3093 3572 4678– Mean data rate: 254 Mbps 282 Mbps 344 Mbps– Hours normalized to 256 Mbps: 3069 3934 6290
• What is the bottleneck? (1 year ~ 8700 hours)– Media and correlator throughput currently limit total throughput– Maintenance and test time have impact on available time– Dynamic scheduling does not lend itself to 100% efficiency
• Many projects demand good weather at many sites• Many projects must be run at a predefined time and/or day• However, scheduled projects produce high quality data
– Scheduling efficiency is improving
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VLBA Key Science Projects• Projects addressing unusually important questions assigned KSP
status– Improved scheduling priority & data quality scrutiny– KSPs are carefully considered when making policy decisions
• Want to promote and preserve the best science• Some example KSPs (mostly astrometric)
– Galactic maser astrometry to yield structure of the Milky Way• A strong impetus behind the C-band receiver upgrade
– Stellar astrometry in star forming regions in Gould’s belt• Yields multiplicity of systems, distances (hence luminosity),
3-D structure of star forming region– An accurate distance to the Pleiades star cluster
• To resolve important, long standing dispute – Megamaser cosmology project
• Distances to galaxies with Keplarian H2O maser disks
• Determine central black-hole masses
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