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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