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VLBA Operational Status

VLBA Operational Status. Ongoing and Possible Future Upgrades. Walter Brisken. NRAO, 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

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VLBA Operational Status

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  1. VLBA Operational Status Ongoing and Possible Future Upgrades • Walter Brisken • NRAO, Socorro

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. Wideband C-band Receiver Performance

  9. 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…

  10. 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

  11. 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!

  12. 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

  13. 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

  14. Back-up slides to follow…

  15. VLBA Observing time • Total Hours Used: 200820092010 • 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

  16. 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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