1. Atacama Large Millimeter/submillimeter Array Expanded Very Large Array Robert C. Byrd Green Bank Telescope Very Long Baseline Array 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 2

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 3

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 4

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 5

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 6

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 7

8. Wideband C-band Receiver Performance 8

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… 9

10. Other Possible VLBA Upgrades I. 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 10

11. Other Possible VLBA Upgrades II. 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! 11

12. Other Possible VLBA Upgrades III. 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 12

13. Other Possible VLBA Upgrades IV. 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 13

14. Back-up slides to follow… 14

15. VLBA Observing time Total Hours Used:200820092010 – Hours observed:309335724678 – Mean data rate:254 Mbps282 Mbps344 Mbps – Hours normalized to 256 Mbps:306939346290 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 15

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 H 2 O maser disks Determine central black-hole masses 16