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VLBA: The Very Long Baseline Array

Link to VLBA Web site

I. General project/facility description

  1. Overview of the facility/project
    The VLBA is an instrument devoted to Very Long Baseline Interferometry (VLBI), with 10 antennas distributed throughout the United States in a configuration that optimizes the distribution of baseline lengths and orientations. It is the only such dedicated VLBI user facility in the world. The VLBA has baselines between 200 and 9000 km, which provide angular resolution as fine as 0.1 milliarcseconds at 86 GHz, by far the highest resolution imaging capability available in astronomy. Antenna stations are located in St. Croix VI; Hancock, NH; North Liberty, IA; Fort Davis, TX; Los Alamos, NM; Pie Town, NM; Kitt Peak, AZ; Owens Valley, CA; Brewster, WA; and Mauna Kea, HI. The shorter baselines, and hence the highest concentration of antennas, are near the VLA for optimal joint observations. The antennas are 25 meters in diameter and of an advanced design which allows good performance at 43 GHz and useful performance at 86 GHz. The antennas are operated remotely from the Socorro Array Operations Center (AOC); local intervention is required only for recording media changes, routine maintenance, and troubleshooting. The current recording rate is limited to an average of 128 Mbit/s which fills two tapes every 24 hours. New, disk-based, recorders are being installed that should eventually permit recording rates up to 1024 Mbit/s with media changes once every 24 hours (or longer).

  2. Managing institution and organization
    The VLBA is a facility of the National Radio Astronomy Observatory, managed by Associated Universities Inc. under a cooperative agreement with the National Science Foundation. NRAO is a federally funded research and development center (FFRDC), while AUI is a non-profit university-based consortium. The VLBA is operated from the Array Operations Center in Socorro, NM, while NRAO is headquartered in Charlottesville, VA. AUI offices are located in Washington, DC.

  3. Funding source(s)
    The VLBA is funded almost entirely by the National Science Foundation, through the NRAO Operations budget. Small amounts of funding, on the order of $100,000-$200,000 in a given year, may be received from NASA or other entities for specific projects. For example, in FY04, NASA funded a pilot project for spacecraft navigation using the VLBA, with a total budget allocation of $240,000. Additional funding of $48,000 was received from NASA, plus a comparable value of hardware from the European Space Agency, in order to support tracking of the Cassini Huygens Probe during its descent through the atmosphere of Titan in January, 2005. In 2000, the Max Planck Institut für Radioastronomie supplied $200,000 in funding for hardware to expand the VLBA 86 GHz capability from four to eight receivers. For the period 1993-2002, significant VLBA funding was derived from NASA for support of the Japanese VLBI Space Observatory Program (VSOP). Funding directly to the VLBA for VSOP support peaked at nearly a million dollars annually, tapering off during the last several years of the VSOP mission.

  4. Construction history and cost
    The VLBA was recommended by the 1980 Astronomy & Astrophysics Survey Committee ("Field Committee"). It was the 2nd-ranked new program, and the 1st-ranked ground-based program, in the recommendations for the decade of the 1980s. Construction funding from the NSF began in the mid-1980s at $3M, and continued for nine more years at about $9M/year. This flat funding profile was not ideal, but once it was specified, the VLBA construction project met both the final budget and the final construction schedule. In FY89 Dollars, the final construction cost was $84M.

  5. Operational history and cost
    The VLBA has operated as the world's only full-time, centrally-operated VLBI observatory since 1993. Scientific observations were limited by a correlator backlog until 1996 but since then, the fractional time spent doing scientific observing has been fairly steady at 50% to 55% of the hours in a calendar year, roughly 4500 to 5000 hours per year. This total observing time is limited by maintenance requirements and tape-recording capacity. By comparison, the next most active VLBI user facility in the world is the European VLBI Network, which observes for three 3-week sessions per year, totaling about 1200-1500 hours of scientific observing annually. Most VLBA scientific observing is carried out with the 10 antennas unstaffed locally (except for scheduled tape changes), monitored only by a single telescope operator located in the AOC. This design feature of the VLBA is a unique capability among the antennas used around the world for VLBI.

    The VLBA pioneered the routine use of "phase-referencing" observations in VLBI, whereby a strong compact radio source is used to calibrate the atmosphere and electronics for a weaker target source. The phase-referencing technique enables integration far beyond the atmospheric coherence time, enabling observation and imaging of radio sources that are orders of magnitude weaker than those that can be observed without phase referencing. Currently, more than 50% of all VLBA programs employ this technique. The VLBA also was the first VLBI array to make routine cross-polarization observations. In the late 1990s, the VLBA was the first major radio telescope to move significantly into dynamic time allocation based on weather and other conditions. At present, roughly 75% of VLBA programs are allocated dynamically, with a lead time of a few hours to 24 hours (up to 60 hours on weekends), to take maximum advantage of weather conditions. The only observations not scheduled dynamically are those that are time-critical (often to get the right time spacing in multi-epoch observations) or those that are coordinated with other participating radio telescopes (e.g., the Green Bank Telescope, the Phased VLA, Arecibo, Effelsberg, and the European VLBI Network). The VLBA also was the major ground element operating in conjunction with the Japanese Halca satellite in its VSOP Space VLBI program, from 1997 through 2001. VSOP could not have succeeded in Space VLBI imaging without the VLBA as its major ground element. NRAO recently has instituted the "High Sensitivity Array," in which the VLA, GBT, Effelsberg, and Arecibo may be added to the core VLBA, dramatically increasing the imaging sensitivity of the VLBA and providing capabilities for new classes of scientific investigations.

    VLA Operations and VLBA Operations take place within common technical divisions of NRAO's New Mexico (VLA/VLBA) Operations, as does EVLA development. Costs are reduced by sharing of personnel among the operations and development projects. The direct total VLBA Operations Costs in FY05 are estimated to be $6.65M. For reasons of cost effectiveness, NRAO centralizes a number of functions such as human resources, fiscal and purchasing, library services, postdoc and student programs, tenured scientific staff, and the Central Development Laboratory. NRAO has not established a formal means of charging internal overhead ("indirect") costs for these functions, but the best estimate is that NRAO-wide support of each operating telescope or project requires about $3M in indirect costs. Using this estimate, the indirect costs of $3M are an additional 45% on top of the direct costs of $6.65M, and the total cost of operating the VLBA in FY05 will be approximately $9.65M.

II. Technical details

  1. Specifics of telescope/instrument
    The VLBA operates with receiving systems at 10 different bands, from 330 MHz to 90 GHz, and has an angular resolution varying from 22 milliarcseconds (mas) at the lowest frequency to 100 microarcseconds at 90 GHz. The VLBA observing bands, resolution, and image sensitivity are shown below. For the calculations of image noise, the sustainable data rate of 128 Mbit/s is assumed for all bands except 0.6 GHz (limited by interference) and 80-96 GHz (4 hours at 256 Mbit/s assumed).

    Frequency Range
    8-hr Image Noise
    0.312 to 0.34222350
    0.596 to 0.62612700
    1.35 to 1.755.046
    2.15 to 2.353.249
    4.60 to 5.101.448
    8.0 to 8.80.8549
    12.0 to 15.40.4790
    21.7 to 24.10.32114
    41.0 to 45.00.17214
    80.0 to 96.00.101200

    The VLBA correlator is located at the Array Operations center (AOC), and is able to correlate as many as eight input data channels from each of 20 antennas simultaneously. For most modes, the correlator can provide 1024 spectral points per baseband channel, and up to a maximum of 2048 spectral channels per station or baseline can be provided for each recorded signal. In order to join its long baselines with the shorter baselines of the VLA and perform high-sensitivity imaging over a wide range of scales, increased bandwidth is critical for the VLBA. This will require a substantially increased capability for VLBA correlation, which can probably be done most cost-effectively by an allocation of spare station resources on the EVLA correlator now under development.

  2. New capabilities anticipated/planned in next 5-10 years

III. User profile

  1. % of "open skies" time
    The VLBA presently operates with 100% of its scientific observing time allocated in an open-skies, peer-reviewed mode.

    In the future, about 300-400 hours per year may be pre-allocated to NASA for high-frequency catalog and spacecraft observations. The anticipated NASA-funded hardware upgrades, specifically the Mark 5 recording system, should increase the overall observing efficiency of the VLBA sufficiently that the total number of hours open for the peer-reviewed science increases rather than decreasing.

  2. Institutional affiliations of users
    According to the NRAO Observing Summary for 2003, 400 individual astronomers from 122 institutions used the VLBA for scientific programs. Fifty-three of these institutions, or 43% (same percentage as the VLA), are located in the U.S. Typically, 50% of the PIs of VLBA proposals are from the U.S., and 50% from foreign countries. About 200 VLBA proposals are received each year, and about 70% include at least one U.S. investigator. Counting all accepted proposals that received observing time during 2003, there were a total of 400 observers on the VLBA, including 367 users from outside NRAO, 27 permanent NRAO staff, and 6 NRAO postdocs.

  3. Student access, involvement, usage
    Out of the 400 observers using the VLBA in 2003, 45 were students. Those students at U.S. institutions are provided travel and housing support if they wish to visit Socorro to analyze their VLBA data. In the NRAO summer student program during 2004, 4 of the 11 students in Socorro did projects involving VLBA observing and data reduction. Three of these were in the NSF Research Experiences for Undergraduates program, while the fourth was a graduating senior funded by NRAO. In addition, the 11 students were granted VLBA time to make an observation as a class project. In 2003, 8 summer students at NRAO (7 in Socorro and 1 in Charlottesville) worked on VLBA projects.

    Over the lifetime of the VLBA, at least 100 Ph.D. dissertations have been completed, based at least in part on data acquired using the VLBA.

IV. Science Overview

  1. Current forefront scientific programs

  2. Major discoveries (through 1999)

  3. Science highlights of last 5 years

  4. Main future science questions to be addressed
    Many of the goals listed below are adapted from the recent report "Mapping the Future of VLBI Science in the U.S." found at:

  5. Synergies with other major forefront facilities
    Synergies between the VLBA and other facilities come in several forms. First there is the synergy of observing at multiple wavelengths to understand specific objects. Next there is the synergy of using multiple facilities as part of a larger instrument. Finally there is the synergy of providing enabling support for other types of observations.

    The VLBI technique may be viewed as one that enables high-energy astrophysics to be done using low-energy photons. Thus one example of an important synergy for the VLBA is that with the high-energy space astronomy missions of present and future. Collapsed objects typically give off copious X-ray and gamma-ray radiation from the inner portions of their associated accretion disks and produce jets which can be studied at high resolution with the VLBA. Such objects range from collapsed stellar remnants in our galaxy to super massive black holes in distant galaxies. A full understanding of the disk/jet system takes high frequency observations with satellites such as Chandra and the upcoming Gamma-ray Large Area Space Telescope (GLAST) in addition to the VLBA imaging.

  6. Unique contributions

V. Education/Outreach activities

  1. Visitor facility
    The VLBA shares the public programs of the NRAO, most notably at the VLA. See discussion in the EVLA document.

    Specific to the VLBA, the site technicians give tours and public presentations upon request and as time allows. In 2004, 1000 visitors participated in tours at the VLBA sites.

  2. Student programs
    Every other year, NRAO conducts the Synthesis Imaging Summer School, an 8-day school at which attendees are taught the basics of radio interferometry, as well as specialized techniques relating to both VLBI and connected element interferometers, including applications and tutorials for VLBA, VLA, and ALMA. The 9th such school was held in 2004, and had 148 attendees of whom most were graduate students or postdocs. The NRAO Jansky fellowship program typically employs four new postdoctoral fellows per year, for a three-year period. A large majority of the Jansky fellows use the VLA or VLBA (or both) as significant tools in their research.

    University classes may be granted several hours of observing time on the VLA or the VLBA upon certification that the observing time will be the basis for at least 10 hours of classroom instruction. The VLA is a more popular choice, but the VLBA also is used at times. Agnes Scott College, Harvard University, Stanford University, and Haverford College are examples of the institutions that make use of this program.

    See a summary of all student programs at NRAO, including the VLBA.

  3. Other (as apply)
    NRAO staff have also been involved in many other activities such as the "Enchanted Skies Star Party", a 2-week "Radio Astronomy for Teachers" course and a Chautauqua short course, "Interferometry in Radio Astronomy". Travel support is provided for U.S. observers using the VLBA, and page-charge support is provided for their subsequent publications.

    NRAO has a regular program of press releases and other press information, including regular press conferences that feature NRAO results at the semi-annual American Astronomical Society meetings. Typically, there are a half-dozen releases per year featuring VLBA results. from astronomers throughout the U.S. and around the world. The NRAO press releases may be found at

VI. Documentation/website URLs

  1. URL of facility website

  2. URL of EPO website or for general NRAO programs.
    Most of the "Useful Links" from the facility website are oriented to the general public, including press releases, the NRAO image gallery, and frequently asked questions. In addition, note that the VLBA facility website gives prominent links to on-line tours of a VLBA antenna and of the Array Operations Center from which the VLBA is operated.

  3. URL(s) of any brief overviews of project/facility

  4. URL(s) of miscellaneous documentation
    Information for users, proposers, and other astronomers may be found via the "Astronomers" link under the main facility website, at The most complete technical description of the VLBA is located at; this "Observational Status Summary" is updated approximately annually. A global document entitled "What Does NRAO Offer You," including a description of visitor programs and benefits (e.g., page charge and travel support) may be found at In 2003, the director of NRAO and the director of MIT-Haystack Observatory jointly commissioned a report to outline the future of VLBI science in the U.S. A 9-member community group was formed, co-chaired by NRAO and Haystack scientists. Information about the activities of this committee, as well as the final copy of the report, are available at

This page created and maintained for the RMSPG by Martha Haynes Last modified: Fri Feb 11 08:53:00 MST 2005 by Jim Ulvestad