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VLBA: The Very Long Baseline Array
VLBA Web site
I. General project/facility description
- 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).
- 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.
- 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.
- 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.
- 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
- 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).
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.
|Frequency Range |
|8-hr Image Noise|
|0.312 to 0.342||22||350|
|0.596 to 0.626||12||700|
|1.35 to 1.75||5.0||46|
|2.15 to 2.35||3.2||49|
|4.60 to 5.10||1.4||48|
|8.0 to 8.8||0.85||49|
|12.0 to 15.4||0.47||90|
|21.7 to 24.1||0.32||114|
|41.0 to 45.0||0.17||214|
|80.0 to 96.0||0.10||1200|
- New capabilities anticipated/planned in next 5-10 years
- Mark 5 implementation
The VLBA currently is limited by an obsolescent tape-recording
and playback system that has become a technological dead end. A much
less expensive, and more expandable "Mark 5" recording system,
developed by Haystack Observatory, now is available commercially.
- Mark 5 upgrade to 512 Mbit/s
- Software enhancements
- 86 GHz antenna improvements:
Highest resolution system
- 32 GHz implementation:
Extremely sensitive high-frequency capability.
- 22 GHz amplifier replacement
- 43 GHz amplifier replacement
Because of the potential for using the VLBA for spacecraft tracking,
discussion with NASA are underway for improvements to the recording system
and possibly receiver upgrades (32 GHz).
III. User profile
- % 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.
- 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.
- 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
- Current forefront scientific programs
- Global astrometry and geodesy
- Pulsar parallaxes and proper motions
- Long-term monitoring of AGN (Active Galactic Nucleus) jets
- Evolution of structures in nearby supernovae
- Monitoring supernovae in merger galaxies
- Measurement of water maser motions in AGNs
- Astrometry of the Local Group, for measurement of dark-matter distribution
- Astrometry of GP-B reference star
- Imaging and monitoring of galactic microquasars
- Imaging and size measurement of Sgr A*
- Major discoveries (through 1999)
- Keplerian rotation and black hole mass in NGC 4258
- Measurement of expansion of Supernova 1993J
- Primary contributions to fundamental Earth and celestial reference frames
- Correlation of radio-jet ejection with gamma-ray flares in blazars
- First measurement of magnetic field of star beyond the solar system
- Detection of solar system rotation about the Galactic Center
- First routine Space VLBI imaging, with VSOP
- Tracing of gas motions in evolved supergiant stars (e.g., TX Cam)
- Imaging the collimation region of the M87 jet
- Science highlights of last 5 years
- Determination of geometric distance to NGC 4258, and recalibration
of HST distance scale
- Discovery of pulsar/black hole at center of Supernova 1986J
- Bending and depolarization of 3C120 jet when encountering
- Discovery of supernova factories in Arp 220 and Arp 299
- Measurement of pulsar parallaxes out to 0.5 kpc
- Measurement of intrinsic size of Sgr A*
- Correlation of radio jet ejection with X-ray variability
states in microquasars and AGNs
- Movie of precessing jet in SS433
- Measurement of microquasar paths in Milky Way, and extrapolation
back to birthplaces
- Astrometry of gamma-ray burst, ruling out major class of models
- Direct measurement of size of gamma-ray burst afterglows
- Discovery of circular polarization in variety of AGN jets
- Imaging of 6-image gravitational lens
- Discovery of very fast jets (v/c >25) in gamma-ray blazars
- Discovery of large magnetic-field changes on parsec scales
through Faraday measurements of AGN jets
- Determination of physical properties of AGN accretion disks/tori
via HI & free-free absorption imaging
- 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:
- How massive are central black holes in AGNs? To date, only two
supermassive black holes, in the Milky Way and NGC 4258, have had their
masses measured directly by motions of gas within a parsec of the center.
Several weak water megamasers with strong indications of Keplerian rotation
as in NGC 4258, have been discovered recently by the GBT and the 70m NASA
antenna in Australia. Imaging of these sources with the VLBA and other
large antennas may provide additional high-accuracy black-hole masses.
Direct distance measurements also may be made at distances several times
larger than for NGC 4258, providing better calibration of the expansion
rate of the Universe.
- How are relativistic, collimated flows generated? In the most favorable
cases, such as M87 where a wide opening angle is seen at high resolution,
it appears that the region in which jets are collimated is starting to be
resolved. Observations of the structure, polarization and dynamical
properties of such regions provide constraints on magnetohydrodynamical
models and simulations of the process of jet formation. Limited work of
this sort can be done with current sensitivities and considerably more
will be possible with increased sensitivity.
- What are the physical conditions in and around these jets?
Very basic physical properties of relativistic jets, such as speed,
temperature, composition, and density, are poorly known. Those
properties, for both the jet and the surrounding medium, are expected
to influence the internal structure and evolution of the jets in ways
that can be modeled analytically and numerically. VLBA movies with
good resolution along, and especially across, the jets can be
compared with theory to try to understand the physical conditions.
Imaging of large samples of milliarcsecond jets from AGNs whose
gamma-ray emission is detected with GLAST will provide key insights
into the production of high-energy gamma rays via particle acceleration
in the jets.
- Supermassive binary black holes: How common are they?
Binary black holes form as a product of galaxy mergers;
they will eventually coalesce and give off strong gravitational
radiation. Almost no binary black holes on scales less
than a kiloparsec are yet known. VLBI surveys have the potential
to detect such systems, either through the detection of proper motions,
or of appropriate compact doubles if both black holes are AGNs. Once
detected, the dynamics of such systems, and their evolution in strong
- What are the kinematics of the Galaxy and the Local Group?
It is now possible to measure proper motions of masers in a very few
members of the Local Group of galaxies. With longer time baselines,
it will be possible to measure more, allowing distance measurements
and orbit determinations. This will provide key parameters for
measuring the mass content and distribution in the Local Group.
Within the Galaxy, proper motion and parallax measurements of
masers and pulsars will help refine knowledge of the structure and
kinematics of the galaxy. The improved understanding of the
distribution of dark matter in the Local Group would have much
broader cosmological implications.
- Gravitational lenses - Where is the dark matter?
Studies of the structure of lensed sources at high resolution
can yield strong constraints on the distribution and substructure
of dark matter in galaxies. Also, increased sensitivity will
allow detection of weak central components expected in lenses,
which will constrain the mass distribution close to the centers
of lensing galaxies.
- How rapidly do GRB ejecta expand?
A single, relatively nearby gamma-ray burst appears to have
been slightly resolved by a VLBI array including the VLBA.
With the recent launch of Swift, we expect that considerably
more nearby gamma-ray bursts, including weak bursts associated
with supernovae, will be detected and localized. The VLBA is
the only instrument with the potential to resolve
the radio emission from these bursts as they evolve, thus providing
unique tests of the models of the gamma-ray burst explosions and
the environments surrounding the burst progenitors.
- What really happens when galaxies collide?
Galaxy collisions lead to large accumulations of gas in the nucleus
of the merging system causing intense star formation and growth
of supermassive black holes in compact regions that are heavily
obscured in the optical. With VLBI, young supernovae can be
identified and monitored while the AGN can be studied through
emission from jets.
- What is the role and effect of magnetism in stars? Magnetic fields
play an important role in star and planet formation by affecting the
gas dynamics. Magnetic activity is also seen at other stages of
stellar evolution in many types of stars. Bright, polarized radio
emission is often seen from the magnetized plasmas in these systems.
Current VLBI can only target the brightest systems, which are not
necessarily typical. As the sensitivity increases, a wider variety
of systems can be studied in detail.
- What are properties of the coupling between the Earth's core
and mantle? Joint observations with the VLBA and the geodetic
VLBI network provide the best available precision of determination
of nutation angle offsets. Such observations allow the study
of retrograde free core nutation, which cannot be predicted but
can be determined only with VLBI. This poorly understood process
has a variable amplitude with period around 430 days. Analysis of
nutation amplitudes provides valuable constraints on parameters of
geophysical models, such as the electromagnetic coupling on the
Earth's core-mantle boundary.
- 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.
- Unique contributions
- The VLBA is the highest resolution imaging instrument in astronomy,
in space or on the ground. With its sub-milliarcsecond resolution, this is
likely to remain true for at least the next decade, until (and if) NASA's
multi-billion-dollar planet-imaging missions are launched.
- The VLBA is the only full-time instrument that can image the sites of gamma-ray
emission in blazars, as well as measuring magnetic field structures on
- The VLBA is the only full-time VLBI user facility in the world,
and will remain so at least until (possibly) superseded by the SKA.
If the high-frequency SKA is built in the U.S., the VLBA probably
will be the high-resolution backbone of the SKA.
- The VLBA is scheduled dynamically and thus is able to respond easily
to targets of opportunity (e.g., GRBs, microquasars) and to take advantage
of the best weather conditions for the highest priority scientific programs
- The year-round operation of the VLBA permits the scientifically required
cadence of regular, repeated, and
commonly calibrated observations of variable and moving sources, thus
enabling imaging of galactic and extragalactic superluminal sources, as well
as evolved stars, at time intervals ranging from days to years.
- The routine monitoring of the VLBA properties, accountability of its
correlator models, and repeatability of observations make it a superb
instrument for sub-milliarcsecond astrometry. This provides unique science
such as geometric distance measurements for extragalactic water masers,
generation of celestial reference frames and plate-tectonic measurements,
as well as parallax measurements within the Galaxy and the Local Group that
illuminate galactic structure and the distribution of dark matter.
- Only the VLBA is an array of VLBI telescopes capable of observing
at frequencies up to 43 and 86 GHz, thus achieving the best angular
resolution and the ability to peer through the optically thick regions
of many extragalactic radio sources.
- The AIPS software developed for the VLA by NRAO has been extended
to support VLBI observations, enabling all VLBA observers worldwide
to analyze their data on desktop computers at their home institutions.
V. Education/Outreach activities
- Visitor facility
The VLBA shares the public programs of the NRAO, most notably at the VLA.
See discussion in the
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.
- 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.
- 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
- URL of facility website
- URL of EPO website
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
and of the Array Operations Center
from which the VLBA is operated.
- URL(s) of any brief overviews of project/facility
- 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
http://www.vlba.nrao.edu/astro/. 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
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