The National Radio Astronomy Observatory's 27 antenna Karl G. Jansky Very Large Array (NRAO VLA) has been successfully transitioned to a broadband system. As part of this transition, the US Naval Research Laboratory (NRL) worked with NRAO to develop, install, and commission a new commensal low frequency system on the VLA. The VLA Low-band Ionosphere and Transient Experiment (VLITE) has dedicated samplers and uses spare NRAO optical fibers to transmit the signal from 10 low band receivers on VLA antennas to a dedicated real-time DiFX correlator. For these 10 antennas, this observing mode provides simultaneous data from both the low frequency receivers through the VLITE correlator and from the VLA higher frequencies receivers (150 GHz) through the NRAO WIDAR correlator. During the first 1.5 years of operation, VLITE recorded data at roughly 70% wall-time, providing 64 MHz of bandwidth centered at 352 MHz with 2s sample time and 100 kHz spectral resolution. VLITE operations require no additional resources from the VLA system and greatly expand the capabilities of the VLA through value-added PI science, stand-alone astrophysics, the opening of a new window on transient searches, and serendipity. We present an overview of the VLITE program, discuss the sky coverage and depth obtained during the first 1.5 years of operation, and brie y outline a possible path forward to a full 27 antenna LOw Band Observatory (LOBO) which could run commensally with all VLA operations.
Ionospheric phase errors degrade high-resolution radio images below
100 MHz, and they differ significantly from the tropospheric errors
which dominate at high frequencies. The ionosphere is so high
(~400 km) and the VLA primary beam is so wide (~0.2 rad) that
the intersection of the beam with the ionospheric screen is larger
than the "isoplanatic patch" size, a phase coherent region on the
sky. Antenna-based calibration techniques developed at higher
frequencies cannot be used because ionospheric phase errors vary
significantly across the field-of-view of each antenna. This paper
describes the "field-based calibration" technique adopted for the
74 MHz VLA Low--frequency Sky Survey (VLSS) being made with the 10 km
"B" configuration. This technique is useful for a range of array
sizes but fails on baselines longer than the linear size of the
isoplanatic patch, a few 10s of km at 74 MHz. Implications for
designing larger low-frequency arrays are discussed.
The 74 MHz system on the National Radio Astronomy Observatory's Very
Large Array (VLA) has opened a high-resolution, high-sensitivity
window on the electromagnetic spectrum at low frequencies. It
provides us with a unique glimpse into both the possibilities and
challenges of planned low-frequency radio interferometers such as
LOFAR, the LWA, and the SKA. Observations of bright, resolved radio
sources at 74 MHz provide new scientific insights into the structure,
history, and energy balances of these systems. However many of these
scientifically motivated observations will also be critical to testing
the scientific fidelity of new instruments, by providing a set of
well-known standard sources. We are also using the 74 MHz system to
conduct a sky survey, called the VLA Low-frequency Sky Survey (VLSS).
When complete it will cover the entire sky above -30 degrees
declination, at a 5σ sensitivity of 0.5 Jy/bm-1, and a resolution of 80" (B-configuration). Among its various uses, this
survey will provide an initial grid of calibrator sources at low
frequency. Finally, practical experience with calibration and data
reduction at 74 MHz has helped to direct and shape our understanding
of the design needs of future instruments. In particular, we have
begun experimenting with angle-variant calibration techniques which
are essential to properly calibrate the wide field of view at low
The Low Frequency Array (LOFAR) will be a radio astronomy interferometric array operating in the approximate frequency range 10-240 MHz. It will have a large collecting area achieved using active dipole techniques, and will have maximum baselines of up to 500 km to attain excellent spatial resolution at long wavelengths. The Sun will always be in LOFAR's beam during daylight hours, and particularly during periods of high solar activity the Sun will be a prominent (and highly variable) feature of the low-frequency sky. A diverse range of low-frequency emissions is generated by the Sun that carry information about processes taking place in the Sun's atmosphere. Study of these emissions with LOFAR will make possible major advances in our understanding of particle acceleration and shocks in the solar atmosphere, and of coronal mass ejections and their impact on the Earth. In this paper we summarize LOFAR's capabilities and discuss
the solar science that LOFAR will address.
The first, serendipitous, radio-astronomical observations by K. Jansky were at decametric wavelengths. However, after the initial pioneering work, long-wavelength radio astronomy was largely abandoned in the quest for higher angular resolution because ionospheric structure was thought to limit interferometric imaging to short (< 5 km) baselines. The long-wavelength (LW, 2 - 20 m or 15 - 150 MHz) portion of the electromagnetic spectrum thus remains poorly explored. The NRL-NRAO 74 MHz observing system on the Very Large Array has demonstrated that self-calibration techniques can remove ionospheric distortions over arbitrarily long baselines. We describe the scientific justification and initial technical design of the Low Frequency Array (LOFAR) -- a fully electronic, broad-band antenna array operating in the 15 - 150 MHz range with a collecting area of 1 km2 at 15 MHz. The longest baselines may be 500 km, providing an angular resolution of 10' at 15 MHz and 1' at 150 MHz. The combination of large collecting area and high angular resolution will enable LOFAR to produce images with sensitivities of order 1 mJy at 15 MHz and 300 (mu) Jy at 150 MHz. As such LOFAR will represent an improvement of 2 - 3 orders of magnitude in resolution and sensitivity over the state of the art. A key operational goal of LOFAR will be solar observations -- both passive imaging and radar imaging. In the latter mode LOFAR will serve as the receiver for bi-static observations of the Sun, with particular emphasis on the imaging of coronal mass ejections. LOFAR will serve as an astrophysical laboratory to study the origin, spectrum, and distribution of the Galactic cosmic ray electron gas and as an instrument to probe the high-redshift Universe.
We present the scientific motivation and design for a Long Wavelength Array (LWA) to open a new, high resolution window on the electromagnetic spectrum from 15 - 150 MHz. This region has been poorly explored because ionospheric turbulence has limited imaging to very course angular resolution. New phase compensation techniques now make it possible to explore this region at unprecedented angular resolution. We describe a large (greater than 100 km), completely electronic instrument capable of imaging radio sources across the sky and spectrum rapidly, but which could be built at a fraction of the cost of higher frequency systems of comparable size or sophistication. The LWA will be a powerful instrument for delineating the interaction between nonthermal emitting plasmas and thermal absorbing gas, for differentiating between self-absorption processes, and for exploring the universe for coherent emission processes. For both Galactic and extragalactic work it will provide unique information on the distribution of ionized gas, relativistic particles, and magnetic fields. For solar physics the LWA will be the ideal solar radar receiver and can be used to image Earth-ward bound Coronal Mass Ejections for accurate geomagnetic storm prediction.