The first station of the Long Wavelength Array (LWA1) was completed in April 2011 and is currently performing observations resulting from its first call for proposals in addition to a continuing program of commissioning and characterization observations. The instrument consists of 258 dual-polarization dipoles, which are digitized and combined into beams. Four independently-steerable dual-polarization beams are available, each with two tunings of 16 MHz bandwidth that can be independently tuned to any frequency between 10 MHz and 88 MHz. The system equivalent flux density for zenith pointing is ∼3 kJy and is approximately independent of frequency; this corresponds to a sensitivity of ∼5 Jy/beam (5σ, 1 s); making it one of the most sensitive meter-wavelength radio telescopes. LWA1 also has two "transient buffer" modes which allow coherent recording from all dipoles simultaneously, providing instantaneous all-sky field of view. LWA1 provides versatile and unique new capabilities for Galactic science, pulsar science, solar and planetary science, space weather, cosmology, and searches for astrophysical transients.Results from LWA1 will detect or tightly constrain the presence of hot Jupiters within 50 parsecs of Earth. LWA1 will provide excellent resolution in frequency and in time to examine phenomena such as solar bursts, and pulsars over a 4:1 frequency range that includes the poorly understood turnover and steep-spectrum regimes. Observations to date have proven LWA1's potential for pulsar observing, and just a few seconds with the completed 256-dipole LWA1 provide the most sensitive images of the sky at 23 MHz obtained yet. We are operating LWA1 as an open skies radio observatory, offering ∼2000 beamhours per year to the general community. At the same time, we are operating a backend for all-sky imaging and total-power transient detection, approximately 6840 hours per year (∼78% duty cycle).

Abstract-LWA1 is a new radio telescope operating in the frequency range 10-88 MHz, located in central New Mexico. The telescope consists of 258 pairs of dipole-type antennas whose outputs are individually digitized and formed into beams. Simultaneously, signals from all dipoles can be recorded using one of the instrument's "all dipoles" modes, facilitating all-sky imaging. Notable features of the instrument include high intrinsic sensitivity (≈ 6 kJy zenith system equivalent flux density), large instantaneous bandwidth (up to 78 MHz), and 4 independentlysteerable beams utilizing digital "true time delay" beamforming. This paper summarizes the design of LWA1 and its performance as determined in commissioning experiments. We describe the method currently in use for array calibration, and report on measurements of sensitivity and beamwidth.

Abstract. Water vapor measurements made by the Halogen Occultation Experiment (HALOE)fromwater vapor, thus contributing to the observed increase in water vapor. The increase in water vapor observed by both instruments is larger than that which would be expected from the sum of all of the above effects. We therefore conclude that there has been a significant increase in the amount of water vapor entering the middle atmosphere. A temperature increase of--0.1K/yr in regions of stratosphere-troposphere exchange could increase the saturation mixing ratio of water vapor by an amount consistent with the observed increase.

The Naval Research Laboratory and the National Radio Astronomy Observatory completed implementation of a low-frequency capability on the Very Large Array at 73.8 MHz in 1998. This frequency band offers unprecedented sensitivity ($25 mJy beam À1 ) and resolution for low-frequency observations. The longest baselines in the VLA itself provide 25 00 resolution; the system has recently been extended to the nearby Pie Town antenna of the Very Long Baseline Array, which provides resolutions as high as 12 00 . This paper reviews the hardware, the calibration, and imaging strategies of this relatively new system. Ionospheric phase fluctuations pose the major difficulty in calibrating the array, and they influence the choice of calibration strategy. Over restricted fields of view (e.g., when imaging a strong source) or at times of extremely quiescent ionospheric ''weather'' (when the ionospheric isoplanatic patch size is larger than the field of view), an angle-invariant calibration strategy can be used. In this approach a single phase correction is devised for each antenna, typically via self-calibration; this approach is similar to that used at higher frequencies. Over larger fields of view or at times of more normal ionospheric weather when the ionospheric isoplanatic patch size is smaller than the field of view, we adopt a field-based strategy in which the phase correction depends on location within the field of view. In practice, we have implemented this second calibration strategy by modeling the ionosphere above the array using Zernike polynomials. Images of 3C sources of moderate strength are provided as examples of routine, angle-invariant calibration and imaging. Flux density measurements of a subsample of these sources with previously well-determined low-frequency spectra indicate that the 74 MHz flux scale at the Very Large Array is stable to a few percent and that flux densities tied to the Baars et al. value of Cygnus A are reliable to at least 5%. We also present an example of a wide-field image, devoid of bright objects and containing hundreds of weaker sources, constructed from the field-based calibration. The paper also reviews other practical aspects of lowfrequency observations, in so far as they differ from those encountered at higher frequencies, including aspects of interference excision and wide-field imaging. We close with a summary of lessons that the 74 MHz system offers as a model for new and developing low-frequency telescopes.

at most altitudes, there is no significant instrumental drift between the HALOE measurements and the measurements obtained from the two WVMS instruments over these time periods. We also find that at altitudes below $60 km, where the solar cycle effects are unimportant, there were no significant trends in either the HALOE or WVMS data sets over the periods of intercomparison. This is in marked contrast to the early 1990s where large trends (2%/year) were obtained from both HALOE and WVMS measurements. Similarly, we compare the HALOE data with the high-latitude 1998-2002 POAM data set at equivalent latitudes from 45N-55N. Again, the trends are small, and the instrumental trend differences are at most only slightly >1s. We also look at the HALOE water vapor and H 2 O + 2CH 4 data globally and find that, after a large increase from 1991-1995, there is almost no trend below $60 km from 1996-2002. The water vapor trend over the entire HALOE measurement period (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002) is <1%/year. The unusual stratospheric dynamics at the beginning of the 1990s caused large changes in upper stratospheric and lower mesospheric water vapor that were unrelated to the amount of water vapor entering the stratosphere, hence in this region the less dynamically sensitive quantity H 2 O + 2CH 4 probably provides a better measure of water vapor entering the stratosphere during this period. The trend in H 2 O + 2CH 4 over the 1991-2002 HALOE measurements period is $0.5%/year.