Since its development in the 1960's, the technique of obtaining high-resolution radio images of astronomical objects using Fourier synthesis has advanced sufficiently so that today such images often provide better angular resolution than is obtainable with the largest optical telescopes. A synthesis array measures the Fourier transform of the observed brightness distribution by cross-correlating the signals from antennas separated by distances up to tens of kilometers. The antennas must be equipped with low-noise receiving systems and connected together by phase-stable transmission links. Wide-bandwidth digital correlators are used to perform the cross correlation. The data-reduction algorithms and computing system play a critical role in determining the quality of the images produced by the array. The Very Large Array (VLA) synthesis telescope, recently constructed in New Mexico, consists of twenty-seven 25-m-diameter antennas arranged in a Y-shaped array. Each arm of the Y is approximately 21 km long and the antetmas can be moved to various positions on the arms by a rail-mounted transporter. The antennas are equipped with cryogenically cooled receiving systems and are interconnected by low-loss, TE 01 -mode, large-diameter waveguide. The cross-correlation products for each of the 351 pair combinations of antennas are measured for 4 IF signals by a 50-MHz bandwidth digital correlator. In this paper we discuss the design of synthesis arrays in general, and describe the design and performance of the VLA in particular, under the seven headings: array geometry design, sensitivity considerations, phase stability requirements, signal transmission system, delay and correlator system, control system, and data-reduction requirments. In each section, we review the underlying instrumental requirements and provide details of how the VLA was designed to meet them. Recently developed data-reduction algorithms provide effective ways of correcting synthesis images for the effects of missing Fourier components and instrumental and atmospheric amplitude and phase errors. The power of these algorithms is demonstrated using actual VLA images.

The very large array: Design and performance of a modern synthesis radio telescope

A statistical model of water vapor fluctuations is used to describe the effect of the dynamic wet troposphere on radio interferometric measurements. It is assumed that the spatial structure of refractivity is approximated by Kolmogorov turbulence theory, and that the temporal fluctuations are caused by spatial patterns moved over a site by the wind, and these assumptions are examined for the VLBI delay and delay rate observables. The results suggest that the delay rate measurement error is usually dominated by water vapor fluctuations, and water vapor induced VLBI parameter errors and correlations are determined as a function of the delay observable errors. A method is proposed for including the water vapor fluctuations in the parameter estimation method to obtain improved parameter estimates and parameter covariances.

The effect of the dynamic wet troposphere on radio interferometric measurements

Water vapor radiometric and surface meteorological measurements taken during three GPS geodetic experiments are used to calculate process noise levels for random walk and first-order Gauss-Markov temporal models of tropospheric path delays. Entire wet and combined wet and dry zenith delays at each network site are estimated simultaneously with the geodetic parameters without prior calibration. The path delays and corresponding baseline estimates are compared to those obtained with calibrated data and stochastic residual delays. In this manner, the marginal utility of a priori tropospheric calibration is assessed given the ability to estimate the path delays directly using only the GPS data. Estimation of total zenith path delays with appropriate random walk or Gauss-Markov models yields baseline repeatabilities of a few parts in 100 million. Results suggest that GPS data alone have sufficient strength to resolve centimeter-level zenith path delay fluctuations over periods of a few minutes.

Stochastic estimation of tropospheric path delays in global positioning system geodetic measurements

A theory is developed for the response of a two-element radio interferometer to a partially coherent field, without restriction as to bandwidth or antenna properties. It is shown that for a completely incoherent source the narrow-band interferometer output is a component of the Fourier transform of the source brightness, which can therefore be mapped by repeated interferometer observations. A partially or completely coherent extended source, on the other hand, cannot be mapped by this technique. The history of radio interferometry in astronomy is discussed and examples are given of the current uses of the technique in the precise measurement of source positions, in the measurement of the diameters of extremely small sources, and in the detailed mapping of complex sources.

The interferometer in radio astronomy

The article presents observations of turbulence-induced tropospheric phase fluctuations measured at 5 GHz on the near-vertical paths relevant to many astronomical and geophysical measurements. The data are summarized as phase power spectra, structure functions, and Allan variances. Comparisons to other microwave observations indicate relatively good agreement in both the level and shape of the power spectrum of these tropospheric phase fluctuations. Implications for precision Doppler tracking of spacecraft and geodesy/radio interferometry are discussed.

Observations of tropospheric phase scintillations at 5 GHz on vertical paths

Simultaneous measurements of phase and amplitude variability were made on a 64 km slant path using five radio frequencies: 9.55, 19.1, 22.2, 25.4, and 33.3 GHz. The three middle frequencies were chosen for their relation to water vapor absorption. The amplitude data show occasional fades in excess of 20 dB superimposed on ubiquitous smaller scintillations of several decibels. The latter generally increased with radio frequency and the fluctuations at the different frequencies were only moderately correlated (e.g., < 0.7). Neither phase nor amplitude variability show effects on the molecular resonance of water vapor.

Phase and amplitude scintillations in the 10 to 40 GHz band

Techniques for reducing propagation-induced phase fluctuations in frequency distribution systems are discussed and compared. Examples of both microwave and optical methods are presented. Stabilization to a few parts in 10^{8} over a 15-km path seems to be feasible.

Phase stabilization of widely separated oscillators

Deep fading, e.g., > 40 dB, was observed at 9.6 GHz on a 150 km path with terminals at 3025 and 3300 m elevations and with no significant illumination of the earth's surface. Fades at two antennas separated horizontally by 100 m were largely uncorrelated. Phase variability was also measured and is presented in terms of spectra and coherency between antennas with 10 m and 100 m horizontal spacing.

L. Wood

Papers

Phase and amplitude scintillations in the 10 to 40 GHz band

Phase stabilization of widely separated oscillators

Phase and amplitude scintillations at 9.6 GHz on an elevated path

The Group Refractive Index of Air