The Japanese VLBI project VERA (VLBI for the Earth Rotation study and Astrometry) is being designed. It will be dedicated to regular monitoring of the Earth's rotation and astrometry. VERA will be composed of two antennas spanning about 2300km and a correlator of multi-station capability. The diameters of the antennas are 35m and 15m.

Progress on the Australia Telescope, due for commissioning in 1988, is on schedule. When complete, the Australia Telescope will consist of a 6-km compact array and a Long Baseline Array which will, in collaboration with the University of Tasmania and NASA, use baselines up to 1400 km. Here I focus on the novel features of the Long Baseline Array, which is planned to use satellite linked local oscillators and VLBA data acquisition terminals to provide a VLBI network capable of expansion both within Australia and overseas.

The Parkes - Tidbinbilla Interferometer is a radio-linked interferometer using the 64-m telescope at Parkes together with one of the NASA antennas (34-m or 64-m) at Tidbinbilla. With a baseline of 275km, it is currently the world's longest real-time interferometer, and is usable at frequencies of 1.6, 2.3, and 8.4 GHz to give angular resolutions of 0.13, 0.09, and 0.03 arcsec respectively, with a sensitivity of 1–2 mJy rms in 5 minutes at a bandwidth of up to 10MHz.

A new VLBI system has been developed and put into operation. The system has three distinctive characteristics as compared with current VLBI practice, namely, the use of a wavefront clock at each station, the capability of a burst mode operation with high speed sampling, and the use of multiple 12 Mb/s data recording channels on commercial VCR's. The main use of the system is to be in geophysical investigations where these new features may prove of value.

Considerable progress has been made over the last few years in ground based optical interferometery. The Mark III optical interferometer at Mount Wilson which at this time is operating with a 12 meter north south baseline is capable of measuring amplitude and phase. Experience gained from this facility will allow for the development of a large optical array. Preliminary plans are described for a facility that will be capable of milliarcsecond imaging of celestial objects at wavelengths from 0.4 to 10 microns. If funded this array will be in operation by 1993.

The Torun VLBI Station is situated in Piwnice near the site of the Torun Radio Astronomical Observatory on the Nicolaus Copernicus University Land Estate, about 10 km north of Torun. The VLBI station development started in 1976, when the rubidium frequency standard was ordered and the ground was broken for the foundation of the 15-m equatorially mounted dish. The construction of the Mark II-C VLBI system began in 1977, and the completed VLBI station was first tested in 1980. The first fringes were obtained on May 31, 1981, with the Effelsberg 100-m dish. This experiment started our work with the European VLBI Network (EVN), and later in 1984 with the Global VLBI Network. We now have five radiometers and have received funding for the construction of a new VLBI station, with a 32-m dish, which is now in the final design stage.

Since the very beginning of the Itapetinga 45-ft antenna operations in 1971, there were plans to use it as a southern hemisphere VLBI terminal for high north-south resolution. The first successful VLBI tests were performed on water vapor lines in cooperation with Haystack Observatory in 1978 in one of the last runs using the MkI system. This cooperation led to the construction of a MkII terminal for Itapetinga years later (1981–1983). Astrophysical VLBI runs have been carried out since 1984, with the cooperation of Caltech, producing maps of unique resolution on stronger quasars, especially on 3C273 whose evolution has been followed since then. Recently, Bonn joined the collaboration in astrophysical VLBI programs. Itapetinga also participated successfully in geodetic surveys, and in the Vega-Venus mission, in collaboration with JPL and French organizations CNES, Observatoire de Paris and IGN. Plans for continuation and expansion of the Itapetinga VLBI participation in international efforts depend strongly on support to maintain and upgrade instrumentation at the Brazilian Radio Observatory.

At the beginning of 1980 the Istituto di Radioastronomia of the Consiglio Nazionale delle Ricerche (C.N.R.) started the Italian VLBI Project.

The first antenna was completed in October 1983 (see Table 1 for telescope characteristics). The first VLBI Mark2 experiment was performed in March 1984. In December 1984 we had the first Mark3 experiment. From that time our Institute is officially member of the European VLBI Network (EVN) and since January 1986 it is also Associate Member of the US Network.

A series of front-ends utilizing small closed-cycle refrigerators and very low-noise, high-electron-mobility transistor (HEMT) amplifiers have been developed for use in the Very Long Baseline Array (VLBA). The frequency bands, amplifier noise temperatures, expected system temperatures, and current status are shown in Table I. The receivers are designed to be light weight (∼ 55 pounds except for 105 pounds at 1.5 GHz) for ease of installation and maintenance, are easily remotely controlled and monitored, and provide dual-channel circular polarization capability. Detailed descriptions of some of the front-ends are given in VLBA technical reports.

The wideband Mark III recorder has been improved by the development at Haystack Observatory of a narrow-track recording system. Using 56 MHz bandwidth, more than 3 hours of VLBI data may be recorded on one tape. The same headstacks with new, thinner “digital video” tape allow recording at 128 Mbit/sec for 16 hours on one reel for the VLBA. New electronics for control, signal switching, and record/playback have been designed. Up to 2 Gbit/sec could be recorded by modular expansion.

The Mark IIIA correlator system, developed at Haystack Observatory, is a second-generation version of the original Mark III correlator which adds significant new capabilities, such as double-speed operation, longer integration periods, and improved internal modelling. One Mark IIIA correlator is currently operational at the U.S. Naval Observatory in Washington, D.C., and another is being readied to replace the Mark III correlator at Haystack.

The Mark IIIA correlator can simultaneously process up to 10 baselines of 24-MHz BW data from 5 stations. Experiments including more than 5 stations may be processed with multiple passes through the correlator. The architecture of the correlator allows a future expansion to a maximum of 16 stations. Due to a simple modular design and the low cost of required computer-support equipment, expansion is straightforward and relatively economical.

Both astronomy and geodetic data processing are supported by the Mark IIIA system, including spectral line and pulsar gating. In addition, recent work has led to support of space-based VLBI observations; this capability was critical in supporting the first successful space-based VLBI astronomy observations between ground-based antennas and an orbiting satellite in August 1986.

For the NASA Crustal Dynamics Project VLBI network, signals from quasars are recorded simultaneously at widely separated antennas. It is well known that hydrogen maser frequency standards provide the stable frequency reference used to precisely measure the difference in arrival time of the radio signals at the different antennas, enabling the determination of precise distances between the antennas. This paper reviews the practical requirements for maser support of VLBI for the Crustal Dynamics Project and describes the means used to meet these requirements for a network of eight fixed and three mobile stations which participate in approximately 200 VLBI experiments per year at locations in North America and the Pacific.

The present status of research and development in atomic hydrogen maser clocks gives stability of 3×10−16 in Δf/f for intervals of several hours. Long-term drift rates are in the order of 1×10−15/day and uninterrupted operating lifetimes of more than 4 years are fairly common with SAO's VLG-series masers. Experiments with improved techniques for separating atoms in the desired state confirm that stability can be achieved with existing technology at levels below 1×10−16. A four-fold improvement beyond this level appears feasible with more smoothly applied storage volume coatings of fluorinated materials. A recent breakthrough in technology has been made by SAO/Harvard, M.I.T. and, University of British Columbia, by operating hydrogen masers at 0.4K using storage coatings of superfluid liquid helium. This promises stability better than 1 × 10−17 in Δf/f for intervals longer than 30 seconds. Features of a hydrogen maser operating at room temperature, developed at SAO for long-term (∼7 years) unattended operation in space, are described. Possible applications of such masers with stability ∼1×10−16 for intervals 104–105 seconds in a deep space network of 4 (or more) radio astronomy antennas interconnected by time correlated one-way and two-way Doppler systems are discussed. By comparing the phase of signals propagated in both senses around the four (or more) closed triangular paths, changes in the magnitude and direction of the baselines can be determined with reference to inertial space defined by the constancy and isotropy of the velocity of light. The system is also sensitive to pulsed gravitational radiation at wavelengths less than the inter-terminal distances. Sensitivity is limited by the stability of the clocks and the fluctuations of ionization in the paths between stations. Multiple station time correlation of Doppler fluctuation in the signals between the six (or more) inter-terminal paths can be used to determine the direction and polarization of the pulsed gravitational radiation.

We review recent progress in the practical aspects of making VLBI images from continuum data.

Recent VLBI development provides opportunities for aperture synthesis, or super synthesis, with terrestrial interferometer antennas, and also in the near future with orbiting antennas. In this connection it is useful to consider the potential mapping capability of a single interferometer. Usually one would assume the use of image restoration procedures to be indispensable for observations with a single interferometer, because the uv-plane is covered only on an elliptical track, and is very much unfilled. High and distant sidelobes of the synthesized beam, caused by gaps in the coverage of the uv-plane, are usually considered to be influenced only to a small degree by any weighting function by which the visibility function is multiplied in the Fourier image formation algorithm. It has been shown that the mean-squared value of the point-source response function is invariant with respect to the weighting function (Doubinsky 1983). However, this does not prohibit the modulation of the point-source response by the visibility weighting function. This study shows that one can suppress the sidelobes in two symmetrical sectors of the point-source response with the help of such modulation. Thus for a line-shaped source model (that is, linear within the angular resolution of the synthesized beam) one could map without excessive distortion by Fourier transformation alone (Doubinsky 1979). The exclusion of restoration procedures, which are largely empirical, increases the reliability of the image when the source structure is extended and the signal-to-noise ratio is low. It also saves much computer time.

The technique of global fringe fitting has proven to be very valuable for very-long-baseline interferometry (VLBI). It allows weaker fringes to be detected than is possible with conventional single-baseline fringe fitting algorithms, and thus improves the sensitivity of present VLBI arrays. Global fitting was developed for VLBI by Schwab and Cotton (1983, Astron. J.88, 688), and has been incorporated into the NRAO Astronomical Image Processing System as program VBFIT. As Schwab and Cotton point out, this is a generalized from of self-calibration in which closure relations for residual fringe delay and rate (frequency) are satisfied in addition to those for fringe phase and amplitude. These additional closure relations allow station-dependent delays and rates to be solved for, using data from all baselines with sufficiently strong fringes. Then it is possible to calculate where fringes will be on any less sensitive baselines between these telescopes, so that very narrow search windows in delay and rate can be used.

An observing scheme and software for single frequency switched phase-referencing observations have been developed and tested on a source pair separated by 0.5° (0248+430, 0249+436). The phase connection was done utilizing closure properties of phase and phase-rate. A comparison of the phase-reference map with a hybrid map shows the success of the method.

The accuracy of VLBI measument has reached centimeters (Carter et al., 1985). But in VLBI observation the observable are broken sometimes because of the variations of the clock phases or frequencies. This is the first problem met in VLBI data processing.

This paper proposes a “half-sample delay switching” to suppress aliasing ghosts caused by finite frequency sampling without the clumsiness of a sharp anti-aliasing filter.

Correlator type processors have been currently used in VLBI, in which the number of correlators increases as N(N-1)/2 where N is the number of antennas. In a large elements VLBI array in future, this will introduce difficulty in constructing the processor. If we took the grid array configuration of antenna, FFT processor could be used in VLBI processing instead. FFT processor is used as a digital lens for a directional finding facility of the arrival waves. In FFT, the number of multiplications increases only as N log N. Although it gives maximum redundancy of Fourier components, the next step of the research on crustal motion will require measuring the velocities as a vector fields. Higher S/N ratio is also obtained in the VLBI maps.

The present processor, which was originally constructed for a pilot system of the radio patrol camera at Waseda University, is formed by a FFT Processor and eight Complex Amplitude Equalizers(1,2). When it is used as an VLBI processor, phase and amplitude fluctuations due to local oscillators and/or propagation effects are absorbed by the quick control of the Complex Amplitude Equalizers.

Here, we show examples of the computer controlled phase adjustment using the Equalizers(3). Our telescope is an 8 elements Very Short Baseline Interferometer (only 2m) at 10 GHz. Fig.1 is the block diagram of the present system. In VLBI system the recording facilities should exist between A/Ds and the Equalizers. The FFT processor is an error free ideal lens, and from the sampling theorem it makes eight independent beams in our system. Fig.2 is the dirty beams before removing phase errors, and Fig.3 the beams after the error removed. Both are 180 deg beam switched, while non switched in Fig.4.