This paper presents a realization of our IP based realtime VLBI (Very Long Baseline Interferometer) observation testbed with the highest sensitivity in the world. Today's rapid deployment of high-speed wide area networks will give a major breakthrough in VLBI astronomy in terms of its observational sensitivity and immediateness. VLBI requires huge amount of data transfer from several radio telescopes located separately each other for calculating cross-correlation. High-speed networks can be applied to such data transfer instead of conventional magnetical tape recording and physical transportation, which cause a serious performance bottleneck. We have newly designed and implemented a special component named gigabit network access node, which can exchange 2.048 Gbps telescope data through a 2.488 Gbps OC-48c/STM-16c SONET/SDH link. We have also constructed the world's first multi-gigabit-rate VLBI observation testbed using actual high-speed wide area optical networks and successfully conducted several real observations.

A new system, in which a real-time VLBI (very-long-baseline interferometer) is utilized, for real-time monitoring of atmospheric disturbances on a very-long baseline has been developed. In this system, beacon waves from geo-stationary satellites are used for received signals and public communication lines are used for data transmission. Connecting the system to the 6-m Kagoshima and the 10-m Mizusawa radio telescopes enables atmospheric disturbances to be observed. The cross-correlation phase was calculated from the received signals, and the Allan standard deviation of the phase was obtained. It was found that the Allan standard deviation across almost the whole region of the time interval reflects atmospheric disturbances.

The Real-time VLBI Correlator (RVC) is a new type processor for the Very-Long-Baseline Interferometry (VLBI). This correlator was primarily designed for supporting the VLBI Space Observatory Programme (VSOP). Two particular techniques, the fringe rotator after correlation and the lag-time extension technique, are newly developed for the RVC. The correlation circuit size of VLBI correlator is reduced to half by introducing the new fringe rotator, and it makes possible to realize a large delay window being essential in finding a cross correlation in real-time. The delay window can be changed flexibly with the lag-time extension technique, and its technique is useful to detect the fringe peak in a VSOP observation. The new correlator was installed at the Usuda Deep Space Center in Japan, and is used in VSOP and other domestic VLBI observations. In this paper, the key features of the Real-time VLBI Correlator (RVC) focusing on these advanced techniques are presented, and the results of its performance test are shown.

VLBI is an important application of ATM technology because it can transmit huge amounts of data. A single VLBI experiment typically generates data (which must be recorded and transported until they are cross-correlated) of tera-bit order at each separated observing site. Conventional VLBI not only requires manpower but also limits the maximum observation data rate. Therefore, a realtime VLBI using a private ATM network was developed recently, but it could not be utilized for regular VLBI experiment. Since utilization of public ATM is most realistic solution for realtime VLBI between ordinary observing sites, we have developed an interface equipment that connects VLBI observation and processing equipment to a public ATM network and demonstrated a successful experiment. This equipment supports VLBI's standard bit rates as 128 Mbps and 256 Mbps, though data rate for user's payload in 155.52 Mbps (STM-1/OC-3) ATM network is actually only 119.5 Mbps. It can easily step to higher networks as 622 Mbps.

The VSOP terminal is a new data-acquisition system for the Very-Long-Baseline Interferometry (VLBI). This terminal was primarily designed for ground telescopes in the VLBI Space Observatory Programme (VSOP). New technologies; higher-order sampling and digital filtering techniques, were introduced in the development. A cassette cart was also introduced, which supports 24-hour unattended operations at the maximum data rate of 256 Mbps. The higher-order sampling and digital filtering techniques achieve flat and constant phase response over bandwidth of 32 MHz without using expensive wide base-band converters. The digital filtering technique also enables a variety of observing modes defined on the VSOP terminal, even with a fixed sampling frequency in an A/D converter. The new terminals are installed at Nobeyama, Kashima, Usuda, Mizusawa, and Kagoshima radio observatories in Japan, and are being used in VSOP and other domestic VLBI observations. In this paper the key features of the VSOP terminal focusing on these advanced technologies are presented, and the results of performance tests are shown.