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.

The purpose of this paper is to improve the detection sensitivity of radio interferometry. The development of wideband signal processing techniques is necessary for high-sensitivity radio interferometry. The higher-order sampling technique can achieve wider bandwidth than expensive wide-baseband conversion system. The over sampling technique can improve the quantization loss from 11.89% to 3.99% at the 4-level quantized sampling. Moreover, the digital filter can reduce the folding noise incurred by the non-rectangular frequency response at a sampling process. It is confirmed that the wideband and low-loss A/D conversion system may be realized by implementing the higher-order sampling, over sampling and digital filtering techniques. In this paper, the key features focusing on these advanced techniques for radio astronomy are presented in detail.

The Communications Research Laboratory (CRL), the National Astronomical Observatory (NAO), the Institute of Space and Astronoutical Science (ISAS), and the Telecommunication Network Laboratory Group of Nippon Telegraph and Telephone Corporation (NTT) have developed a very-long-baseline-connected-interferometry array, maximum baseline-length was 208 km, using a high-speed asynchronous transfer mode (ATM) network with an AAL1 that corresponds to the constant bit-rate protocol. The very long baseline interferometry (VLBI) observed data is transmitted through a 2.488-Gbps [STM-16/OC-48] ATM network instead of being recorded onto magnetic tape. By combining antennas via a high-speed ATM network, a highly-sensitive virtual (radio) telescope system was realized. The system was composed of two real-time VLBI networks: the Key-Stone-Project (KSP) network of CRL (which is used for measuring crustal deformation in the Tokyo metropolitan area), and the OLIVE (optically linked VLBI experiment) network of NAO and ISAS which is used for astronomy (space-VLBI). These networks operated in cooperation with NTT. In order to realize a virtual telescope, the acquired VLBI data were corrected via the ATM networks and were synthesized using the VLBI technique. The cross-correlation processing and data observation were done simultaneously in this system and radio flares on the weak radio source (HR1099) were detected.

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.

This paper describe a new analysis on nonlinearity of a satellite born transponder by using Chebyshev transform technique and shows that the nonlinearity is measurable even in case of the satellite being on orbit and/or an AGC amplifier being equipped in the transponder.

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.