Since 1995 Nippon Telegraph and Telephone Corporation (NTT) has been conducting experiments on real-time VLBI (very long baseline interferometry) using a large scale network testbed having the maximum speed of 2.4Gb/s. With the real-time data transmission using high-speed communications network, the bottleneck resulted from the limited data rates with the conventional magnetic tape based VLBI system can be removed. Two applications of VLBI, geodesy and radio astronomy, are being pursued with our trial and extensive research items regarding the real-time VLBI technology are being conducted. So far, through the experiments using the developed real-time VLBI system, great improvement in observation performance has been achieved. Now we are concentrated in developing an economical VLBI data transfer system using advanced IP (Internet Protocol) technologies to achieve greater connectivity to other research organizations.
We have carried out the experiment of real-time space VLBI by using a high-speed ATM network. A space VLBI program is carried out with the HALCA satellite, which has an 8-m diameter radio telescope. Downlink data is transmitted to Usuda station, which is a Japanese data link station. And Usuda 64-m telescope is used as a ground radio telescope. They were used for this experiment. It is the first experiment that an optical fiber network was applied for a real-time space VLBI. The ATM optical-fiber network has 2.4 Gbps transmission capability. For the real-time space VLBI experiment, 128 Mbps data are transmitted. The VLBI correlator at NAOJ is used, which is usually used for the tape based ground and space VLBI observations. And we have succeeded to detect fringes using this network with a satellite downlink station and a ground radio telescope by test data, which are play-backed with recorded tapes. Unfortunately we have not tested actual observations because of a serious trouble of the satellite.
The Communications Research Laboratory (CRL), the National Astronomical Observatory (NAO), the Institute of Space and Astronautical Science (ISAS), and the Telecommunication Network Laboratory Group of Nippon Telegraph and Telephone Corporation have developed a real-time VLBI array, maximum baseline-length was 208 km. The very long baseline interferometry (VLBI) observed data is transmitted through a high-speed asynchronous transfer mode (ATM) network (2,488- Gbps [STM-16/OC-48] ATM network) instead of being recorded onto magnetic tapes. The system was composed of two real-time VLSI networks: the Keystone Project 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). The acquired VLBI data were corrected via the ATM network and the cross-correlation processing were done simultaneously. A radio flares on the weak radio source (HR1099) and weak radio sources were detected.