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.
Tropospheric phase fluctuation due to the water vapor content is one of difficult problems which degrades imaging performances of radio interferometry. One of the potential solutions is differential radiometry observations to measure the differential water vapor content along the line of sights. We developed a 22-GHz-line radiometer to be mounted on a ground data-link antenna which supplies the timing reference signal for a space VLBI satellite, HALCA. This system will allow us to compare directly the atmospheric phase fluctuation with the water vapor content along a single line of sight measured by the radiometer.
The Institute of Space and Astronautical Science (ISAS) launched the first space VLBI (Very Long Baseline Interferometry) satellite, HALCA, in February 1997. After completing a series of engineering experiments to verify space-VLBI observations, the first VLBI fringes and images were obtained in May and in June, respectively. HALCA has now been operated for science observations at 1.6 and 5 GHz for the VSOP (VLBI Space Observatory Programme) project in cooperation with many organizations and radio telescopes around the world. In this paper the current science activities of the mission are reviewed and results presented.
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.