DEVELOPMENT OF AN INTERFACE SYSTEM TO ENSURE COMPATIBLITY OF THE EXITING VARIOUS VLBI RECORDING SYSTEMS FOR GLOBAL GEODETIC OBSERVATIONS

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BackgroundVery Long Baseline Interferometry (VLBI), which was brought into regular operation in early 1980's, has made a number of remarkable scientific accomplishment in the field of geodynamics. They are, first detection of the contemporary plate motions and motions along the plate boundaries, insight of the earth's core by means of nutation, polar motion and earth's variable rotation, precise understanding of angular momentum exchange of solid earth, atmosphere and oceans, establishment of the astronomical reference frame based on precise positioning of extra-galactic radio sources, establishment of terrestrial reference frame based on precise determination of the station coordinates and motions, etc.However, due to the advent of GPS (Global Positioning System), the role of terrestrial reference system has been gradually taken over by GPS.Hence it is the time for VLBI to clearly define its future scientific targets for the next decade. This will be certainly realized by establishing … More a high-accuracy global VLBI networks for continuous monitoring of the earth's rotation.On the other hand, according to the history of VLBI system development made in various countries, the exiting recording systems are mutually incompatible. For example, Japanese VLBI community has developed K-4 recording system whose recording format is different from that of US developed and preceding Mark-III system. Also Canadian VLBI community has developed S2 system which has different recording format compared to the Japanese and US systems. This situation has made it difficult to make a concerting global VLBI observation for geodynamic research.AccomplishmentsAs the geotic VLBI group of the National Astronomical Observatory, Japan (NAOJ) has only K-4 system, we have been unable of participating global VLBI observations. In order to resolve the problem, we have made effort to deploy K-4 recording system to overseas antenna sites. By the end of the 1995 academic year, we deployed our system to the Urumqi Astronomical Observatory of China and the Crimean Astrophysical Observatory of Ukraine and initiated K-4 global observations.However, VLBI activities of NAOJ is still restricted to this K-4 network and cannot participate in the Mark-III VLBI networks. Hence we planned to develop an interface system which enables us to handle both K-4 and Mark-III format through this two-year research project.In this research project, we introduced a set of high density digital tape recorder (DR-101B,made by INTERFEROMETRICS), and developed and installed an interface circuit which is possible to read and write both K-4 and Mark-III formats. This has successfully been accomplished and at present we can operate a network composed of two different recording systems.Future PlanDuring the term of our research project, NASA announced the plan of forming Mark-IV VLBI global networks for continuous 24-hour observations, which is composed of seven networks. Mark-IV recorder is an up-graded version of Mark-III and has a bit rate of 0.5Gb/s. In order to join the NASA Mark-IV network, we still have to grade up our newly developed interface system. Also NAOJ is collaborating with the National Institute for Polar Research to conduct geodetic VLBI observations at the Antarctic Syowa station from the next year. In this project, we are planning to make joint observations with Australian VLBI community. The presently used VLBI system of Australia is Canadian S2 system. Hence we need to develop another interface to make compatibility with S2 system. Thus we are planning to extend our research project to make compatibility among K-4, Mark-IV and S2 systems. This system called 'Multi-Format System' will be completed during1997 to 1998 academic year. Less