Navigation and Guidance Problems of Japanese Space VLBI Satellite - VSOP

Abstract

The Institute of Space and Astronautical Science (ISAS) of Japan is planning to launch a space VLBI (Very Long Baseline Interferometory) satellite called VSOP (VLBI Space Observatory Programme) in 1995. The weight of this VSOP is about 800kg, carrying a gigantic parabolla antenna, 10m in diameter, and its trajectory is1,000 km in perigee and 20,000 km in apogee.Simultaneously combined with many ground radio observatories over the world, this satellite will receive signals from various radio sources such as quasars and radio galaxies. When these observed data are transmitted to the ground in 64 MHz wide band-width channel, they are correlated with the ground observations, yielding precise maps representing internal structures of such radio sources.Navigation and guidance problems of this VSOP will be discussed in this paper. Particullary the attitude control problem that requires one hundredth degree in pointing of the antenna imposes severe restriction on the guidance system, because of flexible structures of the large antenna and solar cell paddles.(Fig.1)Also the clock synchronization, i.e. maintaining precise time-tags on the observed data by the satellite, synchronized with the very stable atomic clocks such as hydrogen masers on the ground, will become a crucial, technological problem. For this purpose, a beacon carrying precise clock information will be transmitted to the satellite and it will be returned to the ground. Later, these signals are calibrated using informations of the trajectory, troposphere, ionosphere, etc., in order to detect exact timing on the spacecraft. Hence very accurate orbit determnation of the spacecraft is required. In order to augment the radiometric data such as range and range-rate data, the GPS(Global Positioning System) receiver may be placed on-board