A highly stable crystal oscillator applied to the VLBI reference clock

Research into crystal oscillators has made remarkable progress in recent years, and the stability of selected crystal oscillators can reach 3/spl times/10/sup -13/, a value comparable to the stability of the atmosphere. Instead of the hydrogen maser, a carefully selected crystal oscillator which is phase locked to a cesium frequency standard for 100-second time scales was adopted to the time and frequency standard of a geodetic VLBI experiment. The frequency stability of the integrated frequency standard system is poorer than that of the normally used hydrogen maser, but the overall results of geodetic VLBI measurements are comparable to those obtained from the normal experiments, provided a properly designed observation schedule is used. VLBI experiments were carried out for four years from 1988 using this system at one of the 1000 km baselines, and we successfully measured the position of Marcus Island within 5 mm and its site velocity within 5 mm/year. The first VLBI experiment using this system on the 11000 km baseline between Japan and Antarctica was carried out successfully in January 1990.