Development of Broadband VLBI System and Report of Experiments for Geodesy and Frequency Comparison

Abstract

We have developed a new broadband VLBI system GALA-V for distant frequency comparison project. The GALA-V broadband VLBI observation system is designed to be compatible with the concept of the next generation geodetic VLBI system: VGOS (VLBI Global Observing System). The GALA-V system contains advanced technologies, such as (1) unique broadband feed, which enables upgrading standard Cassegrain telescope to broadband radio telescope, (2) RF direct sampling and broadband bandwidth synthesis. We have originally developed broadband feed (NINJA Feed), which has been installed to Kashima 34-m antenna, and we have successfully realized high-gain large diameter broadband antenna with Cassegrain optics. The combination use of RF-Direct sampling technique and broadband bandwidth synthesis have achieved stable high precision group delay measurement. The GALA-V system demonstrated sub-pico second precision delay measurement by one second of VLBI observation of extragalactic radio source with Kashima 34 m - Ishioka 13 m baseline. Our project is targeting to make precision frequency comparison and geodetic measurements with transportable small diameter antennas. We have conducted a series of broadband VLBI experiments between two small telescopes installed at NICT (Tokyo) and NMIJ (Tsukuba) for measurement of clock difference between UTC (NICT) and UTC (NMIJ). These experiments demonstrated that broadband observation enables one pico-second or better delay precision measurement even by using small diameter radio telescopes. Weighted RMS of post-fit delay residual of a pair of 1.6 m & 2.4 m antennas was about 14 psec in the session on 25th Nov., which is the same level with the best precision current geodetic VLBI observation with large diameter antennas. The experiment results suggest that analysis errors of broadband VLBI are no more limited by delay measurement precision, but totally dominated by atmospheric delay uncertainty.