VLBI phase-calibration system suitable for very-wide-band and ultra-high-frequency operation

The key factors determining the final performance in geodetic measurements made using very long baseline interferometry (VLBI) are the total instantaneous bandwidth and the actual value of the observation frequency. The first is strictly related to the uncertainty measurement of the group-delay, while the second is related to the ultimate resolution achievable with this measurement technique. The main advantages of using a higher frequency are a large improvement in the calibration of the ionospheric correction, the avoidance of interference from strong man-made radio signals, and the possibility of having a still wider receiving band. In contrast, implementing a scheme to fully calibrate the instrumental delay would be much more difficult. The authors have developed a new scheme, which expands the operation bandwidth and, in practice, does not have an upper limit on the frequency. It works by up-converting the comb pattern of the phase rails by a local generator, which can be in common with the main receiver or a standalone device. The results of the first VLBI experiment at 22 GHz by stations in Italy and Japan in 1995 demonstrated the effectiveness of this technique. It is currently used in the VLBI Space Observatory Program, which began in 1997.