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 instantan eous bandwidth and the actual value of the observation frequency, The first is strictly related to the uncertainty measurement of the group-delay, whi le the second is related to the ultimate resolution achievable with this me asurement technique. The main advantages of using a higher frequency are a large improvement in the calibration of the ionospheric correction, the avo idance of interference from strong man-made radio signals, and the possibil ity of having a still wider receiving band. In contrast, implementing a sch eme to fully calibrate the instrumental delay would be much more difficult. We 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 result s 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.