Biases of digital VLBI observables due to small signal approximation and imperfect fringe rotation
The analysis presented shows that the usual small signal approximation in VLBI is acceptable in the majority of applications. However, it leads to nonlinear systematic overestimation of the correlation coefficient, which in some cases cannot be considered negligible. For example, the correlation of about 0.26 estimated from the fringe amplitude of 0.1, as measured with the Mark II system, is 3.4 percent too high. Formulas suitable for accurate conversion of measured amplitudes into the correlation coefficient for a few useful fringe rotation schemes are given. The analysis also brings evidence for a small bias in the phase measurements made with present day VLBI correlators, which is entirely due to imperfect fringe rotation. Arguments are presented in favor of five-level rotators.