The VLB tape recording path does add an additional source of error, the scaling of the surface error. In conventional holography processing the scale factor is set simply by the wavelength, and so there is no problem.
However, VLB system uses 1-bit sampling, which means that all amplitude is contained in the system temperature data, along with the additional problems which arise when non-gaussian signals are presented to the sampler and correlator. If uncorrected, we obtain a map which is honest in a relative way: the surface error maps are correct in their morphology, but will have an incorrect scale. This is the reason for our cautious approach of recording two IF streams from the 100m - the low sensitivity channel is entirely honest in the region of the boresight; the high sensitivity channel is honest outside the immediate vicinity of the boresight.