In VLBI observations, the telescopes are disconnected and real-time correlation of the signals from the individual telescopes is not possible. At each telescope, the signals are recorded on tape synchronous with the time signal provided by the Hydrogen-maser. In MkIII mode observations, the data are available as 28 channels of 4MHz bandwidth each; the bandwidth of the recording is = 284MHz = 112MHz. The 28 tracks are recorded simultaneously on magnetic tape. The required bitrate of the recording is
For = 112MHz and a sampling efficiency n 1-1.6, the bitrate recorded on tape is 224Megabites/s. mm-VLBI is being upgraded to 256MHz bandwidth, and more (MkIV).
The short-term frequency stability (up to a few 1000s, see Figure 3.5) of a Hydrogen-maser is 10. There are long-term drifts which can be checked against GPS signals (see Figure 3.6) and adjusted so that they are below, say, ten nano-seconds per day. The maximum possible integration time () of an observation is set by the requirement that the relative frequency drift must not exceed, say, 0.2 radians. The integration time then is ([Kellerman & Thompson 1985])
This relation gives (86GHz) 350s 5minutes and (230GHz) 150s 2minutes.
Because of phase variations introduced by atmospheric water vapour fluctuations, the correlation time derived above can be significantly shorter, 10-30s, especially when observing at high frequencies. Because of the scarcity of strong mm-wavelength VLBI sources at sufficiently close distances in the sky, phase referencing as used in connected mm-wavelength interferometry (for instance used on PdB) has not yet generally been applied in mm-VLBI. Efforts are however undertaken to apply phase corrections from local line-of-sight water vapour measurements (sky emission measurements). As an example, the phase stability of a 86GHz and 215GHz measurement between Pico Veleta and Plateau de Bure is shown in Fig.3.7. A typical atmosphere-induced phase variation and phase correction applied to 86GHz VLBI measurements made at Pico Veleta is shown in Figure 3.8.
The recorded mm-wavelength VLBI data are correlated at Haystack (USA) or at Bonn (Germany). The end-product of the correlation are calibrated visibility values (uv-tables) which can be used in the same way as data, for instance, obtained with the PdB interferometer.