The correlation process in Very Long Baseline Interferometry

The correlation process is an important element of geodetic Very Long Baseline Interferometry (VLBI). For many decades there has been constant progress in the capabilities of geodetic VLBI correlators following technical development and the need for ever higher precision of the results. In this publication, we describe the concept and some of the important practical aspects of VLBI correlation for a better understanding of the general geodetic readership. Time-lag correlators provide the easiest approach to the concept and to the individual steps necessary for the operations. More modern approaches then use the path where the data streams are Fourier transformed first before the cross correlation is actually performed. These correlators are called FX correlators, in contrast to XF correlators, where the cross-multiplications are done before the Fourier transform. Both concepts lead to the same results, the cross power spectrum. Following the correlation process proper, the VLBI observables, time delay and delay rate, have to be found in a fringe fitting process.