The essence of the experiment is to measure the voltage beam pattern of the target antenna.
Two antennas are required for the operation - the target antenna (the Effelsberg 100m, in this instance), and a reference antenna whose role is to provide a phase reference signal. Both antennas observe a strong distant transmitter. For this experiment we used the 11.7 GHz beacon on EutelSat-W2. The reference antenna points towards the satellite for the entire experiment; the target antenna executes a raster map centred on the satellite.
The raster data provide a detailed sampling of the beam pattern; its fourier transform is the field distribution over the aperture plane. The phase distribution is the factor of interest - it can be translated to a surface map; and alignment errors have specific phase signatures which can be identified in the map.
The two antenna signals (from target and reference antennas) are processed in some form of vector voltmeter - to obtain the phase difference and the amplitude ratio. The instrument of choice for this is a cross-correlator. In this experiment we used the Bonn Mk4 correlator - which added the complexity of VLB tape recording as an intermediate step. However, this choice has the virtue of flexibility - the reference antenna can in principle be any distance from the target antenna (eg, at a different observatory).