Abstract: Differential VLBI (D-VLBI) is commonly used in VLBI spacecraft tracking in order to reach sufficient accuracy. Despite the simple concept of measuring a spacecraft’s angular distance to a well-known quasar, this method also holds some characteristics that are worth closer inspection. In this paper, the measurement principle is demonstrated and some basic thoughts on the observation sequence and the resultant scan gaps as well as on the separation angle between the reference source and the spacecraft are presented. This is followed by some geometrical reflections on the similarity of the transmission media. The results in this paper will provide technical support for the subsequent spacecraft mission.