Schematic representation of the different coordinate systems and positions. The first satellite and all associated notations are depicted in red, while blue is used for the second satellite. denotes the relative position vector between satellite i and the observer, and correspond to the observer-centred cartesian coordinates of satellite i. and refer to the right ascension and declination of satellite i, as seen by the observer. are the satellites’ central body-centred cartesian coordinates. defines the RSW reference frame associated with satellite i. The vectors , , and correspond to the radial, normal, and axial directions, respectively.