“…When the VCM is selected for optimisation, its direct mathematical relation with the coordinates of the MSs is needed. To derive it, let us expand the VCM by the Taylor series for a node and four MSs of M, N, O and P (see [ 35 , 36 , 40 , 41 , 42 , 43 ]): where is the estimated VCM of the node of the initial design, and are the coordinate updates to the initial locations of the MSs, and and are, respectively, the partial derivatives of the VCM with respect to the coordinates of the i th MS, with the following expressions (see [ 35 , 36 ]): where the elements of and , are partial derivatives of A or the second-order partial derivatives of the observables. Note that the first-order derivative is taken with respect to the transmitter coordinates and the second-order one with respect to x i and y i -coordinates of the i th MSs.…”