1. A system for calculating the difference in speed between a receiver aircraft and a tanker aircraft in an approach operation for conducting a refueling operation using a hose and drogue device, characterized in that it comprises: a) capturing means of a sequence of digital photograms of the approach operation scenario located on a place of the tanker aircraft that enables laterally viewing said scenario, said capture means being calibrated in order to determine the real distance corresponding to a pixel of a photogram; b) computational means implemented for: b1) obtaining the coordinates of the relative trajectory between the receiver aircraft and the tanker aircraft by means of processing a sequence of photograms of the approach operation obtained by said capture means in which: following the selection in the first photogram of a pattern area of a predetermined size in which a portion of receiver aircraft is viewed, the storing of the 2D coordinates of its midpoint and the identifying of a reference frame having the same midpoint, shape and a larger size than that of pattern area by a predetermined scaling factor, in each photogram following the first the identification of a new pattern area as well as a new reference frame of the same pattern area, having the same features as in the photogram, searching in said new pattern area for the image in the reference frame of the previous photogram, the storing of 2D coordinates of the midpoint of new pattern area and the instant in time of the photogram; b2) calculating the difference of the speed between receiver aircraft and tanker aircraft, based on the trajectory defined by the stored coordinates in process b1).