A method to select, evaluate and recommend optimal rod pumping system design for a downhole well. The apparatus further includes: a position correction unit for updating a current position of the mobile object with the absolute position, and correcting the orientation of the mobile object by setting an orientation of the mobile object in the relative position equal to an orientation of the mobile object in the absolute position. If the output frequency increases above the reference frequency, the upper bound of the spread spectrum function is verified. A plurality of these rank-determining devices can be combined to form a rank determining apparatus in order to find the rank of each one of the digital samples in the set of digital samples. The columnar portion is provided with mutually facing flat faces on which a tensile load detector is mounted such that a tensile load applied to the brake cable is detected by the tensile load detector.