Abstract: The invention is a guidance system used in laying undersea pipe or cable from a pipe-laying vessel when radio transmissions are unreliable. The system processes parameters derived from sensors normally aboard a pipe-laying vessel and sensors cooperating with active winches applying tension to the several anchor lines. The vessel's pitch, roll, and heading angles are supplied from gyrocompass and vertical gyroscopic instruments. Anchor line elevation and azimuth angles with respect to deck coordinates are yielded by conventional angle pick offs associated with fairlead devices, as are the line axial tension and the length of anchor line paid out. Initial conditions, including the initial position of the vessel, are easily obtained in the usual manner. From certain of these parameters, the invention derives intermediate anchor line parameters for use in computing fairlead motion.