The patent, for "Method of and apparatus for controlling mechanism of moving vessels or vehicles," was granted November 8th, 1898. For a device like this to work, it needed to be wireless, and Tesla envisioned it powered by electrical waves. From the patent:

In a broad sense, then, my invention differs from all of those systems which provide for the control of the mechanism carried by a moving object and governing its motion in that I require no intermediate wires, cables, or other form of electrical or mechanical connection with the object save the natural modia in space. I accomplish, nevertheless, similar results and in a much more practicable manner by producing waves, impulses, or radiations which are received through the earth, water, or atmosphere by suitable apparatus' on the moving body and cause the desired actions so long as the body remains within the active region or effective range of such on currents, waves, impulses, or radiations.