The paper describes a new method of measuring long distances by means of a light ray polarization-modulated at ultra-high frequencies. The modulation wavelength is determined by reference to cavity resonators, used in such a manner that no atmospheric refractive index correction has to be applied. This leads to a very simple method of obtaining the required distance directly in feet. The theory of the method is given in detail, including the effect of altitude on the measurement. The apparatus itself is also described together with some preliminary results which illustrate the accuracy that can be achieved.