Developed by Phase Laser Systems Inc., this technique makes innovative use of differential phase measurement technology. Light travels in a straight line. This property makes feasible a variety of possible geometric methods for measuring distance, most of which depend on some form of triangulation. These methods are limited in that they require at least two spatially separate reference points.
Laser light is coherent.
The technique, however, is not easily usable for greater distances. Light propagates through space with a very well-known velocity. In principle, this offers an extremely accurate way to determine distances of any required length. Two general methods might be used to accurately measure the time required for light to make a round trip between the measuring device and the object to be measured. Conceptually more complex, but electronically simpler, is to measure the phase of a modulated light beam when it leaves the instrument, and then compare that phase to the phase of the return light.