Abstract: A method and process for the determination of presence and distribution of clouds is presented. The method involves the use of a sensor, such as laser ranger, for the collection of echo location measurement data from a three dimensional volume of the sky. The process involved in this invention utilizes the measurement data to evaluate and test for the presence of cloud banks. The process also utilizes the measurement data to quantitatively and qualitatively determine the cloud coverage amount and distribution. The preferred embodiment includes a laser ranger that samples the three dimensional volume at several azimuth-elevation angles through the application of a device capable of selecting azimuth-elevation angle values.

Abstract: The optical path lengths of a plurality of comparison laser beams directed to impinge upon a common target from different directions are compared to that of a master laser beam by using an optical heterodyne interferometric detection technique. The technique consists of frequency shifting the master laser beam and combining the master beam with a first one of the comparison laser beams to produce a time-varying heterodyne interference pattern which is detected by a photo-detector to produce an AC electrical signal indicative of the difference in the optical path lengths of the two beams which were combined. The optical path length of this first comparison laser beam is adjusted to compensate for the detected difference in the optical path lengths of the two beams. The optical path lengths of all of the comparison laser beams are made equal to the optical path length of the master laser beam by repeating the optical path length adjustment process for each of the comparison laser beams.