Cross beam wind measuring techniques

CER70-71BCH13.CET69-70BCH26.Includes bibliographical references.The usefulness of the cross beam technique for measuring atmospheric wind speeds is discussed. Two remote photodetectors monitor the light scattering mainly from dust, water vapor, and atmospheric pollutants, which serve as tracers for the mean wind speed. The time required for the particles to travel from one beam to another (transit time) is computed by space-time correlations of the two signals. The transit time occurs at the maximum correlation between the fluctuations in the two signals. Since the beam separation is known, the wind speed may be computed, once the transit time is known. Six experimental wind measurements using varying beam separations are discussed. To provide a basis for comparison, the wind speeds were monitored by cup anemometers. Four of the six experiments yielded wind speeds comparable to the dominant speeds obtained from the anemometers. It had been assumed that the common signals yielded positive correlations, but it was suggested that the aerosols might scatter light into one beam and away from the other, making the correlations negative. If there is an equal likelihood of positive and negative correlations by the common signal, the resulting correlation does not have a maximum value at the desired transit time. This may have been the case for the two remaining experiments. The possibility also existed that the beam separations of the two remaining experiments may have been too long, so that there were no signals common to both beams. It is concluded that the cross beam technique for measuring atmospheric winds works some of the time. Although the present setup cannot be used on an operational basis, the concepts may prove successful using artificial light sources or wavelengths other than those in the visual range