Doppler Wind Lidar

Abstract

12.1 Introduction The change of perceived frequency of radiation when the source or the receiver move relative to one another is a well-known phenomenon. First described by Austrian physicist Christian Doppler (1803–1853) for acoustic waves, it occurs for electromagnetic waves as well. If the change of frequency can be measured, the relative speed of the source with respect to the receiver can be determined, provided the group velocity of the radiation in the respective medium is known. As the speed of light in air and vacuum has been known with high accuracy, the optical Doppler effect lends itself ideally to the remote measurement of the speed of very distant or otherwise uncooperative objects. If the object does not move directly toward or directly away from the observer, then the use of the optical Doppler effect clearly yields the component of the speed of the object along the line of sight. It is obvious that for a velocity measurement the object must emit electromagnetic radiation. This is the case for stars and galaxies; perhaps the most spectacular application of the optical Doppler effect was the determination of the shift of light from distant stars, all toward longer wavelengths, leading to our present notion of an expanding universe. Because the relative shift of optical frequencies, , is proportional to v/c, the ratio of the velocity v of the object to the speed of light c, and because very distant stars move away fast, these measurements were comparatively easy to make. Velocity determinations on Earth and in the Earth's atmosphere are more difficult for two reasons. First, the objects whose speed is to be 326 Christian Werner measured must be made to emit radiation. This can be done, e.g., by illumination. Second, the shift of the return radiation with respect to the transmitted radiation must be determined. Velocities of interest on Earth vary greatly with object and purpose. The movement of air masses, e.g., is interesting at velocities of about 0.1 to 100 m/s which, relative to the speed of light of 3 × 10 8 m/s, amounts to a fraction of roughly 3 parts in 10 10 to 3 parts in 10 7 . This is not easy to measure unless very narrow spectral lines and highly sophisticated equipment are used. Although optical Doppler measurements have a multitude of terres-trial applications such as the determination of the speed and vibrations of moving parts in traffic, in industrial production, in machine shops, etc., this chapter is exclusively devoted to the measurement of the movement of atmospheric air masses, or wind and turbulence, from the observa-tion of aerosols. Compared with other Doppler measurements, Doppler wind measurements have the additional problems that the illumination of the air even with powerful sources yields very weak return signals and that the return signals must be analyzed not just for wavelength, but for distance as well. Following this Introduction, Section 12.2 will briefly recall the notations and formulas that will be used in connection with optical Doppler wind lidar. In Section 12.3 different schemes for remote measurements of the wind vector are presented. In Section 12.4 the wavelengths to be used, the different detection schemes and the vari-ous scan techniques for Doppler lidar are discussed. Section 12.5 shows several applications, with main emphasis on heterodyne wind lidar, and Section 12.6 concludes this chapter with a number of new areas in which optical Doppler wind lidar may gain importance in the future. 12.2 The Optical Doppler Effect