干旱区生态环境知识资源中心

An infrared (IR) or optical signal propagating along a 'line-of-sight' horizontal or slant path near the earth's surface can encounter substantial perturbations. These perturbations result in refractive distortions (low-frequency modulations that can amplify or reduce a signal) and scintillation (a higher frequency fluctuation in signal intensity). In an effort to elucidate the above issues a field test was conducted at the Naval Air Warfare Center (NAWC) at China Lake, CA, during July 2001. Transmission and scintillation measurements were made along slant, near-surface paths over land at 1280 m and 3850 m with a SSC (SPAWAR Systems Center) San Diego transmissometer that operated in the IR regime and in an almost aerosol-free environment. The field test has revealed that slow-scale refractive effects can create pronounced changes in the recorded one-minute average intensity of the IR source. Scintillation can also generate signal changes by a factor of 5 to 10 over very short time scales. In this paper we explore the relation between the refractive changes and scintillation, as well as models developed to describe and predict transmission and scintillation effects. Models include the exploitation of the propagation factor (a multiplicative factor defined from the local refractive field and geometry of the measuring system) and the use of wavelets as IR signal processors.