Transhorizon propagation techniques and their usefulness in determining atmospheric parameters are discussed. It is concluded that the state or structure of the atmosphere cannot be inferred unambiguously from individual measurement techniques used in experiments reported to date. However, because of the sensitivity of the transhorizon radio signals to changes in the parameters of the troposphere, it is suggested that combinations of available techniques be pursued to determine the parameters uniquely.tional Academy of Sciences, Committee on Atmospheric Sciences, and was presented to the Panel in March 1968.
Drift (wind) and internal motions as theyaffect the Doppler spectrum or fading of the radio signals; 2. Statistical characteristics or properties of the random fluctuations o.r variations in the atmosphere as they are related to frequency and angular dependence of the mean scattered power; 3. Nonuniformities in the statistical characteristics of the atmosphere such as inhomogeneity or anisotropy (including stratification tendencies) that exist to a scale resolvable by the measuring devices (sometimes antennas) or the technique. Much of the early work in transhorizon radio propagation was concerned with communications system applications, and the parameters measured in the experiments were usually averaged over long periods of time (many hours or even days) and over large regions of space (sometimes from just above the earth's surface to the top of the sensible atmosphere). These measurements were not very useful for determining atmospheric characteristics at any given time or location in space. For an experiment to be useful in determining atmospheric parameters, all information necessary for their determination should be contained in the radio measurements or in other measurements made simultaneously; that is, it should not be necessary to include in the interpretation any assumptions that are not supportable by the measurements themselves. Experiments that deal entirely with parameters of use only to communications system designers are not included in this review. Thus this review concerns itself with the subject of transhorizon propaga-905 906 DONALD C. COX tion only as it relates to the parameters of the atmosphere. BRIEF REVIEW OF THE THEORY Theories that attempt to describe transhorizon propagation phenomena can be classified into two groups. The models used to describe the variations or fluctuations in the refractive index of the asmosphere are the major distinguishing features of the groups. Turbulence theories. By far the greatest effort expended in describing transhorizon propagation phenomena has been based on a statistical description of turbulence in the atmosphere. A rather extensive review of this subject can be found in Staras and Wheelon, [1959]; it has been treated by many authors [e.g., ]. In this approach, the refractive index (or dielectricpermittivity) fluctuations are described as a small random change about mean level. Early work [e.g., Booker and Gordon, 1950] dealt with the time fluc...
