This chapter presents overviews of two of the more “unusual” propagation mechanisms (tropospheric and meteor scatter) mentioned briefly in Chapter 1, along with a discussion of propagation effects for two applications: tropospheric delays in satellite navigation systems and propagation effects on radar systems. While the material is presented only at an introductory level, it is intended to enable the reader to understand the basic aspects of these topics; sources of further information are also provided.


12.2.1 Introduction

At VHF and higher frequencies, the ionosphere is not a viable communications medium. In these frequency bands, the mode of signal transmission is characterized by three propagation mechanisms, depending on the distance, as sketched in Figure 12.1. For LOS conditions, the direct transmission mode of Chapter 5 is applicable, possibly with the addition of reflections from the ground, as discussed in Chapter 6. Beyond the radio horizon, diffraction (Chapter 7) takes over. The attenuation rate of the diffracted signal depends on the nature of the diffracting obstacle and is higher for relatively smooth ground than for a knife-edge-like mountain range. Even in the diffraction range of distances, another signal is present due to scatter from turbulent irregularities in the lower troposphere. At relatively short ranges, diffraction is dominant. Beyond some distance, which depends ...

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