Telecommunication Systems 41 Introduction

Much of this chapter is based upon earlier work [Goodman and Aarons, 1990] published just prior to the peak of solar cycle 22. Since that time, many issues remain the same, but the growth in technology has led to different approaches. The expanded use of GPS within the civilian sector is but one example. Another change is the growth in capability to monitor and assess the real-time environment so that improved predictions can be entertained. But the Internet has arguably provided the most significant leap change in technology, successfully addressing the issue of data transfer, analysis, dissemination, and the potential for real-time forecasting of space weather phenomena. As we migrate through this chapter, the influence of the Internet and the World Wide Web will be evident.

Electronic systems have evolved to address a myriad of problems associated with disciplines such as earth surveillance and mapping, surveying, the maintenance and transfer of time, navigation, search and rescue, emitter location, signal intercept, target tracking, global communication, the command and control of military forces, and electronic warfare, to name a few. Figure 4-1 is a cartoon depicting the various generic systems.

The general topic of ionospheric effects on radiowave systems has been covered in various topical conferences and workshops, the proceedings of which are generally available. Special topics are reported in scientific and technical journals, selected government publications, and certain monographs. The Handbook of Geophysics and the Space Environment [Jursa, 1985] published by the Air Force Geophysics Laboratory (AFGL) remains an excellent resource. A comprehensive treatment of radiowave propagation in the ionosphere is beyond the scope of this book, but the interested reader is referred to a monograph by Davies [1990]. Other books include those by Goodman [1991], Tascione [1994], and Hunsucker [1991]. A readable account of radio propagation in the ionosphere has been written by Bradley [1991]. For those seeking a more succinct discussion of various ionospheric effects, a classic survey of earth-space propagation effects was published by Lawrence et al. [1964], This was updated by Millman [1967] and later by Flock [1987]. Ionospheric effects have also been included in the numerous Solar-Terrestrial-Predictions Workshops, the Ionospheric Effects Symposia, various conferences organized by the IEEE and the IEE (UK), and meetings sponsored by the NATO Advisory Group for Aerospace Research and development (AGARD). Another major source of information is contained in documents published by the Radiocommunication Bureau of the International Telecommunication Union (ITU-R), previously known as the International Radio Consultative Committee (CCIR) prior to reorganization in the 1990s. Indeed, the established international positions with respect to ionospheric phenomena and its impact on radiowave systems are found in published ITU-R Recommendations, Reports, and Handbooks. Of relevance are the following ITU-R handbooks: (i) The Ionosphere and its Effects on Radiowave propagation [ITU-R, 1998], and (ii) Radiowave Propagation Information for Predictions for Earth-to-Space Path Communications [ITU-R, 1996], The ITU-R Recommendations on Radiowave Propagation [ITU-R, 1997] is also an important source of information. For those involved in more fundamental issues of radio science, a bridge is provided by organizations such as the International Union of Radio Science (URSI). The progress of radio science has been reviewed periodically by URSI revealing the state-of-the-art in propagation assessment, modeling, design factors, and mitigation schemes. Some of the more recent editions of the Review of Radio Science should be consulted; i.e., Stone [1999],

Ionosphere Telecommunication
Figure 4-1: The ionosphere has a substantial impact on communication, command, control, and surveillance systems. The cartoon illustrates earth-space paths, skywave (ionospheric bounce) paths, and paths that exploit the earth-ionosphere waveguide. From Goodman and Aarons, [1990],

This chapter examines many of the systems of modern significance vis-à-vis space weather effects, but we will suppress a detailed discussion of the bands from ELF to LF, the so-called longwave bands, given the reduced emphasis on technological systems using that part of the spectrum. Even so, we will identify major effects on longwave systems and provide suitable references to the interested reader. Much of our attention will be directed to the HF communication and surveillance systems, and satellite systems having a variety of missions (i.e., communication, surveillance, navigation, earth observation, and science applications). Other system types will be covered on a case-by-case basis. Satellite systems typically use radio frequencies at VHF-UHF and even higher, and the use of GHz frequencies is substantial. We would expect satellite systems to be less vulnerable to space weather than terrestrial skywave systems since most effects diminish with increasing frequency. Nevertheless we will discover that practical HF systems with space weather compensation are not all that bad, and operational satellite systems are not all that good.

It is well established that the ionosphere is greatly influenced by ionizing radiation emanating from the sun, including both electromagnetic flux and energetic particles. The major sources of this radiation are associated with active regions on the sun that may host a preponderance of sunspots. Over the years, aeronomists and radio engineers have developed algorithms that describe the circumstantial relationship between the number of sunspots and the ionospheric effects that are observed. In recent times, it has become clear that other solar phenomena, such as coronal mass ejections (CMEs) and configurations of the interplanetary magnetic field (IMF), play a prominent role in the ionospheric state, especially in relation to disturbed periods. Even so, sunspots play more than a legendary role in the long-term trends of ionospheric behavior. The daily sunspot numbers were displayed in Figure 27, and many system users are tempted to use these values. Generally this is a serious mistake, since the ionosphere does not respond in accordance with daily values. However the median ionospheric properties can be successfully parameterized in terms of smoothed values of sunspot number. Figure 4-2 depicts the smoothed sunspot number from 1800 to the present time. Climatological models are based upon smoothed values of the sunspot number or solar flux, and even quasi-real time models require a degree of sunspot number smoothing (e.g., 5-7 days) to provide optimal results. Solar activity is closely associated with ionospheric structure and dynamics, and higher values typically imply enhancements in the maximum electron concentration within the various layers. The reader is referred to Chapter 2 (on space weather) and Chapter 3 (on the ionosphere).

Year

Figure 4-2: Pattern of the smoothed sunspot activity from 1800 to the present. See Figure 2-7 for a representation of the range of daily values.

Year

Figure 4-2: Pattern of the smoothed sunspot activity from 1800 to the present. See Figure 2-7 for a representation of the range of daily values.

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