This chapter discusses ionospheric effects. The ionosphere is a region of ionized plasma that extends from roughly 50 to 2,000 km above the surface of the Earth. The boundary of the Earth's magnetic field within the solar wind is known as the magnetopause; between the magnetopause and the ionosphere is the magnetosphere. The magnetosphere extends about 10 Earth radii in the direction of the Sun, acting as a 'bow wave' against the solar wind. On the other side of the Earth, the magneto sphere streams backwards for about 60 Earth radii.

There are a lot of atmospheric particles and possible weather effects that can exist on the path from a satellite to an earth station. We have seen that the clear-sky level of a signal from a satellite measured at an earth station can change appreciably over a day, a season and a year. So what baseline is set for the measurement of the signal attenuation? There are several measures for the change in signal level along the path. In absolute terms, the total attenuation along the path is the difference between the signal level at the earth station that would exist if there were a vacuum along the path from the satellite to the earth station and the signal level that is measured at the earth station. Total attenuation is the sum of every attenuating mechanism along the path. Excess attenuation is normally taken as the dif ference between the clear-sky level (which generally includes some clouds, gaseous attenuation and possibly low-level tropospheric scintillation) and the signal level in a significant attenuating event. If the cloud, gaseous attenuation and other low-level loss components can be assumed to be constant over the attenuating event, then the excess attenuation measures only the contribution due to significant tropospheric scintillation and rain effects. The excess attenuation of a radiowave passing through the atmosphere is made up of two principal components: absorption and scattering. Absorption takes place when the incident radiowave energy is transformed essentially into mechanical energy, thereby heating up the absorbing material. A radiowave is said to be scattered when its energy is redirected from the original propagation direction without losing any substantial energy to the scattering particle, or particles.

Commercial global communications via satellite began with Intelsat in 1965, using large, fixed earth stations (~30 m in diameter) communicating with each other at C-band (6 GHz on the uplink and 4 GHz on the downlink) via spacecraft in geostationary orbit. The almost universal reach of geostationary satellites, coupled with the fact that the majority of the Earth's surface is water, made the move to offer telecommunications services to ships a logical progression. The original geostationary satellites were small and power limited, and there was clearly no way a 30-m diameter antenna could be located aboard even the largest ship. The solution reached was to offer services at L-band (between 1 and 2 GHz) rather than at C-band and to make the services narrowband (one equivalent voice channel) rather than attempting to provide broadband links.

The previous chapters have detailed the impact of various propagation impairment phenomena on the radiowave signals between earth stations (both fixed and mobile) and satellites. In each chapter, the system effects of the impairment in question were discussed and some means of reducing the impact of the impairment alluded to. This chapter will present in some detail the various schemes suggested and, in some cases, already implemented to restore performance during signal impairment.

This appendix considers equations that appear in the text or are referred to in the text of the book.

This appendix covers non-ionized media propagation, data acquisition, data analysis tropospheric propagation, radio links, ionospheric disturbance warnings, transmission loss, ground-wave propagation, microwave interference, ionospheric propagation, VHF propagation, UHF propagation, long-term ionospheric predictions, radio refractive index, and free-space attenuation.