Extrahigh Frequencies

At EHF and above, propagation is primarily LOS, and be­cause of the higher frequencies (f a 300 MHz), these signals are less affected by the ionosphere than lower frequencies. On earth-space paths that traverse the equatorial and/or high — latitude ionosphere, however, the signal quality can be sig­nificantly degraded. These effects will be described below.

To summarize, the radio services most affected by the iono­sphere lie in the frequency range of «1 MHz to 150 MHz [fixed communication services, AM (amplitude modulation) and SW (shortwave) broadcasting, amateur radio]. To a lesser degree, services in the 20 kHz to 300 MHz region (mainly some of the navigation services] suffer some ionospheric per­turbation effects.

There is a plethora of radio instrumentation currently de­ployed globally that operates routinely or on a campaign basis to measure characteristics of the terrestrial ionosphere. It is beyond the scope of this article to describe these techniques, but they have been described in considerable detail in the lit­erature (2-4).

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