High Frequencies – HF covers communication on frequencies 3 – 29,999 MHz. The interval between channels is 1 kHz.
With the ability range of the high frequency radio waves rereflected from the ionosphere and radio communication over long distances is provided (fig. 112).
High Frequency radio waves
Fig. 112. Rereflection of high frequency radio waves in ionosphere
The air in the ionosphere strongly rarefied. Under the influence of solar radiation in the ionosphere of atomic gases released many free electrons, resulting the appearance of positive ions. Ionization of the upper layer of the atmosphere is being. Ionized layer is able to absorb radio waves and distort their way. During the day, depending on the intensity of solar radiation the number of free electrons in the ionized layer, its thickness and elevation change and this change and electrical properties of this layer, respectively, and the ability rereflect radio waves. Due to this effect the range of high frequency radio communication is 3000 km. High frequency radio communication equipment is one of the main means of communication over long distances, which is important, especially in performance of flights between continents.
High Frequency radio Data Link, HFDL
In speech communication for exchanging ARINC has developed a global system of digital data exchange (High Frequency radio Data Link – HFDL) [39]. The system consists of 14 ground stations receiving and transmitting information (fig. 113), connected to a central server, which provides access to on-board equipment ground data network (AviNet®) to obtain the necessary information. Data exchange is at 300, 600, 1200 or 1800 bit / s speeds.
Fig. 113. Location of HFDL ground stations