Communications, Radar & Electronic Warfare (201.. - Index of

148 Communications, Radar and Electronic WarfareDAYNIGHTD Layer200-400 km (daytime)Merged F layer150-400 km (night)F1 layer150-220 km (daytime)E layer 90-120 km (daytime)IE layer' 90-120 km (weaker at night)ID layer90-120 km (daytime only)Lonosphere minimum altitude50 km StratosphereEarth's surface 117 km Troposphere minimumaltitude-at poles17 km Troposphere maximum altitude-atequator- - - --Ground conductivity layerFigure 9.1The ionosphere by day and night.In order to take advantage of HF high atmosphere skips, radio energy must beinserted into the con'ect angle and within a specific frequency band so that it does notpass straight through the reflecting regimes, refract into the wrong locations or beabsorbed by the D-Iayer. This is illustrated in Figure 9.3. The first transmission (1) isinserted at a high angle and above the highest refracting frequency. The highest usablefrequency is more normally known as the Maximum Usable Frequency (MUF).Transmission (2) is inserted at a lower angle, but still does not refract sufficiently toreturn to the surface of the Earth. Transmission (3) is below the MUF and is reflectedback towards the Earth's surface. The range achieved depends on the geometry of thelink, so it can be seen that the angle of transmission must be determined in order toreach the intended target range.HF energy refracted from the ionosphere back to the surface of the Earth will reflectback at the same angle. The energy is then refracted back down and so forth, producingmultiple 'hops'. In between these areas are regions where little or no energy is present.These are known as 'skip zones'. This is illustrated in Figure 9.4.