TABLE OF CONTENTS The over-the-horizon (OTH) radars we shall discuss bounce signals off the ionosphere, scatter them from targets at or near the ground, and detect these targets by observing the return path via the ionosphere [Fenster, 1977]. Technically, these are over-the-horizon backscatter (OTH-B) radars. Their operational concept is shown in Figure 7.1.
High-frequency (HF) communications have made use of ionospheric refraction to obtain spectacularly long ranges for most of the twentieth century. Ham radio operators talk to the world with their relatively low-power rigs. Even though the very first radars were in the HF spectrum, this was viewed as a disadvantage. The idea of actually radiating via the ionosphere and detecting the backscatter returning by the same path did not seem credible until coherent processing became practical in the early 1960s. The next two decades saw a slow but steady progress, as digital signal processing has permitted unlimited extension of coherent integration times. OTH-B radars exist, and the United States has deployed systems that will provide warning of an air attack while the attackers are still several hundreds of kilometers away. OTH-B radars are also used for detection of drug smuggling aircraft.
What should interest us, however, is how the principles developed for radars at microwave frequencies apply at frequencies that are two to three orders of magnitude lower. We shall examine OTH-B antennas, waveforms, and hardware.
Because the HF region available for OTH-B radars is about 6 to 30 MHz...
