TABLE OF CONTENTS In this chapter we will discuss techniques for geolocating HF signals using only a single-sensor site. As will be seen, this is possible because long-range HF signals are reflected (refracted actually) in the ionosphere back to the Earth, and measuring the angles of arrival in three dimensions allows calculation of the PF of the transmitter when the height of the point of reflection is known. This height information is not required, however, when there are two or more ray paths arriving at the sensor sight and the angles of arrival of these rays can be measured and discerned.
Under some circumstances, HF signals can propagate for considerable distances. This long-range communication capability of such signals was used by navies around the world for ship-to-shore and ship-to-ship communications before satellite systems were available. In fact, it is still used for that purpose.
The ionospheric layers that surround the Earth are responsible for the phenomenon. During the daytime there are four layers involved in the process, the lowest being the D-layer, the next the E-layer, and then two F-layers: the F1 and F2. At night the two F-layers combine into one and the D- and E-layers disappear, as shown in Figure 4.1. These layers are regions of ions that have been charged by bombardment of atoms with photons from the sun and the resulting free electrons. It is widely believed that it is the free electrons that are responsible for the refraction phenomenon; the positively charged ions have...
