An evaluation center is associated with each DF network for the purpose of furnishing fixes
and other navigational information to operations offices.
Radio direction-finding devices are used by ground force units for homing or positioning of
landing. In many cases these devices are not complete DF units but simple devices attached
to a common field radio receiver. DF is used to locate enemy aircraft in flight, ground control
stations, and radio navigational aids. The results of DF correlated with other information
comprise one of the most important sources of combat intelligence from which it is possible
to determine the enemy order of battle when there is no physical contact. This information is
usually more comprehensive than that derived from enemy radio stations. These stations are
then monitored by DF for possible movement.
As demonstrated earlier, there are many types of direction finding systems. In some, only the
azimuth of the arrival angle can be measured. In others, the azimuth and the elevation angle
can be obtained. When only an azimuth is resolved, two or more independent DF stations are
needed to determine the location of the transmitting antenna. Systems that measure both
azimuth and elevation angles are called single station locator (SSL) systems. The SSL was
developed because of the problems of audibility with the traditional DF network. Sometimes
the signal could only be heard at one DF site.
The SSL system direction finder is a phase measuring interferometer. Location data include
an azimuth measure on the target signal and a range estimate based on the measured
parameters at the DF site. The combination of azimuth and great circle range to the target
produces the location output from the system's computer.
Because the SSL system depends on ionospheric propagations, it is designed for use against
the high frequency (HF) spectrum sky wave transmissions. As shown in Figure 1-23, the
combination of an azimuth and great circle range to the target produces a location from the