Long term azimuth and elevation measurements in the HF band

Even in the time of satellite communication long distance links in the HF band (3-30 MHz) via the ionosphere are still of great significance. There is also a need for reconnaissance and location in this frequency band. One approach is the single site location (SSL) technique which is capable of estimating the location of transmitters with only one direction finder (DF). A prerequisite for this method is the ability of the DF to measure both azimuth and elevation angle of the incoming wave. This is for example possible by using the interferometric DF. The SSL method works if: the wave is propagating via the ionosphere (sky wave propagation); the virtual height of the ionosphere at the point of reflection is approximately known; and the wave is propagated with a single "hop" (the number of ionospheric reflections) between transmitter and DF. Clearly these assumptions may not be satisfied in practice. If we do not know the position of the transmitter (which is the normal situation in reconnaissance) we will not know the point of reflection. Hence it is not possible to measure or estimate (by means of an appropriate propagation prediction program) the ionospheric behaviour at this point. Furthermore the number of hops is unknown. Nevertheless the SSL method works satisfactory in some cases. We show results from a long term measurement campaign made in the winter of 1995/1996 (azimuth and elevation, signal amplitude) and a measurement campaign in November 1994. The data is used to check the validity of a new prediction program which is actually under development.