about 30 MHz are limited to line-of-sight. However, depending on ionosphere 

 conditions, radio frequencies below 30 MHz, HF, may propagate by skywave 

 well beyond line-of-sight and international boundaries. Because of extended 

 range skywave propagation, HF direction frequency (DF) has the potential 

 to cover large areas of earth. However, this area coverage is constrained 

 by signal frequency, time of day, season of the year, latitude, and sun- 

 spot cycle. Thus, areas of immediate interest may not be accessible for 

 hours, months, or years from a land-based DF station. 



There are many users of the HF radio spectrum, and it 

 is common for more than one user to time share a frequency. Furthermore, 

 there are users that do not abide by international treaties and regula- 

 tions. Thus, it is unusual to find a clear HF radio channel and to 

 identify a signal source solely on the basis of frequency occupancy. 

 Hence, call sign, language spoken, and other features of the signal be- 

 come important parameters for identification of HF radio signals. 



The line-of-sight range for signals above about 30 MHz 

 requires that the direction finder be within line-of-sight for signal 

 intercept and DF. This range can be extended by the use of airborne 

 and space platforms. The mobility of these DF platforms have a further 

 advantage in that the DF station may be moved to the geographical area 

 of current interest. HF DF can also be accomplished from airborne 

 platforms. 



Generally HF DF is not feasible from space platforms because the plat- 

 form is above the ionosphere, which makes the propagation path frequency 

 dependent. 



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