Figure 9 is a plot showing the general performance 

 to be expected of ground-based HF DF as a function of distance. The 



areas of good, fair, and poor position location accuracy is approximately 



2 2 2 * 



1000 miles , 100,000 miles, and 10 million miles , respectively. 



(2) Airborne DF Techniques 



DoD, Army, Navy, and Air Force have airborne HF, VHF, 

 and microwave DF systems. Modern systems use on-board digital computers 

 to tune radio receivers, make DF measurements, apply known corrections, 

 and to exchange data between other airborne systems for real-time tri- 

 angulation. 



Assuming favorable geometric conditions for the 



intersection of the lines of bearing, a line-of-sight range of 200 nmi,^ 



+ 

 and a navigational accuracy of 2 nmi for the DF platform position, a 



CEP of the order of 5 nmi is possible. The mobility of the platform 

 permits the CEP to be reduced to nearly the CEP of the platform naviga- 

 tion system by homing on the signal source. 



As discussed in the previous section, the propagation 

 ranges for HF signals is not limited to line-of-sight. However, the 

 airborne HF DF accuracy will be best when the airborne platform is within 

 line-of-sight of the signal source. 



An airborne platform at 20,000 ft altitude will have 



a line-of-sight range of 175 nmi and will be able to receive radio signals 



2 

 from an area of 95,000 miles . With a platform speed of 300 kts it is 



Assuming favorable geometry for the intersections of the lines of bear- 

 ing and a spacing between DF stations that is compatible with the dis- 

 tance shown in Figure 9. 



Platform altitude of 20,000 ft. 



Typical for LORAN-C and OMEGA. 



82 



