14-5] NAVIGATION SYSTEM ERRORS CAUSED BY INTERACTIONS 747 



very slight shift of the center of gravity of the received doppler spectrum 

 and hence of the measured velocity. These errors, though very small, de- 

 pend on beamwidth, increasing with increasing beamwidth. Flight calibra- 

 tion can completely "bias out" the terrain bias error over land. 



Over water, the situation is more serious. The velocity accuracy is 

 decreased by three causes: (1) an increased terrain bias error, (2) an error 

 incident to surface droplet motion of the water, and (3) an error incident to 

 water current motion. 



The terrain bias error is perhaps the most important. It results from the 

 marked change in back-scattering coefficient with looking angle over the 

 extent of the radiated beamwidth. Figure 14-10 is a set of plots of the 



-20 



10° 



20 



30 40 50 60 



RADAR DEPRESSION ANGLE 



70" 



Fig. 14-10 Scattering Coefficients for Land and Water, X Band. (Courtesy, 

 General Precision Laboratory, Inc.) 



measured back-scattering coefficient /(i/') versus the radar depression angle 

 (see Fig. 4-4) for various Douglas sea state conditions and several types of 

 terrain at X-band frequencies. The cross section per unit area cr" is related 

 tof{4') by the expression o-" = 2/(;^) cos \p. Douglas sea state 1 is normally 

 defined as "smooth sea with wave heights less than 1 foot" and Douglas sea 

 state 4 is normally defined as "rough sea with wave heights from 5 to 8 

 feet." (Because of the manner in which Douglas sea states as well as the 

 so-called Beaufort sea states are defined — on the basis of wind speed and 

 wave height — they do not appear to be quite fine enough from the 

 viewpoint of radar scattering. A more applicable, fine scale of sea states 

 was developed by J. Campbell of General Precision Laboratory, Inc., and is 



