14-17] 



SYSTEM CONSIDERATIONS 



777 



airplane can cause distortions in the map, since in most systems altitude is 

 used to set in the range scale factor. Angular distortion of the map can 

 occur also if there is no compensation made for airplane drift. This effect 

 can be compensated for in several ways if the drift angle is known. In 

 forward-looking systems the sector scan can be repositioned along the 

 ground track of the airplane. In side-looking systems where the orientation 

 of the antenna may not be changed easily, correction for the drift angle can 

 be obtained, for example, by rotating the CRT trace line by an amount 

 equal to the drift angle. 



Stabilization errors occur when the antenna is not stabilized for aircraft 

 pitch and roll displacements. Such errors produce distortion and smearing 

 of the map. The degree of distortion or smearing that will occur is difficult 

 to define analytically. However, some idea of the nature of the problem 

 can be obtained by considering the range vector to some point on the 

 ground, as shown in Fig. 14-20, and the displacement produced in it by 



Fig. 14-20 Airplane Coordinates. 



airplane pitch and roll displacements. It can be shown that when the 

 airplane pitch angle Oa and roll angle <pA are small, the components of the 

 displacement Ari, Ary, and Ark along axes parallel to the airplane set are: 



Ar,- = dA[<t>ARG sin ^ly -\- h] (14-25) 



Arj = <PAh (14-26) 



Ark = RgI^Pa sin yf^ - Oa cos ^] (14-27) 



where the various quantities are defined in the figure. These equations 

 apply to forward-looking systems; for side-looking systems, where xj/ = 90°, 

 the above equations reduce to 



