444 REGULATORY CIRCUITS 



gain necessary to provide isolation from the expected aircraft angular 



Search-Pattern Velocity Errors. In addition to providing isolation 

 from aircraft motion, the steady-state error caused by the antenna scanning 

 motion must also be considered. Usually the antenna motion is uniform, 

 and in the azimuth or horizontal space direction it is a constant angular 

 velocity. The steady-state position error is the velocity divided by. the 

 velocity constant. ^^ The constant antenna sweep velocity is determined as 

 discussed in Paragraph 5-7 and is usually between 75 and 150°/sec.2* 

 Therefore, to keep the error below 0.25° for a 100° /sec search velocity, the 

 velocity constant must be 



^^qIP"" = 400 sec-i = K^. (8-34) 



Dynamic Errors Due to Search-Pattern Velocity Changes. Some 

 distortion is likely to occur at the ends of the search pattern because 

 physical control loops cannot follow the rapid changes in command signal 

 which are used to change the antenna velocity and position at the end of a 

 horizontal sweep to another horizontal sweep. ^^ The transient at the end of 

 the sweep requires a longer time to reach a small steady-state value than 

 does the vertical-motion transient because the vertical motion is usually a 

 small position step instead of a large velocity reversal. Since it is desired 

 to resume the steady-state error in the shortest time after the sweep 

 direction is changed, both azimuth and elevation control loops may be 

 designed to realize the desired sweep transient. If there were no aircraft 

 roll, each control channel would have different characteristics. But because 

 a large roll angle transfers much of the horizontal motion to the elevation 

 channel, both channels should be designed to have the characteristics of the 

 azimuth channel in horizontal flight. 



2^The specification derived in this manner is somewhat pessimistic because the antenna is 

 not always at the large lead angle used to obtain the data in Table 8-3. 

 241 f the open loop has the form 



i(l +s/wi)il + Vcos) 



K^ is the velocity constant. 



-^The search pattern may have several forms — a horizontal Palmer scan with several 

 horizontal sweeps spaced at the beam width and a diagonal return, a horizontal sweep and 

 return spaced at the beamwidth, a spiral scan, a circular scan, a vertical scan, or combination 

 of these types. 



^^he command changes can be made gradually to eliminate transients, but this usually 

 involves a more elaborate signal generator. Even then, the ideal pattern will not necessarily 

 be obtained, and more time may be required for the antenna sweep to resume the desired 

 constant velocity. 



