the bottom. Information regarding altitude 

 is also required when a mission calls for 

 flying or hovering while maintaining con- 

 stant distance from the bottom. The device 

 used is invaribly a downward-looking echo 

 sounder similar in operation, display and 

 constraints to that described for the depth 

 indicating echo sounder. Indeed, DS-4000 

 uses the same transducer for looking up or 

 down by hydraulically turning it in the de- 

 sired direction. 



For precision control, however, the sonic 

 devices' inaccuracies may be unacceptable, 

 and when the submersible's transducer is too 

 close to the bottom the return signal can be 

 masked by the outgoing signal such that no 

 altitude values are obtained. In a few cases 

 where level flight, regardless of distance-to- 

 bottom, was desired a device called a differ- 

 ential pressure gage was employed. Bass and 

 Rosencrantz (10) described such a system 



they used to fly on a horizontal plane or 

 isobaric (equal pressure) surface to conduct 

 photographic missions with ASHERAH. A 

 diagram oi ASHERAWs so-called isobaric al- 

 timeter system is presented in Figure 10.14. 

 The system provides data under the assump- 

 tion that at 100 feet or more depth (for sea 

 state 2) pressure fluctuation, due to waves, is 

 attenuated such that a horizontal plane par- 

 allel to mean sea level is defined by an 

 isobaric surface. When the bypass valve is 

 open, seawater enters the system, rises in 

 the pressure accumulator, and compresses 

 the entrapped air such that the pressure is 

 equal to the ambient sea pressure. The dif- 

 ferential pressure gage reads zero because 

 the pressure is the same on both sides. When 

 the submersible reaches a desired flying 

 height, the bypass valve is shut. The pres- 

 sure accumulator now provides a constant 

 pressure reference for the desired flying 



PRESSURE ACCUMULATOR 



BYPASS VALVE 



.*► TO SEA 



DIFFERENTIAL PRESSURE GAGE 



Fig. 10.14 The Isobaric altimeter system. [From Ref. (10)1 



486 



