(Doppler shift) A f = -^ f (1) 



Two pair of beams are used, one for the fore-aft and one for the port- 

 starboard direction. This geometry tends to minimize the effects of vessel 

 motion. Processing of the Doppler frequencies is performed in a small electronic 

 console to yield the two velocity components and the resultant velocity over the 

 bottom. The velocity is integrated once to obtain the distance traveled, and its 

 phase relative to the fore-aft velocity components indicates the drift. Mechanical 

 counters display both the drift and the distance traveled to six digits with a reso- 

 lution of ± 0.001 nautical mile (±6 feet). 



The maximum depth where over-the-bottom information may be obtained 

 is approximately 250 feet. This limit is set by limitations in power level and the 

 high frequency used. At depths greater than 250 feet, sufficient return may be 

 available from solid matter distributed within the water. However, velocity, in 

 this case, is measured relative to subsurface currents. Although these generally 

 have velocities between and 0.5 knot, in some areas the velocity of subsurface 

 currents may be considerably greater than this, and will produce an error in true 

 velocity. 



The system accuracy is 1-2% of the distance traveled. The ±0.3° 

 drift error specified for the equipment is considerably lower than the error in 

 many of the heading references available. Velocity measurements have an ac- 

 curacy of the order of 1% with a minimum threshold of about 0. 1 knot. 



b. Raytheon AN/SQS- 12 



The SQS-12 is a more sophisticated acoustic Doppler navigation system 

 which is being developed for Navy use in shallow waters. Early tests of prototype 

 equipment have indicated that this system can provide automatic dead-reckoning 

 information with an error of less than 1%. 



This developmental system uses an array mounted to a fixture on the 

 bottom of the vessel in such a manner that the four transmitting-receiving pairs 

 are arranged in two mutually perpendicular vertical planes. To increase the 

 system accuracy, a stabilizing system can be employed to keep the 8-transducer 

 transmitting and receiving array oriented in the true vertical direction and inde- 

 pendent of small motions of the vessel. The array is also maintained in a north- 

 south orientation by a servo mechanism which is referenced to a Mark 19 or 

 the smaller, less accurate, Mark 26 gyro compass. It is the heading reference 

 or compass error which limits the over -all accuracy of this system. 



101 



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