J. ACOUSTIC DOPPLER 



1. INTRODUCTION 



Both the Janus JN-400 and the Raytheon AN/SQS-12 acoustic Doppler 

 navigation systems are self-contained active systems that operate on the Doppler 

 or frequency- shift principle. In operational use, sound heads are positioned on 

 the navigating vessel to transmit acoustic energy toward the ocean floor and to 

 receive energy reflected from it. A precise measurement of the frequency shift 

 of the reflected signal provides an accurate measure of the vessel's velocity over 

 the bottom. An accurate magnetic or gyro compass is required for determining 

 the heading reference. 



These systems are capable of supplying the three basic quantities 

 required for accurate navigation, i.e. , speed relative to the bottom, distance 

 traveled, and drift angle. In the Janus system, this information is displayed 

 in digital form, and the course and position are manually plotted. Automatic 

 track-following and plotting equipment has been developed for the more accurate 

 Raytheon AN/SQS-12,. Both systems are presently limited to operation in shallow 

 waters of about 250-300 feet, where measurements of velocity and distance 

 traveled have been demonstrated to be accurate to within 1%. 



2. DESCRIPTION 



a. Janus JN-400 



The JN-400 equipment was developed for use on small craft in shoal 

 waters less than 250 feet deep. The vessel's true speed over the bottom, the 

 total distance traveled, and the drift angle with respect to a lubber line are dis- 

 played to the navigator in digital form. The true course over the bottom is found 

 by adding the drift angle to the compass reading. The ship's relative position with 

 respect to the point of departure is manually plotted by taking a digital reading 

 (in miles) from the distance -traveled counter. This counter may be reset as 

 required. 



The basic track information is obtained by a fixed mounting of four 

 transmitting transducers which are arranged to direct acoustic energy toward the 

 ocean floor (see Figure IV-15). Energy striking the bottom is reflected and is re- 

 ceived by four separate receiving elements. When the vessel is in motion, the 

 received signal ( A f) is Doppler -shifted from the transmitted frequency (f) by an 

 amount directly proportional to the velocity component (V) over the bottom and 

 inversely proportional to the acoustic propagation velocity (C). 



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