VI. Navigation and Positioning 



The requirement for accurately monitoring the ship and sweepline array 

 positions during the recovery operation dictated the need to deploy an acoustic 

 transponder net in the vicinity of the MAVA. The critical assumption had been 

 made that the one funv^tioning anchor release on the MAVA would continue to 

 function long enough to deploy the transponder net. This assumption was made 

 following an analysis of the life expectancy of the transponder batteries. It 

 was further assumed that the MAVA transponder would function long enough to 

 complete the retrieval operation. Thus it was decided to use the MAVA trans- 

 ponder as one corner of a triangular net therefore reducing the number of 

 transponder arrays which had to be deployed. A backup transponder array was 

 available, however, in the event that the MAVA transponder failed during the 

 operation. 



The general configuration of the transponder net is shown in figure 17. 

 The transponder arrays were rigged such that the transponder was positioned 

 100 meters from the bottom. Transponders "A" and "B" were deployed 1.5 miles 

 from MAVA to form a right triangular navigation net. The line from transponder 

 "A" to MAVA was oriented with respect to the prevailing wind direction for the 

 area during the month of December. 



Determination of the exact geometry of the navigation net was done in 

 several steps. First, the depth of each transponder ("A", "B", MAVA) was 

 determined by taking depth soundings above each transponder and also by measuring 

 the minimum slant range to each transponder. The length of each leg of the 

 triangular net was determined by measuring the slant ranges to the transponders 

 at each end of the leg while slowly crossing that leg. These slant range data 

 pairs were then converted to horizontal ranges and added together. When the 

 sum of the two horizontal ranges reached a minimum, the length of that leg 

 was defined. By determining the length of each side of the triangle in this 

 manner, the net geometry was defined. 



Orientation of the navigation net relative to magnetic north was done 

 by running a straight course between two satellite fixes and continuously 

 plotting horizontal distances to MAVA and transponder "A" from dead reckoning 

 (DR) positions along the ship's track. The bearing of the line between "A" 

 and MAVA was thus determined, figure 18. 



A precision, hull mounted speed sensor had been installed for the 

 purpose of determining the DR positions during the operation. The sensor was 

 designed and built at NAVOCEANO using a 4 cm. diameter rotor. The low thres- 

 hold (3 cm/sec) and high accuracy (+2 cm/sec) of the sensor was particularly 

 suited to the low speed recovery operation. 



The sentinel was instrumented with an acoustic transponder and pinger. 

 The former permitted acoustic ranging to the transponder net while the latter 

 provided the heigh+ off the bottom data required to convert slant ranges to 

 horizontal rangt. The altitude of the sentinel was also used in order to 

 define the catenary shape of the sweepline array. 



11 



