A sample of the array depth excursion data recorded from the 

 Vibrotron* pressure sensor is shown in Fig. 5. The average 

 peak-to-peak fluctuation in depth on the trace is about \ m with a 

 10 to 12 sec period. At the time of this plot^the sea state was 

 about 7»with steady 30 kn winds gusting to 40 kn, and waves 10 

 to 16 ft in height. Gradual, random excursions of up to 4 m with 

 periods in hours were recorded. These were attributed to hori- 

 zontal drag forces created by shear currents, because rapid 

 changes in array orientation had direct effects on maximum depth. 

 Satellite fixes showed that the drift rate of the spar was about 

 0.2 kn. 



414 r- 



406 h 

 405 



400 



2 METERS 



/^^*'v^n-v^/^/^''^/VV/'nAA^_Aw'v/vwv^vvv«*-»A»A,/VA/>/^^ 



2 3 4 5 6 

 TIME IN MINUTES 



Figure 5. Vibrotron Depth Trace 



Station-Keeping 



Maintaining station on the spar in rough weather proved to be 

 very difficult. On a dark night the spar light was easy to see 

 for 2 miles or better, but was of little use in breaking seas with 

 bright moonlight, in heavy rain or in fog. Neither of the radar 

 reflectors used could be seen reliably on the ship's radar in 

 breaking seas because of the backscatter. Also, the portable 

 lifeboat reflector was too fragile to reliably withstand being 

 dunked during spar launch. These navigation aids were very 



The Vibrotron output is digitized for recording pressure once per 

 secondhand the resolution in depth is better than io.l m.-"- 



178 



