APPENDIX D 



OPERATIONAL USE OF FREE-DRIFTING, SATELLITE-TRACKED BUOYS 



C. R. WEIR, LT, USCG 

 U.S. Coast Guard Oceanographic Unit 



The 1976 Ice Patrol season initiated the Coast 

 Guard's use of the Buoy Transmitting Terminal 

 (BTT) buoy system. This system is capable of 

 drifting with the ocean currents and transmit- 

 ting information via satellite. The information 

 is then relayed to ground stations where environ- 

 mental data and buoy position are determined. 

 A BTT buoy, called the Conshelf Drifter, is 

 shown in figure D-l. This buoy is manufactured 

 by Polar Research Labs of Santa Barbara, Cali- 

 fornia. The buoy used in 1976 was manufactured 

 by NOVA University, Fort Lauderdale, Florida 

 and was very similar in shape and design. A 

 window shade drogue 13 meters long and 2 

 meters wide was used to increase water drag. 

 A full description of this type of drogue is con- 

 tained in Vachon (1975). 



The position fixing capability and the trans- 

 mission of environmental data are accomplished 

 through the use of the Nimbus-6 Satellite Ran- 

 dom Access Memory System (RAMS). The 

 technical specifications are given by Sissala 

 (1975). Basically, the BTT buoy broadcasts a 

 frequency stabilized UHF signal for 1 second 

 every minute regardless of whether or not the 

 buoy is within sight of the satellite. Contained 

 within this signal is a platform identification 

 number and four eight-digit words. During a 

 satellite pass the spacecraft receives this infor- 

 mation and accurately determines the frequency 

 at which it was received. The doppler shift in- 

 formation is used to determine the buoy's posi- 

 tion. 



The buoy used in 1976. platform I.D. 0177, 

 was deployed on 4 April in position 46°59.2'N, 

 47°15.1'W along standard section A-2. Excel- 

 lent data were received through 13 April. Dur- 

 ing this time 5 to 11 positions were obtained 

 every day. On 11 April a storm moved the buoy 

 westward and up onto the Grand Banks. The 

 depth in this area decreases to 100 meters or less 

 and may have interfered with the drogue. After 



13 April the buoy experienced an intermittent 

 electronic failure and positions were obtained 

 only on the days shown in figures D-2 and D-3. 

 These figures show the BTT movement relative 

 to general ocean currents. The last transmission 

 from the buoy was on 15 September 1976. A 

 detailed analysis of the data that were obtained 

 from this system will be the subject of a separate 

 report. 



The data from the buoy yielded three im- 

 portant results. The first was the buoy's appar- 

 ent response to wind currents. On April 4th, 

 5th and 6th the buoy drifted northwest and not 

 southwest as indicated by the dynamic topog- 

 raphy. During this same period the wind was 

 from the southeast at speeds up to 35 knots. The 

 second interim finding was that the buoy's drift 

 direction during periods when the wind was less 

 than 20 knots closely followed the dynamic to- 

 pography. The third result was that during the 

 period of moderate winds the buoy moved along 

 the edge of the Labrador Current at an average 

 speed of about 30 cm/sec. The dynamic topog- 

 raphy of a survey taken just previous to this 

 experiment gives an average speed of about 25 

 cm/sec. 



These three results have a major impact on 

 iceberg drift and they further confirm our pre- 

 viously held beliefs; that is, wind generated cur- 

 rents must be considered when determining 

 iceberg drift, that dynamic topography is quite 

 accurate in determining the baroclinic compo- 

 nent of the current direction, and that dynamic 

 topography produces a current speed that is too 

 conservative. 



REFERENCES 



Vachon, William A. 1975. Instrumented Full-scale 

 Tests of a Drifting Buoy and Drogue, Charles Stark 

 Draper Laboratory, Inc., R-9-17. 



Sissala, J. E. 1975. The Nimbus 6 Users Guide. Avail- 

 able from LANDSAT/Nimbus 1'roject, Goddard Space 

 Flight Center, Greenbelt, AID. 



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