1989-1990 CHANGES 



The permanent 

 changes made in 1989-1990 

 were based on a combination 

 of the drift of the satellite- 

 tracked buoys that Ice Patrol 

 uses in its operations and in- 

 formation gathered from the 

 scientific literature. This sec- 

 tion describes the processing 

 of the archived drifting buoy 

 data and presents a brief chro- 

 nology of the modifications 

 made in 1989 and 1990. 



DATA PROCESSING 



The drifting buoy 

 data set, which extends back 

 to 1976, now has 125 buoy 

 trajectories with a total of 

 nearly 5000 buoy-days of drift. 

 The trajectories are described 

 in the yearly Ice Patrol Bulle- 

 tins. The buoy configuration 

 and the position data quantity 

 and accuracy varied over the 

 collection period. 



The standard buoy 

 configuration is a 3-m-long 

 spar hull with a 1-m-diameter 

 flotation collar at the water- 

 line. Most of the buoys were 

 equipped with a 2-m x 10-m 

 window shade drogue (some 

 slightly larger) tethered to the 

 hull. Over the 15-year period, 

 the tether length varied some- 

 what, starting at 10 m (1979- 

 Page 82 



1982), then to 30 m (1983- 

 1984), and finally to 50 m 

 (1985-present). Only data for 

 the period from March through 

 August, roughly correspond- 

 ing to the Ice Patrol season, 

 were used in the data set. 



The number of buoy 

 positions received each day 

 and the accuracy of these fixes 

 varied over the years. During 

 the firstfewyears (1976-1 979) 

 the buoys were tracked by the 

 Random Access Measure- 

 ment System (RAMS) on the 

 NIMBUS-6 satellite. The ac- 

 curacy of this system was 

 about +/- 5 km and typically, 1 - 

 2 fixes were received each 

 day. In 1 979 Ice Patrol began 

 tracking buoys withthe TIROS 

 satellites, first using a local 

 userterminal atthe U. S. Coast 

 Guard Oceanographic Unit 

 (1979-1981), and finally 

 through Service ARGOS 

 (1982-present). This most 

 recent period has the highest 

 quality and most densely- 

 sampled data in time, with 

 position accuracy of about 350 

 m and 6-10 fixes per day for 

 each buoy. About 70 percent 

 of the data used in the modifi- 

 cation are from this period. 

 The trajectories were treated 

 as a homogeneous data set. 



All records were 

 scanned and obviously bad 

 positions were removed. The 

 quality controlled position data 



were then fitted to a cubic 

 spline curve to arrive at an 

 evenly-spaced record with an 

 interval of 3 hours. For most 

 of the tracks, this resulted in 

 no net increase in the number 

 of fixes per day. FortheNIM- 

 BUSdataandthe early TIROS 

 data, this process resulted in 

 an increase of data points in 

 the interpolated records. The 

 interpolated position records 

 were then filtered using a low- 

 pass cosine filter with a 

 cut-off of 1.17 X 10-5 Hz (one 

 cycle/day). This filter removes 

 most tidal and inertial effects. 



The filtered, interpo- 

 lated drifter data were aver- 

 aged in bins bounded by the 

 midpoint between adjacent 

 grid points of the iceberg drift 

 model current field. For most 

 of the IIP operations area, this 

 resulted in the grid point that 

 was centered in the bin. Along 

 the boundaries between the 

 two different longitudinal 

 grid spacings (along 46 W 

 and 50 W), this resulted in 

 grid points that were slightly 

 off center in the east-west di- 

 rection. First, the average 

 speed and direction of an indi- 

 vidual buoy that moved 

 through a bin was calculated. 

 Then, the movement of all the 

 buoys that drifted through the 

 area represented by the bin 

 were averaged to arrive at a 

 single mean vectorforthe bin. 

 This mean vector was calcu- 



