ked 
rison 
intervals 
The field 
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1g drifter 
han most 
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than the 
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rchers at 
mass and 
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eters, by 
y results 
n in the 
ublished 
ade with 
and by 
methods 
  
are considered to provide accurate and useful 
indications of current speed and direction. The 
current meters, however, measure the current 
passing by at a fixed location in time, while the 
drifters are considered to accompany a parcel of 
water along its course. When current 
measurements are made in a region where the 
horizontal circulation is reasonably uniform, the 
methods are expected to provide very similar 
data. A comparison of such paired data was 
made by Collin et al (1968). A comparison of 
drifter and moored current meter measurements, 
based on theoretical considerations was made 
by White (1986). 
On the continental platform off SE Brazil, 
however, the presence and interaction of 
vortices with the Brazil Current result in a 
complex circulation that is still poorly 
understood. It is important therefore, to know to 
what extent the velocities of ocean currents 
measured by anchored current meters and by 
drifting buoys in COROAS are similar, and 
when necessary can be substituted for one 
another. The objective of this report is to make 
a preliminary comparison of the surface layer 
ocean current data obtained from satellite 
tracked drifters with data from an anchored 
current meter located in the same region. 
STUDY AREA 
In order to maximize results from different 
research components of COROAS, most of the 
field work was done within the area delimited 
by 23.0?S, 44.8? W;.. 25.1? 8, 43.1? W; 26.8? S, 
45.2? W; and 25.1? S, 47.0? W (Fig. 1). 
DRIFTER DATA 
Positional and sea surface temperature data 
were obtained from 15 WOCE standard, low 
cost drifters (LCDs) launched in groups of 5, in 
the vicinity of 24.8?S, 44.3? W, in February and 
July 1993 and January 1994. The drifters were 
Set to measure the current at 15m depth. The 
drifters used ARGOS compatible data 
collection platforms (DCP's) and transmitted 
655 
their data via CLS ARGOS equipment aboard 
NOAA-11 and NOAA-12 satellites. Data were 
subsequently obtained via telephone modem 
and PC computer from Service ARGOS, 
Toulouse, France. Data time series from the 
drifters were limited to those portions of the 
time when the drifters were within the 
previously defined area of study (Fig. 1). For 
this report data sets were limited to series from 
the first. set of 5 drifters: (LCDs 3178-82). 
Initially all of the drifters spent some of their 
time within the study area, while some months 
later several drifters passed to the NE on a 
return course through the western part of the 
study area. 
  
—20 y 
id 
Brazil 
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—35 l 
-55 -5n —45 —40 
longitude (W) 
Figure. 1. Drifter launch positions (+) and 
location of current meter moorings (x). 
Latitudinal and longitudinal data series were 
first split into shorter series corresponding to 
those time intervals when the drifters were 
within the study area. Mean current speed and 
direction were then determined for these series. 
After the individual mean currents were 
computed, the general mean current and its 
standard deviation were determined. Because 
some of the series indicated currents to the 
north and others indicated currents to the south, 
the series were separated by current direction 
and the northerly and southerly mean currents 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996