Drifters have been used to verify that sea surface heights in the ocean can be used to estimate current speed and direction in regions such as the Gulf of Mexico.
This image of the Gulf of Mexico is created from satellite altimeter data. The colored shading represents the relative highs and lows of the sea surface as measured from satellite altimeter data. Currents in the Northern Hemisphere tend to flow clockwise around centers of high sea surface height and counter clockwise around centers of low height. On the image, the arrows or vectors represent the speed and direction of currents calculated using the sea surface height data. The length of the arrows indicates the speed of the current. Notice that the currents are fastest in areas where the sea surface height is changing quickly across a small region (where the strong highs and strong lows get close to each other, a relatively steeper slope in elevation). Drifter data can be compared to the calculated currents to determine how accurate the estimates of speed and direction are.
Comparisons of NOPP Drifter tracks with sea surface height are available at:
http://www-ccar.colorado.edu/~realtime/yoto-drifters