February 8, 2003
where we go
There are a variety of drifter designs, ranging from the simple to the complex. Simple drifters can be cards that are released at a known point with a return address. When the cards are found by somebody, they are returned and the release point and time and the recovery point and time provide information about ocean circulation that moved the card from release point to return point. As an aside, containers of merchandise are occasionally lost from ocean going ships. A well-publicized example is thousands of Reebok sneakers lost in the North Pacific. As the merchandise washes up on shore, information is gained about the ocean circulation from where the container was lost (release point) to where the merchandise washed up (recovery point).
From this very simple drifter,
a piece of paper with a return address, a number of more complicated
designs have arisen. One of the most complicated designs has the drifter
changing its buoyancy and dropping below the surface to a certain depth,
say 500 m. On the descent, the drifter measures temperature, conductivity
and depth, among other things. In essence the drifter acts like a CTD.
Once the drifter returns to the surface, it transmits its data (temperature,
conductivity, and depth) along with its location to a land based receiving
station via satellite. This data is then combined with the data from
hundreds of other similar drifters to provide a large-scale picture
of the temperature, salinity and density in the ocean. Information about
ocean circulation is also gained from the changes in the drifter’s
position over time. These drifters can operate for months to several
The drifter we are following,
however, has a slight difference. In addition to everything described
above, the drifter has a GPS receiver. The Global Positional System
is made up of multiple satellites from which receivers can triangulate
their position. There is a receiver in the drifter’s globe that
determines position every 30 minutes. These positions are good to within
~ 50 m.
The globe transmits data ever 90 sec including the GPS determined positions.
Like the other drifters, satellites receive these data and transmit
to a land-based receiving station and these are emailed to the ship
and stored for later analysis. When we receive these positions via email,
they are several hours old. We also have a receiver on the ship that
picks up the data transmission from the drifter. The receiver passes
the data to a computer where the data is decoded and the real-time position
calculated and displayed. This way we know exactly where the drifter
is and we can follow it, make MOCNESS tows and deploy the CTD.
Dept. of Geology & Environmental Geosciences
College of Charleston
Charleston, SC 29424