Ocean drifters are specifically designed to track the movement of water (currents) within the sea. To do this, they must be carefully constructed and tested to ensure that the movement of the drifter coincides with the flow of water and not that of the wind. There are three main sections on a drifter: 1) a float package, 2) a cable and 3) a drogue.
The float or surface buoy on an ocean drifter serves several purposes. It provides buoyancy for the drogue and sensors, and protects the sensor and transmitter electronics from getting wet.
The transmitter on a drifter transmits data to satellites that determine the buoy's position and relay the data to an ARGOS ground station. The raw drifter data is then sent to a data assembly center where it is processed and distributed. The surface float may also house instruments (sensors) which measure (sense) sea surface temperature, pressure (barometer), ocean color (irradiance), and submergence. A submergence sensor is used to determine if the sail-like drogue is still attached. When the sensor indicates frequent submergence the drogue is present (weight in waves causes submergence). When submergence rarely occurs, it suggests that the drogue has been lost.
A strong cable is used to attach the float package to a sail-like drogue. If more buoyancy is needed then a subsurface float or buoy can be added to the cable. Temperature and salinity sensors can also be attached to the cable. Using noise below the oceans surface scientists have also found a neat way to measure wind. A hydrophone (underwater listening device) is attached to the cable of a drifter. The greater the wind speed, the louder the noise heard by the hydrophone. Using this relationship, wind speed over the drifters can be estimated with a hydrophone.
A drogue is a device on a drifter that acts like an underwater sail. When pushed by an ocean current, the drogue helps a drifter move with the flow of water. Without a drogue, drifters do not depict ocean currents accurately, but become subject to wind and wave action.