Mariners have been studying and measuring currents for hundreds of years. The two main components of currents are speed and direction.
An observer stands on a ship, throws the drifter into the water, and then measures the time that it takes that object to move along the side of a ship. As technology improved over time, oceanographers began using mechanical current meters. A ship would deploy a meter and usually some sort of rotor would turn and measure the currents. This is still the basic process today; however there are more accurate and sophisticated instruments.
Today in the open ocean, a drifter is similar to a buoy in the water that may be equipped with global positioning system technology or satellite communications that would relay data and information. Drifters can also submerge for long periods of time to measure ocean currents at a particular depth. The drifter would then resurface occasionally to send a signal with its data and position to observers on the land.
In addition to buoys, there are other tools that are used to monitor currents. The Acoustic Doppler Current Profiler is commonly used to measure currents. It is normally deployed on the sea floor or attached to the bottom of a boat. It sends an acoustic signal into the water column and that sound bounces off particles in the water. The instrument can calculate the speed and direction of the current by knowing the frequency of the return signal, the distance it traveled, and the time it took for the signal to travel.
Many oceanographers also use radio antennas and high frequency Radio Detecting and Ranging systems (radar) to measure surface ocean currents. Similar to the Acoustic Doppler Current Profiler, these shore-based instruments use the Doppler effect to determine when currents are moving toward or away from the shore or to measure the velocity of a current.
At NOAA, oceanographers use knots to measure current speed. The term knot is defined as one nautical mile per hour. One nautical mile is equal to 1.85 kilometers (1.15 standard miles). One knot is also 51.44 centimeters per second (3.281 feet per minute).
For more information:
Center for Operational Oceanographic Products and Services
National Current Observation Program
Diving Deeper Podcast, Episode15 (August 12, 2009) - What are currents?
Currents Tutorial, NOS Education