Current Measurement Technology

Acoustic Doppler Current Profiler (ADCP)

The Acoustic Doppler Current Profiler (ADCP) measures the speed and direction of ocean currents using the principle of Doppler shift. Anyone who has ever heard a train whistle is familiar with the Doppler effect. When the train is traveling towards you, the whistle's pitch is higher. When it is moving away from you, the pitch is lower. The change in pitch is proportional to the speed of the train. The ADCP exploits the Doppler effect by emitting a sequence of high frequency pulses of sound that scatter off of moving particles in the water. Depending on whether the particles are moving toward or away from the sound source, the frequency, or pitch, of the return signal is either higher or lower. Particles moving away from the instrument produce a lower frequency return and vice versa. Since the particles move at the same speed as the water that carries them, the frequency shift is proportional to the speed of the water, or current. The ADCP's acoustic transducers emit and receive acoustical pulses from different directions. Current direction is computed using trigonometric relations to convert the return signal from the transducers to "earth" coordinates (north-south, east-west and up-down). Because the emitted sound travels through the water column, the ADCP measures the current at many different depths simultaneously. In many instances, it is possible to measure the speed and direction of the current from the surface of the ocean to the bottom.

ADCP pre-deployment	ADCP post-deployment
The profiler is the upright cylinder with the 3 transducers on the top. The yellow horizontal cylinder holds the electronics and batteries. The post-deployment picture is after 4 weeks in the ocean. Note the extensive marine growth.

High Frequency (HF) Radar

HF Radar antennas (WERA) at Pritchard's Island, SC.
A total of 12 antennas are displaced laterally along the dunes.
These antennas receive the radio signal scattered off of the surface waves
and use this information to measure current speed and direction.

HF Radars are used by oceanographers to measure surface currents. Generally speaking, surface currents represent the region within the top few meters of the water column. Like ADCPs, HF Radars use the principle of Doppler shift to measure ocean currents. Unlike ADCPs, which use sound waves scattered from water-borne particles, HF Radars use electromagnetic waves scattered from ocean surface waves. While the medium is different, the principle of operation is identical. Ocean waves propagating on a current directed towards shore will be moving a little faster than waves propagating on an ocean without a current. Ocean waves traveling on a current directed in the opposite direction will be moving a little slower than waves traveling on an ocean without a current. This produces the frequency shift (Doppler shift) needed to extract the current speed.

A typical HF radar includes a set of transmit antennas and receive antennas. The antennas must be close to the water and are usually placed on the high dunes adjacent to the beach. The transmit antenna emits a radiowave out to sea. The radiowave scatters off of the surface waves. Some of the scattered energy is directed back towards the beach, where it is collected by the receive antennas. The return signal is Doppler shifted depending on whether the current is traveling towards or away from the direction of the radiowave.

HF Radars monitor the sea remotely and can measure currents at many individual points over a large area simultaneously. The range is controlled by the radio or operational frequency. Lower frequency radars (5 MHz) have longer ranges, while higher frequency radars (15-20 MHz) have shorter ranges. Long-range systems are able to observe currents out to a maximum of about 120 miles. In the South Atlantic Bight, long range systems can cover almost the entire width of the continental shelf reaching the edge of the Gulf Stream. Because of their wide area coverage, HF Radars are perfectly designed to aid search are rescue operations in coastal regions.

Oceanic Drifters

Drifter and drogue profile
(Image credit: NOAA)

As the name implies, drifters measure water motion by passively drifting with the current. A drifter's velocity is obtained by monitoring its position through time. This determines the path of the parcel of water that encompasses the drifter, and hence produces a measure of ocean currents. Modern drifters are equipped with radio or satellite telemetry that reports their position at regular time intervals. Typically, position is monitored every few hours and drifters can remain in the ocean unattended for periods of several years. In addition to surface drifters, some are designed to "float" at a certain depth below the sea surface. Therefore, drifters can also track the movement of subsurface water masses. It is important to distinguish between the type of flow that is measured by a current meter as opposed to a drifter. A current meter monitors the flow through time at a fixed point, whereas drifters track the position of a water parcel as it moves through the ocean. For more info on drifters visit http://www.aoml.noaa.gov/phod/dac/gdp_drifter.html

Satellite Altimetry

TOPEX/Poseidon Altimetry diagram
(Image credit: NASA/JPL)

Satellite altimetry uses electromagnetic waves to measures the vertical position of the sea surface from a space born platform. While corrections must be applied to account for the atmosphere and the earth's varying gravitational field, the height of the sea surface is determined by measuring the travel time of the electromagnetic pulse between the satellite and the ocean surface. As the satellite orbits the earth, the altimeter continually acquires measurements from points all over the globe. Because of the earth's spin and the satellite's trajectory, a complete map of the earth is acquired every 10 to 15 days. Altimeters can resolve very small differences in sea surface height between two points on the surface of the ocean. This information is used to estimate the pressure gradient, which in turn is used to estimate the current. In technical terms, the current derived from satellite altimetry is denoted as the geostrophic velocity. Because of its wide area coverage, satellite altimetry is a very powerful and robust way to measure large scale circulation patterns such as the Gulf Stream.