Measurement
ADCP prior to deployment.
Description
An ADCP is a current-measuring instrument that transmits high-frequency acoustic signals. Current is determined by a Doppler shift in the backscatter echo from plankton, suspended sediment, and bubbles, all assumed to be moving passively with the mean speed of the water.
The ADCP consists of four acoustic transducers, inclined slightly to the horizontal plane. When combined with the instrument tilt and compass heading, the measured Doppler shift along each beam axis provides an estimate of the three primary current directions: U (east), V (north), and W (upwards). By segmenting the received signal into time-gated windows, an estimate of the velocity is obtained in vertically separated bins above the ADCP.
When plotted against depth, these readings form a velocity profile. A velocity profile is obtained for each acoustic ping, which may be programmed to repeat every few seconds (0.5 – 2 Hz). Averaging profiles from many pings provides a more accurate ensemble profile of the ocean currents. Plotting successive profiles shows the variation of currents at each depth over time.
The ADCPs are mounted to instrument platforms within a gimbaled device. This ensures that the ADCP faces upward, regardless of platform orientation.
