Thursday, April 27, 2017

Absolute velocity estimates from autonomous underwater gliders equipped with Doppler current profilers

Schematics of the 1-MHz Nortek AD2CP and its installation in a Spray glider. (a) Front and (b) side views of the instrument showing the size of the instrument and the geometry of the four transducers. (c) The downward-looking three-beam pattern when the glider is pitch upward at 17.48; the forward-looking beam is not shown, since it is not used for velocity measurements during ascent. (Courtesy Todd et al.)

A study published in the Journal of Atmospheric and Oceanic Technology demonstrates the value of glider-based Acoustic Doppler Current Profilers (ADCP).

ADCPs can take measurements with high-resolutions in the deep ocean, making them an efficient tool for sampling large areas in relatively short times. These profilers measure ocean currents following the premise of the Doppler effect, emitting high-frequency pulses of sound that bounce off moving particles in the water.  

As underwater gliders are increasingly used in oceanography research, glider-based ADCPs are expected to become more common. Researchers of this study stated the techniques they described can be used with various types of gliders. 

Efficiently measuring ocean currents helps determine important ocean dynamics. For example, in the warm Gulf Stream, current movement means heat transport contributing to global climate change. 

This study was supported by the Ocean Observing and Monitoring Division

Read the paper


Doppler current profilers on autonomous underwater gliders measure water velocity relative to the moving glider over vertical ranges of O(10) m. Measurements obtained with 1-MHz Nortek acoustic Doppler dual current profilers (AD2CPs) on Spray gliders deployed off Southern California, west of the Galápagos Archipelago, and in the Gulf Stream are used to demonstrate methods of estimating absolute horizontal velocities in the upper 1000 m of the ocean. Relative velocity measurements nearest to a glider are used to infer dive-dependent flight parameters, which are then used to correct estimates of absolute vertically averaged currents to account for the accumulation of biofouling during months-long glider missions. The inverse method for combining Doppler profiler measurements of relative velocity with absolute references to estimate profiles of absolute horizontal velocity is reviewed and expanded to include additional constraints on the velocity solutions. Errors arising from both instrumental bias and decreased abundance of acoustic scatterers at depth are considered. Though demonstrated with measurements from a particular combination of platform and instrument, these techniques should be applicable to other combinations of gliders and Doppler current profilers.

Tuesday, March 07, 2017/Categories: Climate Observation Division, General News

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