Depth-Average Velocity from Spray Underwater Gliders

TLDR

Depth‑average velocity is routinely derived from underwater glider data by dead‑reckoning the difference between ground‑over‑water and water‑through‑glider velocities, and its accuracy hinges on precise estimation of the glider’s water velocity given GPS precision and typical 3–6‑hour dive cycles. This study describes the method for calculating the Spray glider’s velocity through water. The authors determine a correction gain by comparing water velocities before and after glider turns, analogous to shipboard ADCP techniques, and assess accuracy using deviations of this gain from unity across extensive multi‑year missions off California and Palau. The depth‑average velocity achieved a root‑mean‑square accuracy of 0.01–0.02 m s⁻¹, matching early glider design expectations.

Abstract

Abstract The depth-average velocity is routinely calculated using data from underwater gliders. The calculation is a dead reckoning, where the difference between the glider’s velocity over ground and its velocity through water yields the water velocity averaged over the glider’s dive path. Given the accuracy of global positioning system navigation and the typical 3–6-h dive cycle, the accuracy of the depth-average velocity is overwhelmingly dependent on the accurate estimation of the glider’s velocity through water. The calculation of glider velocity through water for the Spray underwater glider is described. The accuracy of this calculation is addressed using a method similar to that used with shipboard acoustic Doppler current profilers, where water velocity is compared before and after turns to determine a gain to apply to glider velocity through water. Differences of this gain from an ideal value of one are used to evaluate accuracy. Sustained glider observations of several years off California and Palau consisted of missions involving repeated straight sections, producing hundreds of turns. The root-mean-square accuracy of depth-average velocity is estimated to be in the range of 0.01–0.02 m s −1 , consistent with inferences from the early days of underwater glider design.