Glider observations of optical backscatter in different Jerlov water types: Implications to US naval operations

Abstract

Unmanned underwater vehicles (UUVs) are increasingly being used as efficient and cost-effective Intelligent Preparation of the Operational Environment (IPOE) platforms by the Naval Oceanographie Office (NAVOCEANO), which seeks to provide Bat-tlespace on Demand to the US Navy warfighter. One such UUV, the Teledyne/Webb Slocum Electric Glider, was utilized during four US Naval exercises in the past year to provide environmental data in support of Navy operations. Folding Slocum Glider missions into tactical operations has numerous benefits: the Slocum Glider (1) provides high-resolution optical and physical data in shallow coastal environments in real-time, (2) maintains a low profile when surfaced, (3) is piloted remotely with minimal manpower, (4) can be deployed over-the-horizon, and (5) reduces the amount of time an asset spends off-mission for the sake of IPOE measurements. Environmental data include sound velocity profiles, water clarity, and current velocity. The scientific payload for the Slocum Gliders in the NAVOCEANO glider inventory consists of a CTD probe and an optical backscattering sensor. Sound velocity profiles are derived from the CTD, water clarity is inferred from the backscattering data, and depth-averaged currents are determined by offsets between subsurface dead-reckoning position estimates and actual surface Global Positioning System (GPS) position fixes. One issue pertaining to the optical data is that many of the optical products utilized are based on beam attenuation and not optical backscattering. Consequently, optical backscattering data from the NAVOCEANO Slocum glider data must be converted to beam attenuation to be utilized for algorithms that create optical products. The work presented here examines the optical backscattering to beam attenuation relationship of glider-based observations of four distinct coastal water masses, as described by the Jerlov Optical Ocean Water Mass Classification. Optical backscattering data were converted to beam attenuation by (1) using a standard algorithm and (2) scaling backscattering to attenuation. The results indicate that the contribution of optical backscatter to total attenuation depends heavily on the composition of particles in the water column and that concurrent measures of beam attenuation and optical backscattering are needed to provide accurate estimates of relevant optical products.