“AGUGLIA”. why not going straight to density?

摘要

The monitoring of oceans and seas is performed with different platforms, some of which profile the water column, measuring temperature and salinity. An important goal of current monitoring activities is the provision of quasi-synoptic four-dimensional maps of water mass properties. Technological platforms such as lagrangian profilers, or autonomous vehicles, have missions that include parking depths or acquisition of data with predefined density layers. There are still some disadvantages in the actual technologies. Autonomous vehicles have a speed that is insufficient to counter to current fields that might transform their planned Eulerian path into a Lagrangian drifting mode, following the water mass they were supposed to pierce. Other operational limitations include the still high running cost of CTD platforms that are not yet ready for a ?launch it and wait for data? policy. Finally, the achievements of operational missions are partially hampered by the inability to have an exact knowledge of the water mass density. The possibility to directly measure water mass density as opposed to the usual conductivity/salinity measurement is of paramount importance for observational strategies, as well as for numerical models. that the sole equation for density, can eventually replace the three equations for the calculation of temperature, salinity and density. The actual calculation of density from in-situ temperature, conductivity and depth information is affected by uncertainties due to the experimental algorithm in use and the combination of errors generated by the measurement of three parameters instead than one. Historically it is fascinating to note that the approach proposed has already been adopted more than three centuries ago by one of oceanography's founders, the Count Luigi Ferdinando Marsili, in his work on the Bosphorus Strait water circulation. The direct measurement of density also has various practical applications on platforms such as ARGO or AUVs that ca-n profile more precise density ranges. The vehicle described here, AGUGLIA (Garfish), can be launched at sea where it will reach a falling speed that is a direct function of water mass density changes, after a careful and specific density calibration vs. a 1000 m sea depth, simulated in a pressure tank to deduct the unavoidable minor structural compression. A clock and a pressure sensor accurately measure this falling speed. In situ calibration of AGUGLIA is made by changing its density and stopping its run to match the corresponding density of a given water mass (Lagrangian mode). Depth is also double checked by placing two pressure sensors at a known constructional vertical distance in the vehicle hull. Its hydrodynamics, although simplified due to cost minimization, is able to achieve low energy consumption in water penetration and course stability during surface transfers, due to its slender longitudinal hull resembling that of a Garfish. This hybrid capability of being self propelled like an AUV, as opposed to a fully passive glider mode, is an essential feature when one needs fast re-direction for cooperative work as well as when facing a current front or needing to reach the surface fast in order to mark a fix.