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The description of the new platform we have conceived and developed is made here more or less on a chronological order, starting with the design, and then continuing with the successive versions and the various tests that have been performed.
Design concept (full description here)

Platforms developed for oceanographic purposes are rarely adapted to the deployment of radiometers at sea. Indeed, recording the light field within the ocean interior is difficult because the instruments themselves and, more dramatically, the platform onto which they are installed, inevitably introduce perturbations (shadowing in particular). Other difficulties originate from the need to keep the instruments as much horizontal as possible, either because a plane irradiance is aimed at (cosine sensors), or if a given direction (generally nadir) is aimed at. The actual measurement depth is also difficult to accurately assess, because rapid vertical displacements of the instruments sometimes occur, which prevent any precise estimation of pressure, thence of depth. Considering the above observations (among others), a new type of platform has been developed, dedicated to radiometry measurements. This platform is able to minimize shadowing effects, to minimize perturbation of the sub-marine light field, and to warrant the stability of the instruments.

The constraints were:

  1. The data to be measured are Eu, Ed, and Lu(nadir) at two depths, plus at the surface;
  2. Minimizing shading of the instruments;
  3. Maximizing the stability of the instruments; and
  4. Deployment at a site with a depth of 2,440m, and swells up to 8m (but low currents).

The principle is that of a reversed pendulum, with Archimedes thrust replacing gravity. A large sphere (with a diameter of about 1.8m) is stabilized at a depth out of the effect of most swells, at the end of a cable that goes down to the sea floor. This sphere creates the main buoyancy of the system. A rigid, tubular, structure is fixed above the sphere, which hosts the instrumentation onto horizontal arms (at 4 and 9m). The resulting approximately three tons of thrust ensures the stability of the system, which is subject to very limited forces from the so-called transparent-to-swell superstructure. This is a taught mooring, definitely different from what is usually referred to as spar buoys.

With such a design, there is no large body at the surface generating shade, the stability of the instruments is warranted even for quite large swells, and the possibility exists to accurately measuring the water level above the instruments.

Theoretical calculations were performed, by specifying an initial and preliminary design (Fig 6.) and material for the construction, a swell of height 5m and period 7s, typical of the deployment site. The inherent periods of the whole mooring were determined, as well as the period and amplitude of the oscillations and displacements due to swell and currents. These calculations were extremely encouraging in terms of tilt and oscillation, so that it was decided to first build a reduced-scale model in order to perform tests in an engineering pool.

Schematic drawing of the BOUSSOLE buoy

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