A measurement system for in-situ studies of optical water quality, salinity and temperature has been designed and built at FOA Transmission, forward scattering, back scattering and irradiance are measured The system is unique in using a green HeNe laser (543 5 nm) for this application and in providing an airborne instrument for measuring optical water quality data in-situ.

This development had two main objects (1) to map variations in optical water quality as a function of depth, geographical position, season, weather and other hydrological and meteorological parameters and (2) to examine the hydro-optical environment during tests with other optical systems The system has been tested in the Baltic from a helicopter and from ships.

BACKGROUND

Hydro-optical research has been performed at FOA for about twenty years. The optical properties of Swedish coastal waters and open sea have been measured in different ways during these years. Since the Swedish coastline is very long, the measurement positions form a sparse pattern in time and geography Hydro-optical parameters have been measured one by one, and correlations to other hydrological and meteorological parameters have not been systematically studied. The need for a multi-sensor system was obvious.

Many new hydro-optical techniques are based on laser technology In order to predict the performance of this kind of system in Swedish waters, we chose to measure transmission, forward scattering, back scattering, and the irradiance of the downwelling daylight. Temperature and salinity measurements are added to correlate these parameters with the optical ones.

Because FOA budget does not permit extensive exploration of the subsea environment, an instrument should be simple to operate, making it possible for any crew in the Swedish Navy to perform the data collection.

DESIGN
General Remarks

The systems layout is presented in Figure 1. All the subsea sensors are mounted on the subsea unit according to the system block diagram (fig 2) Temperature, salinity (conductivity) and pressure (depth) are measured with commercially available sensors, but the irradiance, scattering and transmission sensors are designed and manufactured by FOA.

Fig 1 System Layout (available in full paper)

Fig 2 System black diagram (available in full paper)

All the sensors are designed to give rapid response. The subsea unit has an open layout to secure a rapid exchange of water around the sensors, making acquisition of data possible during continuous up or down movement of the instrument. A depth profile from the surface to 100m can be recorded within 4 minutes. The data are stored on tape and can later on be fed to a data post processing system for production of water quality maps etc.

The accuracy of the sensors is so far not exactly confirmed, but the ambition has been to keep the errors low without making the instrument too complex or expensive.

Sensor design

Transmission and forward scattering sensors

The transmission and the forward scattering sensors share a light source- a green HeNe laser. This laser was chosen because it is comparatively cheap and it emits in 540 nm which is rather close to a doubled Nd TAG laser (530nm).

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