Precision underwater positioning for in situ laser Raman spectrographic applications

The Monterey Bay Aquarium Research Institute (MBARI) has developed and deployed a laser Raman spectrometer system (DORISS-Deep Ocean Raman In-Situ Spectrometer) for oceanic geo-chemical measurements of sea water specimens. Quality spectra have been obtained on standards carried to the seafloor. The next stage of this development involves the ability to obtain spectra from natural targets of interest in the deep ocean and to maximize signal return from the sample. To accomplish maximum signal return the DORISS probe head must be properly positioned and focused. In addition the probe head must be held steady for several minutes while spectra are being acquired. This is in contrast to laboratory work in which the sample is precisely positioned with respect to the probe head. This requirement has been the driver for a new device, the Precision Underwater Positioning system (PUP). The positioning system has strict requirements for motion about the target. The DORISS requires very exact and repeatable motions with positional accuracies in the <1 mm range over a large workspace. The device must also let the user see where they are focusing, be able to move without disturbing the base position, and to stay stable over a variety of terrains. In addition, the system must be adaptable to other vehicles. Another requirement is to return relative position from a known home position once the PUP is set in place. Moreover, the device has several operational constraints that impacted the design and operation of the system. This paper outlines the science drivers, the operational considerations, and the engineering trades that were made to build the first two stages of the PUP. The test results of this system are also included demonstrating the device's actual performance against the specifications. In conclusion the paper outlines the next tasks and the direction the program is taking to fulfill the complete precision underwater positioning system requirements.