Ancillary Systems

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In addition to the multibeam sonar array, a multibeam mapping survey requires careful integration of a number of ancillary systems. These include: (1) an inertial positioning system (INS) or a differentially corrected Global Positioning System (DGPS); (2) an accurate measure of the heave, pitch, roll, and heading of the vessel, all to better than 0.01°, and the transformation of these measurements to estimates of the motion of the transducer at the times of transmission and reception (motion sensor); (3) a method to precisely determine the sound-speed structure of the water column, using measurements of temperature, salinity, and depth with one, or a combination of, a CTD (an instrument that measures conductivity and temperature vs depth), XSV (an expendable sound velocity profiler), and XBT (expendable bathythermograph), and the calculation of sound velocity profiles (SVP).

The Crater Lake survey was navigated with (INS) provided by a TSS Applied Analytic POS/MV model 320 inertial motion sensor (IMU) as well as dual Trimble model 4000 DGPS with a commercial SatLoc satellite differential station. Spatial accuracy (positions) for the mapping is ±0.5 m. In addition, the POS/MV records vehicle motion (pitch, roll, yaw, and heave) at 100 Hz with an accuracy of 0.02° for roll, pitch, and yaw, and 5% of heave amplitude or 5 cm.

Sound-velocity profiles were calculated each day so that ray-tracing techniques could be used to determine the effect of acoustic refraction in the water. A SeaBird model 19-02 CTD was deployed the first day of operations to get a good reference SVP. Two additional sound-velocity sensors are installed at the transducer to directly determine the speed of sound in water. All the SVP data were fed directly into the Simrad EM1002 processor for instantaneous beam forming and ray tracing of each individual beam.