Three dimensional synthetic and real aperture sonar technologies with Doppler velocity log and small fiber optic gyrocompass for autonomous underwater vehicle

The technologies could be expected a practical application of searching for submarine minerals, such as thermal vents a few meters high, to a depth of 3,000 meters. In order to develop exploration technologies for submarine resources, new sensor technology needs to be developed for use with autonomous underwater vehicles (AUV) or remotely operated vehicle (ROV). We have used an optic gyrocompass CDL Mini POS-II and a Doppler velocity log (DVL) RD WH300 for the research on three dimensional synthetic and real aperture sonar technologies. However, these sensors with pressure housing are too large to mount on autonomous underwater vehicle (AUV). So, in cooperation with IXBLUE and OceanWings, we created a small fiber optic gyrocompass (FOG) with motion sensors based on IMU-50 and packed it in a small pressure housing with an outer diameter 112 mm and a length 295mm. We extracted a unit composed of three-axes angular velocity and three-axes velocity sensors and a serial interface from the IMU-50, and add it to a RS-232C converter board. Then we achieved the original software for measuring heading based on the true-north, roll, and pitch angles using the raw increment data from the small fiber optic gyrocompass. Compared with the high-performance FOG IXBLUE PHINS with accuracy 0.01 degrees, the original software achieved the true north estimation with accuracy 0.25 degrees of the same one as IXBLUE Quadrance based on IMU50 without a pressure housing. As for a small DVL NavQuest 600 is employed for optical gyrocompass and DVL navigation for the SAS processing. With respect to developing and researching on three dimensional synthetic and real aperture sonar technologies, we originally designed and manufactured the five raw hydrophone arrays with 40 hydrophone elements in a shape of triangle and have made several sea tests on targets of breakwaters, fishing banks, and hydrothermal vents. So far, we have developed the software for producing three dimensional backscatter images with the interferometry technique and auto-estimation of detail navigation from overlapped echo signals of the 40 hydrophones suited for the synthetic aperture processing. The operational acoustic frequency is 100 kHz with the result that required position accuracy for SAS processing is normally better than 2 mm. However, DVL navigation accuracy level is equal to next to 5 cm in our processing technology. So we beforehand set 5cm grids of acoustic images from hydrophone signals of continuous 40 pings. Next, for the purpose of achieving the required position accuracy, we estimated position errors from acoustic image signals within overlap area for continuous two pings. By summing the estimated errors, high accuracy positions were calculated and were used for SAS imaging. The SAS interferometry processing software is currently being improved. In addition, we designed and manufactured four raw hydrophone arrays in parallel with each other because the five raw hydrophone arrays are a little too large for the AUV. Furthermore, we add a real aperture interferometory option to the new system using a long projector.