Sea-floor image transmission system for AUV

Autonomous Underwater Vehicle (AUV) has become one of the promising tool for ocean exploration during the last few decades, and, in particular, is the solution for the spatial-temporal investigations in wide areas for a long period. One of the next mission expected from AUV is deep sea specimen sampling, which is currently performed by Remotely Operated Vehicle (ROV) or Human Occupied Vehicle (HOV) where the sampling targets are selected by scientists on-line. In order to establish the similar on-line investigation with AUV system, the sea-floor images have to be transmitted to the scientists on the support vessel by acoustic communication. However, the speed of the acoustic communication is low compared with that of radio communication, and the data can be lost because of the directionality of acoustic modem, the positional relationship between the AUV and the support vessel, attenuation and so on. The robust image transmission system is necessary with acoustic communication for in-situ decision making for sampling by AUV with many tasks. In this paper, we propose a sea-floor image transmission system with image compression, and evaluated by sea trials in Suruga-bay. The image compression method is based on a set of color palettes, where the colors of a color palette are assigned as a set of main colors obtained from the minimum variance quantization, to represents a typical sea-floor image. The colors of the obtained images are replaced by the most similar colors in the color palette. The images compressed by a 16-colors color palette are evaluated by Structural SIMilarity (SSIM) method, and these compressed images have shown the SSIM index of 88.5%. The duration of one image transmission is about 40 seconds in the sea trials and the transmission success rate is 75%.