HERMES—A High Bit-Rate Underwater Acoustic Modem Operating at High Frequencies for Ports and Shallow Water Applications

Abstract

AbstractAn underwater acoustic modem capable of point-to-point transmission of data at high bit rates is presented in this article. The high data-rate acoustic uplink operates between 262 kHz and 375 kHz in three sub-bands. The lower sub-band (262.5 kHz to 337.5 kHz) carries the binary information. The middle sub-band (347-373 kHz) is for detection purposes. The higher sub-band carries a narrow-band, 375-kHz tone designed to improve the Doppler-tracking capability of the high bit-rate acoustic uplink. The acoustic uplink uses phase-modulated symbols of adjustable bandwidth (25 kHz, 50 kHz or 75 kHz). The peak data rate is 87,768 bits-per-second at a maximum range of 180 m using an omni-directional source and an omni-directional receiver. The source level required to achieve this range is 185.8 dB re 1 μPa at 1 m. This modem is also equipped with an acoustic downlink designed for command-and-control in a lower frequency band (62-76 kHz) and at a lower data rate. The focus of this paper is on the high bit-rate uplink. A series of experimental results demonstrate that this underwater acoustic modem can operate reliably in difficult environments such as ports and very shallow waters. High-resolution sonar images are transmitted in real-time from various types of autonomous underwater vehicles during the inspection of ship hulls, walls and sea bottom.