Long range acoustic underwater communication with a compact AUV

Abstract

The operational conditions of an Autonomous Underwater Vehicle (AUV) are desirable information for the AUV operator. When operating at the surface, various through the air communication technologies, mostly based on radio frequencies, can be employed to easily transfer data between an operator and the vehicle. These approaches offer good range and, especially over WiFi, high bandwidth. Once submerged, as radio frequencies do not propagate well in seawater, information transfer between the vehicle and the operator becomes both critical and difficult. This paper describes a method of employing underwater acoustic modems to exchange data and information between a submerged AUV and the operator. Examples of data that would be of interest to an AUV operator could consist of: battery charge state, vehicle heading, vehicle depth, last know waypoint, and cumulative distance traveled. This type of data can be transferred via underwater acoustic communications, eliminating the need for the vehicle to routinely surface and transmit this data through the air. Various operational profiles can be imagined; the AUV operator querying the vehicle for status, the vehicle transmitting status at a predefined interval, or perhaps modifying a mission profile in situ. Underwater acoustic modems can employ low frequency (9 to 13 kHz) acoustics to achieve effective extremely long range communication. Distances of greater than 5000 meters can be achieved with frequencies in this range. Low frequency transducers are typically 4” in diameter, which may prove too large for some AUV's. Higher frequency systems employ smaller diameter transducers. While these may be more compatible with some compact AUVs the higher frequencies result in diminished effective range. AUV's operating in shallow water can utilize higher frequency acoustic modems as the mission profile may be limited to less than 2000 meters from any single point. Teledyne Benthos and OceanServer Technology have demonstrated reliable underwater acoustic communications at ranges exceeding 1700 meters between a submerged compact AUV and a surface acoustic modem. This was demonstrated with an OceanServer Technology IVER2 vehicle fitted with a Teledyne Benthos ATM900 series acoustic modem operating in the 22 to 27 kHz frequency range, commonly known as Band C. During these trials acoustic bit rates of 600 bits per second were observed between the AUV and a surface modem connected to the OceanServer topside control and monitoring system. Data exchanged between the AUV and control system was consistent with what would normally be sent back to the control system through RF systems while surfaced. This paper discusses the results of field tests and demonstrates the improvement seen by application of the Band C modem as compared to other acoustic modem systems employed by compact AUVs. While these results do not challenge the established theoretical understanding of acoustic communications the results are meaningful to a growing compact AUV user community. The shallow water environment these users typically operate in is especially challenging to acoustic communications. The results presented here, derived from the combination of two commercially available products, represent a significant improvement in observed, practical, performance.