Performance Evaluation of a Prototype Underwater Short-Range Acoustic Telemetry Modem

This thesis documents the evaluation of the transmitter performance of a short-range underwater acoustic modem. This prototype modem was fabricated by contractor Teledyne Benthos, Inc. and is identified as Model ATM-90X. It was developed for use in the Seastar underwater Local Area Network LAN. The ATM-90X modem is required to be capable of transferring large amounts of digital data at a range up to 500 m using the 33-55 kHz acoustic frequency band. The modems transmitter performance was evaluated in term of its transmit frequency response, vertical beam pattern, and maximum source level. Underwater acoustic measurements were conducted in an anechoic water tank and the data were analyzed using signal processing techniques including Hilbert transforms, autocorrelation, and cross correlation. The transmission characteristics of the intended underwater acoustic communication channel were modeled to determine the required modem operating performance in the best and worst case situations. The measured performance characteristics were evaluated in the context of the communication link margin i.e., the excess signal-to-noise ratio associated with the modeled channels. The results show that 1 the modem transmit frequency response does not have acceptable flatness across the entire 33-55 kHz band 2 the beam pattern in the vertical plane has a good hemisphere pattern 3 estimated maximum source level is 175 dB re 1 Pa-m at broadside of the modem. Based on the evaluated performance, the ATM-90X modem can provide a successful communication link with the highest data rate at the range of 500 m in the best-case situation least noise. However, in the worst-case situation, the communication link will fail to reach performance objectives.