UNDERWATER VEHICLES
ACOUSTIC ACOUSTIC MODEM The speed of sound in water is limited to around 1500 m/sec, considerably slower that other competitive wireless techniques while also being subject to losses at it travels through the water. It is possible to increase the efficiency by processing the basic signal. This technology is largely borrowed from the telecommunications industry and adapted for underwater use. The signals can be changed or modulated, and then demodulated at the receiving end. This is carried out using a modem. One of the most
common techniques is the Spread Spectrum. This splits the data up and codes it or arranges the way it is packed. Larger amounts of data can then be sent as a number of packages. One of the most common methods is to send data at several different frequencies (Multi-Frequency S hifted Key, MFSK). Another arrangement called Phase Shifted Key, or PSK allows a higher data speed but is less relaible. At the receiving end, the software can look for errors and ask for missing sections to be re-transmitted. This reduces errors but affects the data transfer rate.
Alterntively, Multipath processing essentially sends the same signa numerous times but slightly delayed, and the software can assemble a 'clean' version at the receiving end. Modems normally require line-ofsight communication and most are reliable in distances of around for 200-300m. They are not always ideal when propagating long horizontal distances, particularly in shallow waters where there is greater propensity for losses due to the signal reflecting off seabed and surface boundaries. The ability to communicate acoustically through the water has many applications. These include tsunami warning systems, stress monitoring, positioning, imaging, bathymetric profiling data transfer and control and monitoring.
APPLICATIONS DIVING In recreational diving, helmet microphones allow the diver’s speech is converted into a high frequency sound signal which is transmitted through the water.
EVOLOGICS S2C T MODEMS Earlier this year, EvoLogics introduced its latest generation S2C T modems. Dubbed “tiny”, the light and ultra-compact design represents a size reduction of almost 20% compared to their M-series mini-modems, at only 25 cm standard height and 1200g weight. The S2C T high frequency models are aimed at modern size- and weightsensitive applications with four frequency range/directivity options. The new model features a fully-fledged S2C engine with no compromises in acoustic performance. It is a designed for small AUVs and ROVs where seamless integration of the components is critical. Perfect as transponders for positioning, the S2C T are capable of simultaneous tracking and reliable bidirectional data transmissions with advanced networking.
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Ocean Technology Systems' acoustic comms system for recreational divers. Commercial diving systems, however, send communications through the divng umbilical