Ocean Trials of the S2C Acoustic Link for German-Indonesian Tsunami Early-Warning System

In November-December 2005 a deep-sea version of sweep-spread carrier telemetry link has been applied for data exchange between an ocean bottom unit and a surface buoy comprising the German-Indonesian tsunami early system (GETEWS). So, main development efforts were primarily focused on the solution of problems induced with vertical and vertically inclined hydroacoustic telemetry channels in deep waters. In conditions of most practical interest, rather high bit rates, usually between 1 and 4 kbps (sporadically up to 6.5 kbps), were registered and thus experimentally proved. Keeping in view the limitations on given frequency range (effectively from 8 to 16 kHz) and on power consumption (less than 80 W), the practically achieved performance confirmed advantages of the sweep-spread carrier technology for data telemetry under tricky environmental conditions in practical oceanographic applications. Working from research vessel "Sonne" the surface telemetry unit was exposed to high-level broadband noise produced by ship mechanisms (due to strong wind and waves a positioning system of the ship constantly switched on and off different positioning mechanisms on board). Owing to permanent ship movements, receive and transmit conditions were continuously varying. Strong water flows in vicinity of surface modem provided fast and strong deviations of the modem from its normal (vertical) position. Variations in modem arrangement and presence of multiple reflecting objects provided conditions, in which the multipath structure was especially intricate. The paper contents trials description, theory and analysis of the S2C acoustic link working under given conditions.