Synchronous data transmission and seawater power return cable systems

A wide range of modern in-water scientific systems requires data transport over optical fiber and power over copper conductors. As a standardized approach, a data backbone using synchronous transfer mode (STM) has been utilized for a wide range of applications. The transfer of data from the digitized sensor input to the synchronous backbone is by means of the asynchronous transfer mode (ATM). Power for these systems is over a standard telecommunications single conductor cable requiring a return over a seawater path. Experience with this combination of technologies has led to an increased level of understanding in the interactions between the data, power, and sensor elements. Switching rates in the conversion of analog sensor outputs to digital are usually synchronized to the STM frame rate eliminating noise at the sampling rate. However, the ATM cell loading rates present in the seawater return path create harmonic and intermodulation products that appear back at the analog input. These conditions contaminate the data and need to be minimized in the design of the sensor and power elements.