1.SYSTEM OVERVIEW DESCRIPTION
The DONET is a submarine cabled real-time seafloor observatory network for the precise earthquake and tsunami monitoring. For the purpose of understanding and forecasting the earthquake and related activities underneath the seafloor, the twenty sets of state-of-arts submarine cabled sub-sea measurement instrument will be deployed in seafloor at the interval of 15-20km. The twenty sets of preliminary interface are prepared in consideration of the improvement of observation capability in the future. Operating large-scale subsea infrastructure over a long period of time (20-30 years) is one of a challenge of underwater technology. The increase of measurement instruments has a big influence on the total system reliability, because of the state-of-arts instrument is a bottleneck to maintain long-term reliability. A novel system design concept is necessary for the observatory network development to make two demands such as 'high reliability system design' and 'state-of-arts measurement' united. The observatory network should be able to replace, maintenance and extend while operating, and should be have a redundancy for the internal or external observatory network component failure. To achieve these requirements, the DONET proposes a composition that consists of three major components with different system reliability. There are high reliability backbone cable system, replaceable science node, and extendable measurement instruments. Following figure1 shows the system design plan to be implemented in DONET.
Figure1. System Concept of DONET
2. BACKBONE CABLE SYSTEM
The backbone cable system provides the power feed line and the communications channel to the apparatus in the seafloor. The system brings a recently developed sub-sea telecomm cable technologies to fit for the high reliability requirement for 20years seamless observation. A constant current DC power supply technology provides high robustness against with unexpected power line failures. The DONET backbone cable system allows loading up to 3kW (3kVDC / 1A) electric power in operation. The five science node interfaces are scheduled to be equipped in the system. A duplicated pier-to-pier optical fiber physical communications channel is allocated between science node interfaces and terminal equipments on land to ensure the reliability. The optical amplifiers (repeaters) are prepared every 40-60km optical fiber length interval to transmit the signal longer distance without degradation. These repeaters correspond to the coherent optical time-domain reflectometry (C-OTDR) optical fiber fault detection system. The branching unit (BU) is an interface for science node interface. This unit controls the high voltage power feed path in backbone cable system, and has a function to separate a science node when the node interface one by one becoming unexpected status. For the connection between a BU and a science node interface, a dual conductor light weight submarine cable that met ITU-T recommendations is being developed in this project as Figure2.
Figure2. Specifications of Dual Conductor Light Weight Cable.
3. SCIENCE NODE
The science node is a device with the role of hub that connects the backbone cable system to sub-sea instruments. Many of novel technologies are consolidated in the science node development. A hybrid (fiber optic and electric) / high voltage sub-sea wet mate / demate interface make possible to put on and take off the science node from backbone cable system. Eight hybrid connectors per a science node have been reserved for measurement instruments. The power distribution control, data transmission control, and precise timing control function in the science node are most critical components of DONET development. The power distribution control system (Figure3) receives 500watts of constant current DC power supplied from the terminal equipment, and distributes 45 watts of secondary power output to a measurement instrument as the occasion demands. The secondary power output features a constant current DC power output system to ensure the reliability of sub-sea system and efficiency of power transmission to measurement equipment. The power distribution control system has a mechanism to balance the power consumption of science node constant to prevent the system from unstable power distribution status. This function is essential for monitoring the condition of entire observatory network.
The data transmission control system handle data link and precise timing / clock control between measurement instrument and terminal equipment. The STM (Synchronous Transfer Mode) on SONET / SDH (Synchronous Digital Hierarchy) is selected to realize the precise time synchronization requirement. The data link between terminal equipment and science node is running at approximately 600Mbit/s. The bidirectional data transmission between measurement instrument and science node, is running at 50Mbit/s. precise time synchronization is a key function of science use of submarine cable system. The synchronous transmission system makes possible the high accurate time synchronization between GPS clock on terminal equipment and measurement instrument in seafloor. The timing circuit develops aiming at the accuracy of time synchronization of less than 1microsecond in this project.
Figure3.The Power Distribution Control System