Satellite Link Management for an Ocean Observing Network

The Monterey ocean observing system (MOOS) moored observatory hosts tens of instruments on multiple networked nodes distributed over the sea surface, water column, and seafloor. Commands and data are exchanged between instrument nodes over high-speed copper and fiber-optic links at 10 Megabits per second using TCP-IP protocols. Science and engineering instruments on each node acquire and log data at various rates; the current deployment of five instrument nodes logs tens of Megabytes of data per day. Approximately 5 Megabytes per day of telemetry is required to provide a subset of science data and system status information. The surface node periodically establishes a PPP connection to shore using the Globalstar satellite system, providing a link for remote system control, maintenance, and telemetry retrieval Telemetry retrieval is particularly challenging, given the capacity and cost of the 7800 bits per second communications link. The challenge is compounded by limited satellite availability, wave-driven motion of the surface buoy antenna, and occasional outages of hard-wired network connections between nodes. To address these issues, we have developed software strategies to manage the low- bandwidth satellite link in a highly efficient manner. Elements of our telemetry retrieval strategy include use of data summarization algorithms, PPP compression, multi-threaded utilization of the satellite link, optimized data packet size to reduce protocol overhead, and assertive reconnection of prematurely disconnected satellite links. We discuss the efficiency and trade-offs of various approaches, as well as overall observed improvements in telemetry rates. Our current implementation is capable of retrieving at least 10 Megabytes of telemetry per day, and we discuss further improvements which could substantially increase that rate.