Precision timing in the NEPTUNE Canada network

Delivery of accurate timing to subsea instruments is one of the essential functions of the NEPTUNE Canada ocean observatory. In the context of an ocean observing system, ldquotimingrdquo is understood to mean the ability to timestamp data using a clock which is traceable to Universal Time Coordinates (UTC) within some desired level of precision. Transmission or delivery of timing means transporting the necessary timing signals and data to subsea instruments or data collection processors. Timing signals are conventionally delivered in the form of a logic state transition on a dedicated communications line followed by a data string indicating the time at which the transition occurred. NEPTUNE Canada employs Ethernet communications channels to deliver timing. This avoids the need to provision additional communications channels and ensure that timing signals are available at all points within the network. The disadvantage of this approach is that timing signals must share the communications channels on which data is transmitted and may suffer delays or packet loss. Three timing protocols are employed: Network Time Protocol (NTP) described in IETF RFC 1305, simple network time protocol (SNTP) as described in RFC 2030 and IEEE 1588 precision time protocol (PTP) per IEEE 1588. NTP/SNTP are generally accepted to be accurate to within a few milliseconds and are suitable for a wide variety of applications. PTP provides significantly better accuracy which is required for applications such as seismology and acoustic thermography. Master clocks located in the shore station acquire time from the Global Positioning System for transmission to subsea instruments. Alcatel-Lucent Submarine Networks and NEPTUNE Canada have successfully demonstrated PTP operation on a test bed which is representative of the NEPTUNE Canada network under simulated data traffic conditions, achieving a precision of plusmn10 microseconds or better. Ongoing development of IEEE 1588 will allow precision to within 100s of nanoseconds in future observatories. Science instruments which can take advantage of PTP timing delivery are currently under development.