A paper describes an approach for timekeeping and time transfer among the devices on the CoNNeCT project’s SCaN Testbed. It also describes how the clocks may be synchronized with an external time reference; e.g., time tags from the International Space Station (ISS) or RF signals received by a radio (TDRSS time service or GPS).
All the units have some sort of counter that is fed by an oscillator at some convenient frequency. The basic problem in timekeeping is relating the counter value to some external time standard such as UTC.
With SpaceWire, there are two approaches possible: one is to just use SpaceWire to send a message, and use an external wire for the sync signal. This is much the same as with the RS-232 messages and l pps line from a GPS receiver. However, SpaceWire has an additional capability that was added to make it easier — it can insert and receive a special “timecode” word in the data stream.
Another method is to use the SpaceWire time code features. A standard SpaceWire interface provides four signals: Tick In, Time In, Time Out, and Tick Out. When one end of the SpaceWire link asserts “Tick In,” some small amount of time later (a few microseconds), Tick Out at the other end of the link is asserted. So there is a “virtual” wire connection over the SpaceWire link that can do synchronization (with an uncertainty and latency on the order of a few microseconds). The Time In signal provides an interface to send a 6-bit time code that is transparently inserted in the stream of data and control tokens being carried across the link, and recovered and presented on the Time Out at the destination without needing to create a special “time message.”
This work was done by James P. Lux of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47437
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Time Distribution Using SpaceWire in the SCaN Testbed on ISS
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Overview
The document titled "Time Distribution Using SpaceWire in the SCaN Testbed on ISS" outlines the methodologies and technologies employed for timekeeping and time transfer among devices on the International Space Station (ISS) as part of the CoNNeCT project. It emphasizes the importance of precise time synchronization for various aerospace applications, particularly in communication and navigation systems.
At the core of the timekeeping process is a counter fed by a stable oscillator, which is essential for relating the counter value to an external time standard, such as Coordinated Universal Time (UTC). The document discusses two primary strategies for time synchronization: clock setting and clock offset keeping. Clock setting involves adjusting a clock to a known time, while offset keeping records the difference between the observed clock time and the actual time at the moment of synchronization.
The document details the use of various communication protocols for time transfer, including Mil-STD-1553B and Ethernet. For Mil-STD-1553B, the bus controller sends time messages to sub-addresses, allowing precise timing for synchronization. Although Ethernet lacks sub-millisecond accuracy, software solutions like Network Time Protocol (NTP) can achieve reasonable precision by measuring round-trip times of IP packets. For high-precision timing, IEEE 1588 is mentioned as a standard for time transfer over Ethernet, utilizing specially designed interfaces.
SpaceWire, a key focus of the document, offers unique capabilities for time synchronization. It can transmit timecode words within the data stream, allowing for synchronization with minimal latency. The document describes how a "Tick In" signal at one end of a SpaceWire link can trigger a corresponding "Tick Out" signal at the other end, facilitating synchronization with an uncertainty of just a few microseconds.
Additionally, the document highlights the integration of software drivers at both ends of the link to manage timekeeping and synchronization processes. The JPL radio system is mentioned as an example where the "Tick Out" signal serves as an external synchronization signal, enabling accurate time calculations based on received messages.
Overall, the document provides a comprehensive overview of the strategies and technologies for effective timekeeping and synchronization in space applications, showcasing the advancements made in aerospace technology and their potential broader applications.
