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26-Apr-2017
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SpaceWire - Purpose

The purpose of the SpaceWire standard is:

  • to facilitate the construction of high-performance onboard data handling systems,
  • to help reduce system integration costs,
  • to promote compatibility between data handling equipment and subsystems, and
  • to encourage re-use of data handling equipment across several different missions.

Use of the SpaceWire standard ensures that equipment is compatible at both the component and sub-system levels. Processing units, mass-memory units and down-link telemetry systems using SpaceWire interfaces developed for one mission can be readily used on another mission. This:

  • reduces the cost of development (Cheaper),
  • reduces development timescales (Faster),
  • improves reliability (Better),
  • increases the amount of scientific work that can be achieved within a limited budget (More).

Payload processing involves several functions:

  • Controlling instruments
  • Calibrating instruments
  • Collecting data from instruments
  • Storing the instrument data
  • Processing the data
  • Compressing the data
  • Sending the data to the down-link telemetry transmitter

SpaceWire is able to support many different payload processing architectures using point-to-point links and SpaceWire routing switches. An architecture can be tuned to the requirements of specific missions.

An example architecture is shown in the diagram below, which uses SpaceWire routers to provide the interconnectivity between instruments, memory and processing modules.

architecture diagram, explained below

The Instrument in the top left-hand corner is a high data-rate instrument. A SpaceWire point-to-point link is used to stream data from this instrument directly into the High Capacity Memory Module. This Memory Module is also connected into the rest of the payload processing architecture by a SpaceWire link to a SpaceWire Router. Data from other instruments can be stored in the High Capacity Memory Module using this link. The data stored in memory from the high data rate instrument can also be accessed using this link, for processing, compression and sending to the down-link telemetry transmitter.

A second Instrument is connection to an input/output (I/O) module. This module is used to connect to instruments that do not have direct SpaceWire connections. The instrument passes data to the I/O module, possibly over a parallel data bus. The I/O module forms this data into SpaceWire packets and sends them to the required destination over the SpaceWire network. This may be the High Capacity Memory Module, a Processor Module, or a Data Compression Module.

The third instrument, the Complex Instrument, has many sub-systems that have to be controlled separately. To do this a low data rate bus, for example the CAN bus, is used, to control and collect data from the various sub-systems. A Remote Terminal Interface (RTI) is used to provide the bridge between SpaceWire and the local bus (e.g. CAN). The Remote Terminal Computer device has been specifically designed to support this function.

The final instrument is an instrument that requires substantial processing so may include its own Data Processing Unit (DPU). The DPU is connected to the SpaceWire network providing processed data to the High Capacity Memory Module for storage before being sent to the down-link telemetry system.

There are four different processing modules shown in the example architecture. The Control Processor module controls the complete set of payload instruments according to commands sent over the Spacecraft Control Bus. The DSP Processor module performs digital signal processing on the instrument data to extract important information or to help implement computationally intense control loops for the instruments. It may also perform compression of the payload data if the data rates are low. For higher data rate instruments a dedicated Data Compression may be necessary and for instruments that require specific, demanding processing a Dedicated Processor module may be used.

Data from the instruments, High Capacity Memory and processing units can be sent over the SpaceWire network to the Telemetry Formatter/Encryption module under control of the Control Processor module. The Telemetry Formatter/Encryption module sends the data to a ground station via the down-link Transmitter.

A Context Saving Memory is also shown in the architecture diagram. This memory can be used for periodically saving the context of the payload processing system, so that in the event of a failure a previous context can be restored.

Additional SpaceWire links can be added in the network to provide additional bandwidth or to support fault tolerance. The SpaceWire routers may be stand alone units or may be integrated into the memory, processing or other modules. SpaceWire allows standard instruments, memory systems and processing modules to be developed and reused on several missions.

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