Developed in 1999 under the auspices of the European Space Agency, SpaceWire answered a longstanding spaceflight problem: no standard, high-speed communications protocol existed for flight electronics. Therefore, all spaceflight electronic payloads (such as processing units and onboard computers) were custom-designed, which resulted in long development periods, high costs, and elevated risks. The SpaceWire standard was developed as a network of nodes and routers interconnected through bidirectional, high-speed serial links, limiting the custom-design problem by designing a standard with flexibility, modularity, and reusability.

SpaceWire is defined in the European Cooperation for Space Standardization standard ECSSE50- 12A. The SpaceWire standard was authored by Steve Parkes of the University of Dundee, with contributions from individuals within the European Space Agency (ESA), European Space Industry, NASA, and academia. SpaceWire is the standard for high-speed links and networks for use onboard spacecraft to ease the integration of sensors, processing units, telemetry subsystems, and other micrcoelectronics, and is applicable for spacecraft missions, military hardware systems, and bus systems.

According to the ESA, the purpose of the SpaceWire standard is to facilitate construction of high-performance onboard data handling systems, reduce system integration costs, promote compatibility between data handling equipment and subsystems, and encourage re-use of data handling equipment across several different missions. The SpaceWire standard ensures that equipment is compatible at both the component and subsystem levels.

SpaceWire equipment is connected together using SpaceWire links that are serial, high-speed, bi-directional, and full-duplex. Application information is sent along a SpaceWire link in discrete packets, and control and time information also can be sent along SpaceWire links. SpaceWire supports many different payload processing architectures using point-to-point links and SpaceWire routing switches.

NASA's SpaceWire

According to Glenn Rakow, NASA's SpaceWire development lead at Goddard Space Flight Center in Maryland, this flexible, modular, and reusable design allows aerospace companies to standardize their designs. "SpaceWire lets you create one design that you can go to every time, for every mission," Rakow said.

NASA Goddard's SpaceWire design is the first and most mature of its kind in the United States. New features and enhancements to the NASA SpaceWire core make the design more suitable for spaceflight applications. A sophisticated verification environment includes directed and random testing, helping to detect hard-to-find design bugs and making the design simple to maintain and update. Not all features are included in the SpaceWire standard, but Goddard's design may be configured so that it is completely compatible with it. These significant advantages enable easy system integration and testing of SpaceWire designs while improving reliability and reducing complexity.

Compared to Ethernet networks with pre-set link rates, SpaceWire offers flexible link rates, helping save power and providing more options for high-speed applications. In addition, the standard is topology-independent, meaning that connections between routers or network fabrics can be fashioned in nearly any way that suits the design's needs. Finally, SpaceWire doesn't define a rigid data-packet structure. "It's very scaled down and simple, so it gives the system engineer a lot of flexibility in developing additional protocols," said Rakow.

NASA Goddard also developed a unique SpaceWire link-and-switch implementation. This new design provides for a standard that enables high- and low-rate communication between avionics systems over a network architecture using a first-in/first-out (FIFO) interface. This significant advancement helps reduce the complexity of communication over satellite architecture applications and other spaceflight systems, while improving speed and reliability.

The link and switch is a unique implementation of the SpaceWire specification, which enables avionics computers to seamlessly communicate at varying data rates (2 Mbps to more than 200 Mbps), minimizing interconnects. The communication allows resources to be distributed and provides for redundancy across spaceflight applications. The new implementation can be configured according to number of serial ports, local ports, FIFO sizes, etc., and is applicable to any aerospace or military hardware microelectronics.

Redundant physical interfaces (cables) added to the design simplify operation because the user does not need to interact with the design. Goddard has also designed time-code enhancements to the protocol by allowing multiple time-code masters to operate on the network simultaneously. These low-latency broadcast pulses are sent over a SpaceWire network to signal an event, interrupt an event, or distribute time information. In addition, a zero-jitter time code reduces uncertainty as to when the time-code signal arrives at all destinations. Goddard has also designed the link and switch for high accuracy with design verification using direct and random testing. This means that all parameters of the design are randomized and tested in a continual loop to find errors or situations that may be difficult to anticipate.

Goddard's link-and-switch implementation provides advantages for spaceflight applications over the current SpaceWire capabilities. Specifically, the redundant physical interfaces eliminate the need for the user to be involved in finding the most reliable data connection. These features may reduce the complexity that worm-hole routing may introduce into a network, helping to prevent network blockage.

In addition, unlike the current SpaceWire standard, which only provides for one time-code master, Goddard's implementation allows for up to four masters, or people, on the network who can send time-code signals. For spaceflight applications, this feature allows more out-of-band signaling between nodes in the network, enhancing flexibility in the system design.

Crucial for spaceflight applications is the implementation's provision for both random and directed testing. This combination of testing helps improve the reliability of communications in spaceflight applications in which accurate data transmission is critical.

Commercial Applications

NASA Goddard has entered into Space Act Agreements with a number of commercial companies to further develop the SpaceWire technology. "SpaceWire technology development can be beneficial for wider U.S. industry and government use," explained Rakow. While the standard is tailored to spaceflight systems, the technology behind it may benefit ground applications as well. "The more people we get using it, the more ideas we'll have," said Rakow. "Industry has expertise that NASA doesn't have and vice versa, and that exchange will benefit the SpaceWire standard, as well as everyone who uses it now and in the future."

The Lunar Reconnaissance Orbiter (LRO) is one mission that will incorporate SpaceWire-based components. The LRO mission, scheduled to launch in October 2008, will conduct investigations specifically targeted to prepare for and support future human exploration of the Moon. (NASA Goddard)

Early this year, NASA announced that it is providing support to Harris Corporation so its researchers can understand how Goddard's link and switch SpaceWire router can be integrated into Harris electronics. Harris plans to integrate the technology into its aerospace electronics, including its Space Programmable Modem, enabling connection of digital technologies for communication in space.

This summer, NASA announced two more agreements. The first was with BAE Systems, the fourth-largest defense company in the world, and the largest in Europe. BAE is building a new application- specific integrated circuit (ASIC) design for Goddard's SpaceWire router technology. The SpaceWire router functionality will be integrated into BAE's onboard computer system, making the computer desirable to NASA, the military, and other U.S. aersospace organizations. BAE plans to finish its SpaceWire ASIC design early next year, and should have working boards available for NASA missions and other aerospace companies within two years.

The second agreement will enable Aeroflex, which develops microelectronic and test and measurement products, to develop a SpaceWire-based router with guidance from Goddard. The company will translate the multiport router into ASICs, enabling a variety of applications to connect through the router and communicate with each other, benefiting spaceflight applications for both organizations and the aerospace industry. Specifically, NASA will benefit from being able to purchase ASICs from Aeroflex at a much more affordable rate than producing them in-house.

Rakow explained that these and other U.S. industry partners are developing components to infuse in-to Goddard missions such as the James Webb Space Telescope, Geostationary Operational Enviornmental Satellites — R Series (GOES-R), Lunar Reconnaissance Orbiter (LRO) mission, the Magnetospheric Multiscale Mission (MMS), and Swift, a multi-wavelength observatory dedicated to the study of gamma-ray burst (GRB) science.

According to Rakow, one major improvement has been to define a standard SpaceWire packet format that is consistent for decoding because the standard permits different packet formats depending upon the type of routing address scheme used. The new format supports the development of standard protocols through the assignment of protocol identifiers. As a result, several new protocols have been developed.

"One protocol performs reliable packet delivery that will be used by the GOES-R mission. Another defines memory mapping of packets, which is useful for replacing standard parallel backplanes to increase reliability and reduce mass and power while standardizing transactions between boards and subsystems no matter their location," Rakow explained. "MMS will use this.

"I also recently led an international team of engineers to develop the hardware plug-and-play features to support software's discovery of a network and notification when devices are added or removed. This will first be used by the Air Force Research Laboratory for PnPSat, which is a technology demonstration of a plug-and-play satellite that will be rapidly integrated from standard components. These changes have been incorporated above the (computer communication stack) layers that define SpaceWire."

Improvements to SpaceWire itself include adding a cable redundancy mechanism that was developed for JWST and is being used for a National Reconnaissance Office (NRO) satellite. Other improvements, said Rakow, include "adding the ability to fuse many discrete signals over the SpaceWire cable to eliminate extra wires for side band signaling (one pulse per second is a common example of one of these), and adding features to provide network blockage protection into our SpaceWire router design."

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