CompactPCI Serial reaches out into space

2Recently, the U.S.-based Internet service provider OneWeb ordered 900 satellites to provide additional global broadband. Knowing that this volume is more than half of the total 1,400 satellites already in orbit, and knowing that the cost for sending one into space is about $100 million, the industry needs to start thinking about new technologies that could help manage the mass of satellites that must be produced every year.

The commercialization of aerospace, like this OneWeb project, increases the pressure on established companies to develop competitive products for this market. One possible option is to count on existing international standards in order to save time and cost during both development and during the project life cycle.

Taking such a standard and adapting it to the requirements for space was also the solution a group of global players in space like Airbus, Thales Alenia, and others went for: They decided to go with a rather new and powerful but already industry-proven standard: CompactPCI Serial. Using a proven standard like this also helps diminish the obsolescence problem of avionics hardware while significantly reducing complexity and costs for hardware and software.

In this vein, the new technical PICMG subcommittee “Space CompactPCI Serial” is now extending the current CompactPCI Serial specification by a substandard covering the specific requirements for space applications. Players in this working group include – in addition to Airbus and the initiating companies STI Spacetech, SYSGO and TTTech – Amphenol FCI, EKF, Elma Electronic, ERNI, Fastwel, Harting, HEITEC, Intel, Keysight, Pentair, and Positronic, with MEN Micro acting in a consultative role, having already been strongly involved in the development of the CompactPCI PlusIO and CompactPCI Serial standard.

Availability and open interfaces for space applications

VPX was – with the exception of completely customized solutions – for a long time the only standard for embedded systems in space, but CompactPCI also has a long history in space applications. Famous examples of CompactPCI use include the Curiosity Mars rover in satellite control and its implementation for scientific tasks in the International Space Station. Even the CompactPCI Serial base specification has the mechanical and conduction cooling technologies needed for space already defined and in place.

In addition, compared to VPX, the flexible CompactPCI Serial standard offers an easier-to-use and cost-optimized development.

A typical application for Space CompactPCI Serial could be, for example, the implementation of the platform and the payload controller onboard a satellite.

Space CompactPCI Serial is the most logical choice for a highly sophisticated market, while both reusing and evolving proven industrial technologies and finding significant cost reductions. At the same time, some unused features have been removed from CompactPCI Serial to make the standard more streamlined, while other properties have been added to optimize the standard for use cases.

The two main changes in the extension of the CompactPCI Serial specification are the definition of a dual star architecture for increased availability, and the addition of an open management bus that allows the integration of different communication protocols common in space applications.

The CompactPCI Serial basic specification defines a single star architecture while Space CompactPCI Serial now symmetrically doubles the usage of these interconnects, so if one CPU fails, the functionality of the complete system will not be affected. Having this increase in availability was essential for use in space, especially since it’s a little difficult to simply exchange a CPU card while a satellite is in orbit.

In addition to the system slot (A) on the left side of the system, a second system slot (B) on the right side of the system uses the same routing method. All seven peripheral slots are connected to both system slots, and both system slots are connected to each other. Altogether, these links build a fully meshed interconnectivity network. The full-mesh network is not restricted to any particular protocol and may be used for physical interconnection standards like Ethernet, SpaceWire, TTEthernet, and EtherSpace. (Figure 1.)

Parallel to the full-mesh network, both system slots can be connected to any of the peripheral boards by means of eight specific differential links. These links could be also used for any protocol, depending on the application and the individual boards. This dual star architecture is intended to be used in high-availability solutions.

The result is a parallel and flexible usage of the full mesh Ethernet network via the backplane, as well as the dual-star architecture via PCI Express or any other protocol.

Figure 1: Space CompactPCI Serial defines a second system slot, forming a dual star architecture for PCI-Express.
(Click graphic to zoom by 1.9x)

The specification defines a utility connector, which can be controlled and configured via an open management bus. It takes over the hot-plug functionality, as it was used for CompactPCI Serial, and allows single cards within a system to be switched on and off. Hot-plug functionality is indeed not necessary for satellites in orbit, but can actually be extremely useful during integration while on ground and for test systems, which can be still realized via PCI Express and with common CompactPCI Serial cards.

As Space CompactPCI Serial is intended to be used in a conduction-cooled environment, the mechanical design is fully compatible with CompactPCI Serial, but the board-to-board pitch is 5HP [horizontal pitch = 25.4 mm] instead of 4HP [20.3 mm] to allow a conduction-cooling frame for each board. At the moment, members of the working group are already working on specific standard backplanes for Space CompactPCI Serial.

The high-speed backplane interconnects and the connectors are intended to support data rates of 12.5 Gbit/s per differential pair. The accelerated bandwidth of the full mesh is 400 Gbit/s. Additionally, the dual star interconnect simultaneously supports a 1 Tbit/s through-put rate.

Infrastructure signals allow for comfortable and flexible system management. In addition to an I²C bus, CAN bus is also supported. The power supply is 12 volts; an optional 5-V standby voltage could be used to support suspend modes or sleep modes. Power rails can be either be a single plane, or every board could be supplied and controlled individually. The power supply and the system management are not part of the Space CompactPCI Serial specification.

The harsh environments in space, and especially the vacuum conditions, demand much from the connectors. The material must not outgas, as some outgassed materials can leave deposits on the satellites’ sensitive lenses. An outgassing test had already confirmed the qualification of the connectors used for Space CompactPCI Serial. Other mechanical or environmental measures, like SEU-resistance, are not defined in the standard specification, as these measures depend on the customer’s requirements and the boards’ place within the application and end system.

The working group – which finished the specification recently – plans to bring it into the PICMG ballot during the second quarter of 2017.

Space CompactPCI Serial allows the use of established industrial technology in space, which allows the cost-effective use of the latest technology for space applications. The open standard guarantees the interoperability of different boards from different suppliers, and helps designers reuse solutions from mission to mission.

Manfred Schmitz is the CEO of MEN Mikro Elektronik in Nurnberg, Germany.