Ethernet computer networks date back to the 1970s and are now used virtually everywhere, including in space. Inexpensive, ubiquitous, and easy to use, Ethernet has often connected astronauts’ computers, cameras, and other noncritical devices on the Space Shuttle and International Space Station (ISS).
Until recently, though, Ethernet could never be used for critical systems like the spacecraft avionics responsible for navigation, communication, and other essential functions, because it’s imprecise. There’s no guarantee how long it will take to transmit a message or even whether it will arrive at its intended destination. Messages can interrupt each other or be lost or damaged.
But a sophisticated deterministic Ethernet system developed for NASA’s new Orion space capsule allows all the spacecraft’s systems to communicate on a single Ethernet network—and the technology is also being applied to integrate systems for self-driving cars, industrial applications, defense programs, commercial spacecraft, and more.
Traditionally, critical spacecraft subsystems have been housed on separate computers connected with point-to-point links or dedicated buses, says Andrew Loveless, command and data handling domain lead for NASA’s Advanced Exploration Systems Avionics and Software project and an engineer at Johnson Space Center. For example, he says, the ISS has a highly tiered network, with subsystems communicating indirectly by sending messages up the network “tree” and back down.
“That’s generally not ideal, because you have more complexity, and you have all the size, weight, and power concerns,” Loveless says, noting that the arrangement requires each function to be housed on a separate computer, each with its own power supply.
As they were planning Orion, NASA engineers wanted to take a new approach, replacing this federated architecture with a more integrated network, but the technology to do so didn’t exist in a mature form.
TTTech Computertechnik AG, based in Vienna, Austria, with its U.S. headquarters in Andover, Massachusetts, was already working on technology to build a more reliable Ethernet network. The company had been partnering with Honeywell for aerospace applications since 2000, contributing, among other things, early versions of the company’s Time-Triggered Protocol chips. Cooperation between the two companies started with a five-year NASA project that enabled the application of Time-Triggered Ethernet to several of Honeywell’s engine programs, as well as the Airbus A380, the Boeing 787, Bombardier’s C Series of airliners, and Embraer’s Legacy 450 and 500 business jets.
“What Time-Triggered Ethernet does is create three traffic classes for messages of different criticality,” Loveless explains. Only the most critical data is time-triggered, meaning it’s scheduled into time slots that don’t interfere with each other and is guaranteed to be transmitted and received at specific times. “You’re essentially reserving that bandwidth ahead of time,” Loveless says. Less-critical but high-priority data is rate-constrained, meaning it is transmitted reliably within strict timing constraints. The rest of the bandwidth is available for noncritical messages.
“No matter what you do with noncritical data, you’re not going to interrupt any of those time-triggered messages,” he says. This is why all the systems can run on the same network. “It’s a pretty big departure from the way past space avionics have been developed.”
Honeywell early on became part of the team developing the avionics for Orion, so by the time Johnson Space Center and TTTech signed a Space Act Agreement in 2009, the company’s technology was already being incorporated into Orion’s electronics.
Rather than passing messages up and down tiers, anything on Orion’s network can potentially communicate directly with anything else on the network. The arrangement allows engineers to maximize computing power and reduce the number of computers, because one computer can carry out more than one function. Loveless says this has dramatically lowered the avionics’ size, weight, power consumption, and complexity.
Another major benefit of scheduling high-priority messages is that it allows networks to be built with a modular approach, with individual functions able to be added or removed without interfering with each other or affecting the rest of the network, says Mirko Jakovljevic, engineer and marketing manager at TTTech. “I don’t need to retest everything together when I change a function.”
Loveless says the setup, known as integrated, modular avionics, provides additional flexibility by allowing functions to be moved from one computer to another. “A computer that’s processing sensor data doesn’t have to be collocated with that sensor,” he says. “You’re decoupling functions from the platforms they’re running on.”
Although the ability to move functions around isn’t all that pertinent to Orion’s network, he says, “that’s the direction our avionics are going.”
To help other industries also go in that direction, NASA’s 2009 Space Act Agreement with TTTech set about developing an official standard for Time-Triggered Ethernet through the engineering association SAE International. Loveless says NASA engineers are now helping the Consultative Committee for Space Data Systems create additional international standards enabling the use of Time-Triggered Ethernet for spacecraft.