The Usage Of Embedded Linux In Spacecraft

As the first part of a series, [George Emad] takes us through a few examples of the Linux operating system being used in spacecraft. These range from SpaceX’s Dragon capsule to everyone’s favorite Martian helicopter. An interesting aspect is that the freshest Linux kernel isn’t necessarily onboard, as stability is far more important than having the latest whizzbang features. This is why SpaceX uses Linux kernel 3.2 (with real-time patches) on the primary flight computers of both Dragon and its rockets (Falcon 9 and Starship).

SpaceX’s flight computers use the typical triple redundancy setup, with three independent dual-core processors running the exact same calculations and a different Linux instance on each of its cores, and the result being compared afterwards. If any result doesn’t match that of the others, it is dropped. This approach also allows SpaceX to use fairly off-the-shelf (OTS) x86 computing hardware, with the flight software written in C++.

NASA’s efforts are similar, with Ingenuity in particular heavily using OTS parts, along with NASA’s open source, C++-based F’ (F Prime) framework. The chopper also uses some version of the Linux kernel on a Snapdragon 801 SoC, which as we have seen over the past 72 flights works very well.

Which is not to say using Linux is a no-brainer when it comes to use in avionics and similar critical applications. There is a lot of code in the monolithic Linux kernel that requires you to customize it for a specific task, especially if it’s on a resource-constrained platform. Linux isn’t particularly good at hard real-time applications either, but using it does provide access to a wealth of software and documentation — something that needs to be weighed up against the project’s needs.

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Hackaday Links: October 22, 2023

The second of three major solar eclipses in a mere six-year period swept across the United States last week. We managed to catch the first one back in 2017, and still have plans for the next one in April of 2024. But we gave this one a miss, mainly because it was “just” an annular eclipse, promising a less spectacular presentation than a total eclipse.

Looks like we were wrong about that, at least judging by photographs of last week’s “Ring of Fire” eclipse. NASA managed to catch a shot of the Moon’s shadow over the middle of the US from the Deep Space Climate Observer at Lagrange Point 1. The image, which shows both the compact central umbra of the shadow and the much larger penumbra, which covers almost the entire continent, is equal parts fascinating and terrifying. Ground-based photographers were very much in the action too, turning in some lovely shots of the eclipse. We particularly like this “one-in-a-million” shot of a jet airliner photobombing the developing eclipse. Shots like these make us feel like it was a mistake to skip the 10-hour drive to the path of annularity.

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Voyager 2: Communication Reestablished With One Big Shout

You could practically hear the collective “PHEW!” as NASA announced that they had reestablished full two-way communications with Voyager 2 on Friday afternoon! Details are few at this point — hopefully we’ll get more information on how this was pulled off, since we suspect there was some interesting wizardry involved. If you haven’t been following along, here’s a quick recap of the situation.

As we previously reported, a wayward command that was sent to Voyager 2, currently almost 19 light-hours distant from Earth, reoriented the spacecraft by a mere two degrees. It doesn’t sound like much, but the very narrow beamwidth on Voyager‘s high-gain antenna and the vast distance put it out of touch with the Canberra Deep Space Network station, currently the only ground station with line-of-sight to the spacecraft. While this was certainly a problem, NASA controllers seemed to take it in stride thanks to a contingency program which would automatically force the spacecraft to realign itself to point at Earth using its Canopus star tracker. The only catch was, that system wasn’t set to engage until October.

With this latest development, it appears that mission controllers weren’t willing to wait that long. Instead, based on what was universally referred to in the non-tech media as a “heartbeat” from Voyager on August 1– it appears that what they were really talking about was the use of multiple antennas at the Canberra site to pick up a weak carrier signal from the probe — they decided to send an “interstellar shout” and attempt to reorient the antenna. The 70-m DSS-43 dish blasted out the message early in the morning of August 2, and 37 hours later, science and engineering data started streaming into the antenna again, indicating that Voyager 2 was pointing back at Earth and operating fine.

Hats off to everyone involved in making this fix and getting humanity’s most remote outpost back online. If you want to follow the heroics in nearly real-time, or just like watching what goes on at the intersection of Big Engineering and Big Science, make sure you check out the Canberra DSN Twitter feed.

Discussing The Finer Points Of Space-Worthy Software

At the dawn of the Space Race, when computers were something that took up whole rooms, satellites and probes had to rely on analog electronics to read from their various sensors and transmit the resulting data to the ground. But it wasn’t long before humanity’s space ambitions outgrew these early systems, which lead to vast advancements in space-bound digital computers in support of NASA’s Gemini and Apollo programs. Today, building a spacecraft without an onboard computer (or even multiple redundant computers) is unheard of. Even the smallest of CubeSats is likely running Linux on a multi-core system.

Jacob Killelea

As such, software development has now become part an integral part of spacecraft design — from low-level code that’s responsible for firing off emergency systems to the 3D graphical touchscreen interfaces used by the crew to navigate the craft. But as you might expect, the stakes here are higher than any normal programming assignment. If your code locks up here on Earth, it’s an annoyance. If it locks up on a lunar lander seconds before it touches down on the surface, it could be the end of the mission.

To get a bit more insight into this fascinating corner of software development, we invited Jacob Killelea to host last week’s
Software for Satellites Hack Chat. Jacob is an engineer with a background in both aero and thermodynamics, control systems, and life support. He’s written code for spacecraft destined for the Moon, and perhaps most importantly, is an avid reader of Hackaday.

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Laser Propulsion Could Satisfy Our Spacecraft’s Need For Speed

There are many wonderful places we’d like to visit in the universe, and probably untold numbers more that we haven’t even seen or heard of yet. Unfortunately…they’re all so darn far away. A best-case-scenario trip to Mars takes around six months with present technology, meanwhile, if you want to visit Alpha Centauri it’s a whole four lightyears away!

When it comes to crossing these great distances, conventional chemical rocket technology simply doesn’t cut the mustard. As it turns out though, lasers could hold the key to cutting down travel times in space!

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Mercury Thrusters: A Worldwide Disaster Averted Just In Time

The field of space vehicle design is obsessed with efficiency by necessity. The cost to do anything in space is astronomical, and also heavily tied to launch weight. Thus, any technology or technique that can bring those figures down is prime for exploitation.

In recent years, mercury thrusters promised to be one such technology. The only catch was the potentially-ruinous environmental cost. Today, we’ll look at the benefits of mercury thrusters, and how they came to be outlawed in short order.

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Apollo Comms Flight Hardware Deep Dive

You no doubt recall the incredible Apollo Guidance Computer (AGC) reverse engineering and restoration project featured on the CuriousMarc YouTube channel a few years ago. Well, [Marc] and the team are at it again, this time restoring the Apollo Unified S-Band tracking and communication system flight hardware. As always, the project is well documented, carefully explained, full of problems, and is proceeding slowly despite the lack of documentation.

Like the guidance computer, the Unified S-Band system was pretty innovative for its day — able to track, provide voice communications, receive television signals, and send commands to and monitor the health of the spacecraft via telemetry. The system operates on three frequencies, an uplink containing ranging code, voice and data. There are two downlinks, one providing ranging, voice, and telemetry, the other used for television and the playback of recorded data. All crammed into two hefty boxes totaling 29 kg.

So far, [Marc] has released part 9 of the series (for reference, the Apollo Guidance Computer took 27 parts plus 8 auxiliary videos). There seems to be even less documentation for this equipment than the AGC, although miraculously the guys keep uncovering more and more as things progress. Also random pieces of essential ground test hardware keep coming out of the woodwork. It’s a fascinating dive into not only the system itself, but the design and construction techniques of the era. Be sure to check out the series (part 1 is below the break) and follow along as they bring this system back to life. [Marc] is posting various documents related to the project on his website. And if you missed the AGC project, here’s the playlist of videos, and the team joined us for a Hackaday Chat back in 2020.

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