software bug

2 Articles

The Glitch That Brought Down Japan’s Lunar Lander

June 3, 2023 by 36 Comments

When a computer crashes, it usually doesn’t leave debris. But when a computer happens to be descending towards the lunar surface and glitches out, that’s a very different story. Turns out that’s what happened on April 26th, as the Japanese Hakuto-R Lunar lander made its mark on the Moon…by crashing into it. [Scott Manley] dove in to try and understand the software bug that caused an otherwise flawless mission to go splat.

The lander began the descent sequence as expected at 100 km above the surface. However, as it descended, the altitude sensor reported the altitude as much lower than it was. It thought it was at zero altitude once it reached about 5 km above the surface. Confused by the fact it hadn’t yet detected physical contact with the surface, the craft continued to slowly descend until it ran out of fuel and plunged to the surface.

Ultimately it all came down to sensor fusion. The lander merges several noisy sensors, such as accelerometers, gyroscopes, and radar, into one cohesive source of truth. The craft passed over a particularly large cliff that caused the radar altimeter to suddenly spike up 3 km. Like good filtering software, the craft reasons that the sensor must be getting spurious data and filters it out. It was now just estimating its altitude by looking at its acceleration. As anyone who has tried to track an object through space using just gyros and accelerometers alone can attest, errors accumulate, and suddenly you’re not where you think you are.

We know what you’re thinking: surely they would have run landing simulations to catch errors like these? Ironically they did, it’s just that after the simulations were run, the landing site for Hakuto-R was changed. Unfortunately, nobody thought to re-run the simulations, and now the Moon has a new lawn ornament,

We’ve previously written about why lunar landings are so hard. While knowing what led to the crash will hopefully prevent a similar fate for future missions, the reality is that remotely landing a robot on a dusty world without the help of GPS is fiendishly difficult and likely will be for some time.

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Fixing The Only Thing That’s Slow About Grand Theft Auto V

March 2, 2021 by 13 Comments

The driving, crime, and general mayhem game Grand Theft Auto V is something of a phenomenon that has lasted for the last seven or more years. Whether following the in-game missions, driving around like a hooligan for fun or performing crazy stunts, the depth of detail in its landscapes and the continual improvements to gameplay that have arrived over the years have assured it a massive following across multiple platforms. The game is not without its problems though, one of which is an unreasonably long loading time for its online version. This annoyed [T0st] to the extent that it was worth the effort of looking under the hood to find out where the problem lay.

It was evident that for PC users the effect varied depending on the hardware present. Furthermore AMD processors seemed worse-hit than Intel ones, and indeed they found an entire core maxed out by a couple of processes during the wait. Some diagnostics and disassembly led the trail to some string processing code which was identified as a JSON parser. This was not simply parsing the JSON but also performing a check for token uniqueness in an extremely inefficient manner, causing the whole process to be extremely slow. Sone nifty patching in a DLL containing a much more efficient function with a cache for unique values saved the day, and delivered an impressive 70% speed-up. It’s to be hoped that the game’s developers will take note, and a future GTA V update will deliver a fix.

Driving a car from a third-person viewpoint in a game like GTA V is a hoot. In real life though, not so much.

Thanks [Thanatos Erberus] for the tip.