Don't they use radar altimeters to get altitude? You don't need to rely only on image processing. You can even pull off stereo using a single camera since you are moving and know the altitude at every point.

They definitely combine several sensors to get a proper height reading, but as we've seen with the American and Japanese landers, getting good readings from the sensors, properly accounting for all factors that might affect sensor readings and being able to properly handle sensor disagreements is quite challenging.

Better call my man... Kalman.

https://en.wikipedia.org/wiki/Kalman_filter

For distance during landing I would be thinking a spatially maximally distributed array of quartz-shielded, thermally-supported laser TOF sensors along nominal extremities, but that's just because they're familiar to me, small, power-efficient, highly linear, relatively accurate, and cheap. Unsure if the IC physics assumptions work in non-atmospheric conditions. Perhaps the output can be re-scaled to obtain cheap and accurate enough readings.

A non-dilettante with an actual physics degree would clearly be desirable ;)

The apollo guidance system was one of the first production deployments of Kalman filters. Here's the source code: https://github.com/chrislgarry/Apollo-11/blob/master/Luminar... (it was used for star navigation, not sure about the lander).

A kalman filter wouldn't account for say, the issue that hit the HAKUTO-R lander, where because the reading on the radar altimeter changed too rapidly, the computer assumed it was faulty, or the IM-1 lander, where they initially had a lot of trouble with altitude sensing (in part because they forgot to remove the covers from the laser rangefinders), managed to work around it, and then failed to fully sense and cancel out the lateral velocity, causing it to skid along the surface, snap a leg and tip over.

The first sounds like bad assumption (hard fault limit).

The second sounds like bad process leading to bad input, at which point it becomes garbage in, garbage out. The workaround was untested and insufficient.

While you are of course correct the filter will not fix these, none of these are the fault of the filter, they are all human process issues that are firmly out of scope.

LIDAR/RADAR performance weren't issues with last two attempts. SLIM had a thruster kaboom, IM-1 had human eye safety mechanism wired incorrectly.

This one seem to use scaled down Soyuz capsule as reentry vehicle. They must have reused a lot of proven hardware and software unlike those.

The problem with RADAR on the moon is there's an unknown amount of dust between you and the reflective surface.

Apparently cameras also have issues because the lack of atmosphere means everything is high contrast.

So basically, you don't actually know your altitude, and your visual systems may not be reliable.