[contingencies]

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 ;)

[dotnet00]

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.

[contingencies]

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.

[dekhn]

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).