Lesser, but important devices
Short-range rangefinding devices
We need the usual "ring of dumb obstacle sensors" on the vehicle. Shown here are a few options.
Short-range Doppler radar devices are widely used for sensing obstacles behind trucks and buses. Eaton VORAD makes one, as an adjunct to their forward-looking radar. Unfortunately, all it indicates is presence/absence of a target. (Eaton's engineer tells me that it has a DSP and has distance information internally, but doesn't bring it out to an external connection.) Competing products include the Rostra "RearSentry" and "School Bus Protection System".
These devices have good immunity to dirt, but cannot detect stationary objects. So they're not that useful for side-looking sensors, where the range might be constant, or for looking ahead of the front wheels to detect drop-offs when moving slowly.
There's been some progress since the days when the fragile Polaroid ranging sensors were the only available devices.
These devices vary in beam width. The narrow-beam Senix unit seems ideal for looking for a surface ahead of each front wheel. But it would take too many of those units to provide full perimeter coverage. We still need to find a wide-angle unit that is very rugged.
Murata makes both types of sensors as OEM components, but their web site has problems.
We'll need to measure shaft speeds at the engine and driveshaft, but just knowing what the wheels are doing doesn't mean that much off-road.