I don't think you can do terrain following entirely in software unless you already have an accurate image of what to expect--and a munition will never know the fine detail. (Coarse detail--I would expect you could do a fair job of steering a ballistic missile based on images of the target area. Closest match and figure out in what way the image is stretched is at least theoretically possible with camera + software. Computationally practical, I don't know, nor how accurate it could be.)