Lander landing sensor
Okay A list of ideas now:
Lidar. a few feet is cheap and light, a mile or so is not to bad since sporting good stores sell them for a hundred bucks, but 20 km ? could be a problem. Also really could use multiple beams for attitude determination and doppler for velocity (drift etc)
Radar. Power, weight, availability. Could the communications gear help ?
Image. Some work for attitude and surface navigation is being done at Micro-space M-S Lunar Nav. Other ideas that shouldn't be to hard is a variation on the optical mouse to determine lateral velocity (drift) for final touchdown. Star cameras are known to be used for rendezvous. cameras can also see the horizon pretty well for attitude and limited altitude.
Touchdown sensors might be useful to be sure when to shut off the engines ;)
And avionics should have the usual spacecraft style inertial platform and tracking systems.