Yes, by ‘fixed’ I meant statically situated.
Even indoors, I do like the looks of the Lidar unit, but it is too costly to use more than one, so with that one, you could rotate it (hint: Adafruit sells six-wire slip rings that allow for continuous rotation - maybe sparkfun does too). Rotating ultrasonics has never worked out well for me because the rate at which you can read and move them is too slow - it really limits the speed at which you can move your bot. But rotating sensors, in general, isn’t necessary for basic area coverage and obstacle avoidance.
Consider, as an example, the Roomba. Roombas don’t use SLAM - they just employ a series of coverage patterns and escape maneuvers. They have a Sharp IR sensor on one side (for wall following), mechanical contact sensors (switches), cliff sensors (downward facing IR and wheel-drop switches), and wheel encoders if I recall correctly, and yet they can do an excellent job of covering a room in spite of obstacles. They rely heavily on the contact sensors. There is a tendancy to over-do sensors on hobby robots, so think carefully about what you are trying to achieve.
a) if you just want to move and cover an unknown space without mapping it, you just need a small set of fixed sensors, and contact sensors are cheaper and more reliable than anything else (if less sexy). If you can’t touch obstacles, then choose IR or ultrasonics, but be aware of their blind spots (discussed previously)
b) if you want to move within a map that is know ahead of time, but which may contain unmapped obstacles, then you need wheel encoders and IR/ultrasonics to correct for errors while moving (wheels slipping for instance) - and again, I always recommend contact switches as a failsafe.
c) if you want to map an unknown space, then you need to always know your position within that space and you need to match up current sensor measurements against the map you are building up. That requires fast, dense, accurate, and repeatable measurements, and probably is best done with lidar. SLAM may require more memory and processor power than you will have on most NETMF devices. I won’t say it is impossible, I just think you’ll probably have to use wifi or something to offload some of the processing.
And one final note about ranged sensors like IR and ultrasonics: It’s easy for thin objects (table legs, chair legs, cats, etc) to slip in between their fields of view, so contact sensors are always a good idea.