[genericuser]

First I recognize you said maybe, but the range would need to be much larger than 100 meters.

As out of the box hobbyist drones can already travel at 50 mph [1] (or more I assume). And 100 meters assuming approach of a stationary object would provide just over 4 seconds for the drone to respond and avoid the object.

While 4 seconds is sufficient time, it will lead to drones flying very close to objects before avoiding them which will not inspire public confidence. Also as faster drones appear that window will become even smaller. They should probably have radar with a range measured in nautical miles to avoid having to implement a new system almost immediately. I would say at least 5 nautical miles, although I feel even that would be on the short sighted end of implementation.

[1]http://www.dji.com/product/inspire-1

[Animats]

Radar power needed increases as the fourth power of the range.

The state of the art in radar is that sensing power lines and similar wires is not reliable. 77GHz radars employed in cars can sense people and bicycles, but that's not good enough. There's been much effort put into power line detection for aircraft (helicopters hit power lines about 50 times a year in the US) but no good solution yet. Drones zipping around streets with overhead wires and trees is not yet workable.

[genericuser]

It seems like power lines would be able to be detected with system measuring changes in magnetic fields, perhaps based off current induction sensors. However clearly a system like that does not yet exist and that would not help with other obstacles. And I agree with your statement that this is currently not workable.

[Vexs]

They've been clocked at 80+. Remember though, they're not going to be descending at 80mph, which is really the only time you'd need to worry about collisions.

[genericuser]

That is definitely not the only time you need to worry about collisions, it is the most dangerous time but not the only time, as there are many things taller than 200 ft, as well as birds and unequipped vehicles using the airspace in an unauthorized manner for you to worry about.