I would like to assume the worst possible environment so I can consider all the possible challenges and make a generic solution. For example an indoor environment with lots of partitions and lets assume we want a robot to clean each partition by moving from one to the other. So we would like to know (x,y) of the robot at time t.
Even in an empty room IR would tell existence of an object not (x,y), I don't think that can be used here. A good range(50-100m), best accuracy(<1 inch), beyond line of sight would be my choices. And it is a good idea if the robot can also have active response kinda comm.