Distance sensor 15 m

Hi,
I have been searching for info about the most suitable (and cheap) sensor for measuring distance indoors in bright light up to 15 m. So far I haven't found anything that really seems to fit my needs. So I ask you for help.
I'm constructing a robot that is going to move around in a room with slightly sloping walls, of the size of 60 m x 20 m. The robot has to pass every inch of the floor as fast as possible (much like a vacuumcleaner robot), so I was thinking that it should go along the long sides of the walls checking the distance to the walls on both sides and at each end of the wall move a bit further from wall X closer to wall Y. So the distance that needs to be measured is never more the 10 m but I would like to have a margin of 5 m just in case. As I have understood it a common ultrasonic sensor probably wont do it well since it only can handle up to 10 m (and I can't have that it fails now and then). The best thing I can imagine would be if there was a solution that could keep track of the exakt position of the robot in the room, like a distance sensor in all four directions. In that case I also could maneuver it to it's "home" without a beacon. The most important thing though is to keep track of the long walls.

Any bright ideas?

Thank you!

It aint cheap, but the LiDAR Lite V3 is an amazing long range distance sensor. I use it myself as an altimeter for RC planes.

Thank you! It was exactly what I was looking for but a little expensive, but I guess you can't have everything for free. If anyone knows about a cheaper solution though I'm all ears.