Question about sensor model in range_sensor_layer

[SherlockSheep]

Hi guys, is there any reference about sonar model implemented in the range_sensor_layer?
The implementation is a little bit different from what I've read in 'A comparison of three uncertainty calculi for building
sonar-based occupancy grids' by Miguel Ribo.

[DLu]

I've been digging through my notes and I honestly don't remember exactly where the model comes from. I will keep looking, but alternate models are welcome.