p0 is the occupancy value you're updating
p1 is the sensitivity -- if it's occupied, how likely are you to detect it?
p2 is the specificity -- if it's not occupied, how likely are you to read a "not occupied"?
Note that p2 is really high for cells between you and the echo, but the cells in the "arc" at the echo distance have a much lower p2, because any cell occupied at the distance of the echo could generate the echo, even if the most-straight-ahead cell is empty.
I'd use p1 of 0.8 or so, p2 of 0.8 for the cells "before" the echo distance, and perhaps 0.2 or 0.3 for the cells "at" the echo distance, as a first crude initial guess.