interfade time = fade + nonfade period
Av. number of crossings per sec = [av. interfade time]-1
Number of level crossing per sec is proportional to
speed r' of crossing R (derivative r' = dr/dt) probability of r being in [R, R + dR]
speed r' of crossing R (derivative r' = dr/dt)
probability of r being in [R, R + dR]