The pulses are defined in the following way:

r = radius
phi = azimuth
theta = zenith

r >= 0
0 <= theta < pi
0 <= phi < 2*pi
phi = 0 is true north
theta = 0 is vertical “up” view
ALS scan angle = pi - zenith

Here is a link to an image: http://envisat.esa.int/handbooks/meris/CNTR4-2.htm