Why are laser footprint shape parameters not available for low energy campaigns?

Several parameters help quantify the shape of the GLAS laser footprint, which change slightly for each campaign. These parameters are:

GLAS HDF5 Parameter GLAS HDF5 Parameter Description GLAS HDF5 Data Set
d_tpintensity_avg Transmit pulse intensity - frame average GLAH14, GLAH15
d_tpazimuth_avg Transmit pulse azimuth - frame average GLAH06
d_tpeccentricity_avg Transmit pulse eccentricity - frame average GLAH14, GLAH15
d_tpmajoraxis_avg Transmit pulse major axis - frame average GLAH14, GLAH15

These parameters are valid on most campaigns, but in the later campaigns (after Laser 2D), the laser energy was too weak to be able to compute the pulse shape, resulting in values of 0.