NSA/CEIL/C1 - poor sensitivity at low altitude
The ceilometer has an internal amplifier that it sets to low or high depending on prevalant conditions. Normally, the gain is automatically set high during clear skies. It appears that during these times, a slight degradation in either the transmitter or receiver is being amplified and produces a "ripple" at low altitudes in the backscatter plot. Cloud base height detection is not affected.