TWP/VCEIL/C3 - low receiver sensitivity

The ceilometer reported frequent receiver alarms. There are periods when the receiver 
sensitivity dropped to about 50%. The problem was corrected by replacing the instrument with 
a spare.

• Vertical visibility(vertical_visibility)
• Aerosol volume backscattering coefficient at 355 nm(backscatter)
• Third cloud base height(third_cbh)
• Second lowest cloud base height(second_cbh)
• Percentage of nominal factory setting (0 to 999%)(receiver_sensitivity)
• Sum of detected and normalized backscatter(sum_backscatter)
• Millivolts at internal ADC input (0 to 2500)(background_light)
• Lowest cloud base height detected(first_cbh)
• Altitude of highest signal(alt_highest_signal)