TWP/VCEIL/C3 - Degraded hardware

It appears that the sensitivity of the Darwin ceilometer was degrading over a few month 
period leading up to September 2005.  Beginning on September 16, 2005, the instrument 
shifted to a particularly bad state.  Looking at the backscatter data, I do not see nearly as 
great a day-night difference in the background as I expect and there is very little 
sensitivity to clouds.  There was cross talk error noted with that instrument. The ceilometer 
was replaced on December 12.

• 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)
• Millivolts at internal ADC input (0 to 2500)(background_light)
• Lowest cloud base height detected(first_cbh)
• Altitude of highest signal(alt_highest_signal)