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.

• Aerosol backscatter coefficient at 355 nm(backscatter)
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• Lowest cloud base height detected.(first_cbh)
• Millivolts at internal ADC input (0 to 2500)(background_light)
• Altitude of highest signal(alt_highest_signal)
• Vertical visibility(vertical_visibility)

TWP/VCEIL/C3 - ceilometer lost sensitivity

ceilometer sesitivity decreased suddenly and only very strong cloud reflections are 
observed in the first kilometer.

• Aerosol backscatter coefficient at 355 nm(backscatter)
• Warning and alarm status bits(status_string)
• Flag indicating aeri retrieval status(status_flag)
• Backscatter sum(sum_backscatter)
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• range(range)
• Lowest cloud base height detected.(first_cbh)
• Detection status. See details(detection_status)
• Millivolts at internal ADC input (0 to 2500)(background_light)
• Vertical visibility(vertical_visibility)

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.

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

TWP/VCEIL/C3 - standard instrument installed

A spare ceilometer without the tropics window option was installed. The manufacturer 
recommends the tropics window be used at +/- 25 deg lattitude to prevent direct solar 
radiation into the laser optics when the instrument pointed at zenith. No significant signal 
degradation is apparent during the autumnal equinox but, as a precaution, the unit was 
replaced with the original with a tropics window.

• Aerosol backscatter coefficient at 355 nm(backscatter)
• Percentage of nominal factory setting (0 to 999%)(laser_pulse_energy)
• Laser temperature(laser_temperature)
• Backscatter sum(sum_backscatter)
• Millivolts at internal ADC input (0 to 2500)(background_light)
• Vertical visibility(vertical_visibility)

TWP/VCEIL/C3 - Reduced sensitivity to high clouds

The instrument produced a significant number of optical cross-talk warnings and displayed 
a reduced capacity to detect high clouds.  The internal cables were inspected, the 
compensation fiber and optics monitor were replaced, the crosstalk compensation was adjusted, 
and the system tested with an optical termination hood, all with no improvement.  While 
waiting for a new receiver board, receiver ribbon cable, and compensation fiber cable from 
Vaisala, a spare ceilometer with a standard, "non-tropics", window was installed.

• Percentage of nominal factory setting (0 to 999%)(receiver_sensitivity)
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• Detection status. See details(detection_status)
• Altitude of highest signal(alt_highest_signal)

TWP/VCEIL/C3 - Reduced sensitivity to high clouds

The instrument had reduced sensitivity to high clouds.

The instrument produced a significant number of optical cross-talk warnings and displayed 
a reduced capacity to detect high clouds. The window was cleaned and the instrument was 
power-cycled but with no improvement. Diagnostics indicated the problem was with the 
compensation fiber cable. 

The problem was corrected by replacing this standard-window instrument (that was tilted at 
16 degrees) with a tropics unit that became available.

• Percentage of nominal factory setting (0 to 999%)(receiver_sensitivity)
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• Altitude of highest signal(alt_highest_signal)
• Vertical visibility(vertical_visibility)