TWP/VCEIL/C3 - Degraded Sensitivity, Limited Range

The Darwin ceilometer was displaying a cross-talk warning during this
period. This causes degraded sensitivity and the inability of the 
ceilometer to identify cloud heights that are higher the about 2 km.

• Third cloud base height(third_cbh)
• Lowest cloud base height detected.(first_cbh)
• Second lowest cloud base height(second_cbh)
• Aerosol backscatter coefficient at 355 nm(backscatter)
• Vertical visibility(vertical_visibility)
• Altitude of highest signal(alt_highest_signal)

• Lowest cloud base height detected.(first_cbh)
• Vertical visibility(vertical_visibility)
• Aerosol backscatter coefficient at 355 nm(backscatter)
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• Altitude of highest signal(alt_highest_signal)

TWP/VCEIL/C3 - Data collection problem

There was a bad connection between the ceilometer and the instrument computer which 
resulted in a small amount of data loss on 8 days during the effected period.  The data that 
were collected are not effected by the problem.  On April 11, 34% of the data was lost and 
on April 1, 4% was lost.  On the remaining 6 days, May 1-6, the amount of data lost was 
less than 2%/day.

• null(Raw data stream - documentation not supported)

• Vertical visibility(vertical_visibility)
• Backscatter sum(sum_backscatter)
• Time offset of tweaks from base_time(time_offset)
• Second lowest cloud base height(second_cbh)
• base time(base_time)
• Laser temperature(laser_temperature)
• Altitude of highest signal(alt_highest_signal)
• Warning and alarm status bits(status_string)
• Millivolts at internal ADC input (0 to 2500)(background_light)
• lat(lat)
• Degrees from vertical (0 to +90)(tilt_angle)
• Lowest cloud base height detected.(first_cbh)
• Percentage of nominal factory setting (0 to 999%)(laser_pulse_energy)
• lon(lon)
• Aerosol backscatter coefficient at 355 nm(backscatter)
• Flag indicating aeri retrieval status(status_flag)
• Dummy altitude for Zeb(alt)
• Third cloud base height(third_cbh)
• 6 character string describing instrument measurement parameters(measurement_parameters)
• Millivolts at internal ADC input(window_contamination)
• Percentage of nominal factory setting (0 to 999%)(receiver_sensitivity)
• range(range)
• Detection status. See details(detection_status)

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.

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

TWP/VCEIL/C3 - ceilometer lost sensitivity

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

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

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.

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

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.

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

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.

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

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.

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