TWP/VCEIL/C2 - Ceilometer failure

Large data gaps began occurring frequently at the TWP Nauru site 26 February at 06Z.  
Prior to 25 February, optical cross talk errors had been observed.  Beginning on 25 February, 
data errors were observed.  By 26 February, these errors were exhibiting themselves as 
extended data gaps.  These gaps became increasingly frequent between 2/26 and 3/4.  All 
data are missing after 20050304.1550.

The instrument was replaced on 29 March and after some subsequent configuration work, the 
instrument was revived just after 00Z on 1 April.

• null(Raw data stream - documentation not supported)

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

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/C2 - occasional communication fault

Beginning August 30, 2005, the Nauru Ceilometer periodically went into a strange 
operational mode where the backscatter data exhibits vertical stripes.  Looking at the backscatter 
data - there are high amplitude oscillations occuring in the backscatter as a function 
of time at a given height.  The phenomenon appears to be more common at night.  Dates 
include: 30 Aug, 31 Aug, 1 Sep, 9 Sep, 15 Sep, 20 Oct, 22 Oct, 8 Dec, and others.

The fiber optic link to the ceilometer was faulty and causing periodic data corruption. 
The problem was corrected when the fiber optic line was replaced with a hard wired RS232 
link.

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

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/C2 - Vceil computer disconnected

VCEIL computer was disconnected during this time period.

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

TWP/VCEIL/C2 - Time offset

On 7/14/2006, I found that the ceilometer time was 8m 36s slow, the PC time display (lower 
right taskbar) was frozen at "22:24", and AboutTime was not running. Corrected it on 
7/17 by running AboutTime so that 19:03:29 became 19:12:05. This also corrected the frozen 
display on the Windows98 taskbar. 

I am not sure of the start time of this problem but there were several other Windows 
related issues that had to be corrected on 5/14 with indications of inappropriate access via 
Remote Administrator on 5/13. I do not know if the time offset was a sudden or gradual 
change.

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

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/C2 - optical cross-talk compensation

The ceilometer gave frequent optical cross-talk compensation warnings, was insensitive to 
high clouds, and the tropics window was deteriorated. The instrument was replaced with a 
spare.

• Aerosol backscatter coefficient at 355 nm(backscatter)
• Vertical visibility(vertical_visibility)
• Altitude of highest signal(alt_highest_signal)
• Third cloud base height(third_cbh)
• Second lowest cloud base height(second_cbh)
• Millivolts at internal ADC input(window_contamination)