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.

• receiver_sensitivity
• Normalized corrected relative backscatter(backscatter)
• sum_backscatter
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• Lowest cloud base height detected.(first_cbh)
• background_light
• alt_highest_signal
• vertical_visibility

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.

• status_string
• detection_status
• sum_backscatter
• laser_pulse_energy
• tilt_angle
• alt_highest_signal
• laser_temperature
• vertical_visibility
• range
• Third cloud base height(third_cbh)
• measurement_parameters
• Second lowest cloud base height(second_cbh)
• status_flag
• Lowest cloud base height detected.(first_cbh)
• background_light
• Normalized corrected relative backscatter(backscatter)
• window_contamination
• base_time
• receiver_sensitivity
• time_offset

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.

• Normalized corrected relative backscatter(backscatter)
• laser_pulse_energy
• laser_temperature
• sum_backscatter
• background_light
• 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.

• receiver_sensitivity
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• detection_status
• alt_highest_signal

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.

• Second lowest cloud base height(second_cbh)
• alt_highest_signal
• Normalized corrected relative backscatter(backscatter)
• window_contamination
• vertical_visibility
• Third cloud base height(third_cbh)

TWP/VCEIL/C2 - Receiver failure

The receiver failed and the instrument was replaced.  Data during the specified period are 
not valid.

• sum_backscatter
• Second lowest cloud base height(second_cbh)
• Lowest cloud base height detected.(first_cbh)
• alt_highest_signal
• Normalized corrected relative backscatter(backscatter)
• background_light
• vertical_visibility
• Third cloud base height(third_cbh)

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.

• receiver_sensitivity
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• alt_highest_signal
• vertical_visibility

TWP/VCEIL/C1 - Compensation fiber

The VCEIL was reporting a "receiver optical cross-talk compensation is poor" warning. The 
window was cleaned and the instrument was power-cycled. The problem was corrected when 
the compensation fiber was replaced and ants were removed from communication connector.  
Cloud detection was impaired.

• Second lowest cloud base height(second_cbh)
• alt_highest_signal
• sum_backscatter
• receiver_sensitivity
• laser_pulse_energy
• Lowest cloud base height detected.(first_cbh)
• vertical_visibility
• Third cloud base height(third_cbh)
• Normalized corrected relative backscatter(backscatter)
• background_light

TWP/VCEIL/C1 - Reprocess: Incorrect serial_number and sum_backscatter fields

Attribute serial_number: In most cases, this field is not representative of the actual 
serial number of the instrument because the value did not change when an instrument changed 
locations or was replaced with a spare.
   
Variable sum_backscatter: An out-of-range condition for the "sum of detected and 
normalized backscatter" was reported as 0 instead of missing (-9999). The valid range is 0 - 
0.0999 /srad.
   
This DQR also applies to a1 level data.

• sum_backscatter

TWP/VCEIL/C2 - Reprocess: Incorrect serial_number and sum_backscatter fields

Attribute serial_number: In most cases, this field is not representative of the actual 
serial number of the instrument because the value did not change when an instrument changed 
locations or was replaced with a spare.
   
Variable sum_backscatter: An out-of-range condition for the "sum of detected and 
normalized backscatter" was reported as 0 instead of missing (-9999). The valid range is 0 - 
0.0999 /srad.
   
This DQR also applies to a1 level data.

• sum_backscatter

TWP/VCEIL/C3 - Reprocess: Incorrect serial_number and sum_backscatter fields

Attribute serial_number: In most cases, this field is not representative of the actual 
serial number of the instrument because the value did not change when an instrument changed 
locations or was replaced with a spare.
   
Variable sum_backscatter: An out-of-range condition for the "sum of detected and 
normalized backscatter" was reported as 0 instead of missing (-9999). The valid range is 0 - 
0.0999 /srad.
   
This DQR also applies to a1 level data.

• sum_backscatter