NSA/MPL/C1 - laser head failure

On 2002-04-28 the laser energy was observed to decrease throughout the day.  Although low 
clouds continued to be detected, by the end of the day the output laser energy was about 
10% of nomimal.  This decrease continued throughout 2002-04-29 to the extent that clouds 
were no longer detected.  The source of failure was eventually determined to be the laser 
head, requiring the MPL to be shipped for repair.  The MPL was reinstalled on 2002-08-28 
at about 01:00 UTC.  

MPL NSA data from 2002-04-29 through 2002-08-27 (inclusive) should not be used.

• transceiver_altitude_angle
• base_time
• instrument_temp
• range_bin_time
• range
• filter_temp
• max_altitude
• time
• property
• Preliminary cloud base height(preliminary_cbh)
• Altitude above mean sea level(alt)
• scalar_sync_offset
• laser_scalar_sync_offset
• voltage_10
• voltage_15
• range_bin_width
• range_bins
• Pressure, associated geometric height, at altitude, NGM model output(height)
• East longitude(lon)
• range_offset
• energy_monitor
• shots_sum
• Backscatter, ceilometer, range and sensitivity corrected(backscatter)
• shots_summed
• trigger_freq
• pulse_rep
• total_counts
• transceiver_azimuth_angle
• North latitude(lat)
• laser_temp
• time_offset
• background_signal
• detector_temp
• laser_sync_offset
• deadtime_correction
• voltage_05

NSA/MPL/C1 - Laser Q-Switch Problem

At about 22:52 UTC on 2004/01/19, the NSA MPL appears to have suffered a temporary failure 
of the laser Q-Switch.  This results in a complete loss of lidar data immediately 
thereafter.  The problem was resolved (at least for the time being) by disconnecting and 
reconnecting the Q-Switch cable on 2004/01/24 at about 00:16 UTC.  Data after this time appears 
good.

• transceiver_altitude_angle
• base_time
• instrument_temp
• range_bin_time
• range
• filter_temp
• max_altitude
• time
• property
• Preliminary cloud base height(preliminary_cbh)
• Altitude above mean sea level(alt)
• scalar_sync_offset
• laser_scalar_sync_offset
• voltage_10
• voltage_15
• range_bin_width
• range_bins
• Pressure, associated geometric height, at altitude, NGM model output(height)
• East longitude(lon)
• range_offset
• energy_monitor
• shots_sum
• Backscatter, ceilometer, range and sensitivity corrected(backscatter)
• shots_summed
• trigger_freq
• pulse_rep
• total_counts
• transceiver_azimuth_angle
• North latitude(lat)
• laser_temp
• time_offset
• background_signal
• detector_temp
• laser_sync_offset
• deadtime_correction
• voltage_05

NSA/MPL/C1 - Instrument problem

On 2/18, the energy monitor on the MPL dropped from 11 microjoules per second to 2 
microjoules per second.  This severely affected data quality.  This appears to be have been an 
intermittent cable problem for the laser Q-Switch. A new cable was provided and installed 
on 2-24.

• energy_monitor
• Backscatter, ceilometer, range and sensitivity corrected(backscatter)

NSA/MPL/C1 - Hardware Failure/Missing Data

Data are missing during this extended period due to a hardware failure.

• transceiver_altitude_angle
• base_time
• instrument_temp
• range_bin_time
• range
• filter_temp
• max_altitude
• time
• property
• Preliminary cloud base height(preliminary_cbh)
• Altitude above mean sea level(alt)
• scalar_sync_offset
• laser_scalar_sync_offset
• voltage_10
• voltage_15
• range_bin_width
• range_bins
• Pressure, associated geometric height, at altitude, NGM model output(height)
• East longitude(lon)
• range_offset
• energy_monitor
• shots_sum
• Backscatter, ceilometer, range and sensitivity corrected(backscatter)
• shots_summed
• trigger_freq
• pulse_rep
• total_counts
• transceiver_azimuth_angle
• North latitude(lat)
• laser_temp
• time_offset
• background_signal
• detector_temp
• laser_sync_offset
• deadtime_correction
• voltage_05

NSA/C1 - Site-wide power failure

The Barrow facility experienced a site-wide power failure on 12/13.  Power was restored to 
most instruments between 2250 and 2330 GMT.

• transceiver_altitude_angle
• base_time
• instrument_temp
• range_bin_time
• range
• filter_temp
• max_altitude
• time
• property
• Preliminary cloud base height(preliminary_cbh)
• Altitude above mean sea level(alt)
• scalar_sync_offset
• laser_scalar_sync_offset
• voltage_10
• voltage_15
• range_bin_width
• range_bins
• Pressure, associated geometric height, at altitude, NGM model output(height)
• East longitude(lon)
• range_offset
• energy_monitor
• shots_sum
• Backscatter, ceilometer, range and sensitivity corrected(backscatter)
• shots_summed
• trigger_freq
• pulse_rep
• total_counts
• transceiver_azimuth_angle
• North latitude(lat)
• laser_temp
• time_offset
• background_signal
• detector_temp
• laser_sync_offset
• deadtime_correction
• voltage_05

NSA/MPL/C1 - Frost on Viewport Window

During periods of extremely cold outside temperatures (-30 C or below), the MPL may suffer 
from reduced sensitivity due to obscuration of its viewport window with frost or 
condensation.  

During the winter of 1998-1999, on-site observers noted frost or condensation obscuring 
part or all of the viewport window.  A fan providing blowing heated air on the viewport had 
failed.  Although replacement of this fan reduced the degree of obscuration, the problem 
was not entirely eliminated.  During the summer of 1999 a more robust  forced-air system 
was installed and the problem was essentially solved except for during isolated cases of 
extremely cold weather.  Under these extreme conditions partial or complete obscuration 
of the window is still possible with the result that cloud detection sensitivity will be 
reduced depending on the fraction of the window surface obscured.  Even with significant 
obscuration, the MPL may still detect clouds and the cloud base heights of detected 
clouds will be reported correctly.  This problem is one of detection sensitivity only and does 
not affect the range scale in any way.

• Preliminary cloud base height(preliminary_cbh)
• background_signal
• Backscatter, ceilometer, range and sensitivity corrected(backscatter)

NSA/MPL/C1 - MPL clock drift

The MPL PC clock was noted to be off (slow) by more than an hour over a several day 
period.  It has previously been noted that the time synchronization program "D-4" fails to 
update the PC clock under certain circumstances.  As a result, the program "D-4" was replaced 
by "AboutTime" on 2/5/2004.  

The specifics of this problem are not well understood.  It turns out that the instrument 
has TWO clocks on the instrument computer display - one being displayed by the MPL 
software and the other being the PC clock displayed in the lower right-hand corner of the 
screen. The MPL software clock was the one that is noted on daily rounds and was never off by 
any amount. Apparently the only clock that was off was the computer clock.

What is clear is that between the time the MPL computer was installed on 11-11-2003 and 
the CM log on 1-06-2004 the MPL clock exhibited a discrepancy of over 1 hour and 20 
minutes.  What is not clear is whether this clock discrepancy was also applied to the data time 
stamps.  Furthermore, seven days later the MPL clock exhibited only a 1.77 second drift.  
Not exceptional at all, and not in line with the previous discrepancy of more than 4800 
seconds over a span of 57 days.

Attempts to identify gradual correctible clock drift in the MPL data were not successful.  
As a worst-case scenario, the MPL data may be inappropriately time-stamped by as much a 
1 hour 20 minutes and 41 seconds slow relative to UTC between the dates of 2003-11-11 and 
2004-01-06.  After this time until 2004-02-05 time synchronization was maintained within 
a few seconds a day with local observer assistance.

• base_time
• time_offset
• time

NSA/MPL/C1 - Diode failure

On 3/25, the energy monitor on the MPL dropped from 11 microjoules per second to 2 
microjoules per second.  We determined the problem was the diode; it reached the end of its 
lifetime. A replacement was sent.  Data quality returned to normal.

• energy_monitor
• Backscatter, ceilometer, range and sensitivity corrected(backscatter)

NSA/MPL/C1 - Intermittent Double Pulse from MPL laser

When Spectra Physics discontinued support for their laser diode power supply, an 
alternative was needed to extend the usefulness of the Spectra Physics laser head inside the MPLs. 
 The solution was to use a Coherent laser diode. Unfortunately, the Coherent diode 
lasers do not have the exact same characteristics as the Spectra Physics diode lasers. As a 
result, the output of the Spectra Physics laser head sometimes produces a second pulse 
close in time but lower in intensity from the first main pulse. This is known as “double pulsing”.

Not all of the Spectra Physics/Coherent combination systems produce a double pulse and the 
systems that do produce a double pulse do not always exhibit this behavior. It can be 
identified by a second peak at ~.2km. At first glance the signal in the data will resemble 
a low aerosol or cloud layer. When the double pulse is weak, the atmospheric signal will 
obscure the double pulse signal. At the ARM sites with a sunshade such as TWP and AMF, 
the double pulse can be seen when the sunshade closes during solar noon. When the sunshade 
closes, the MPL does not see atmospheric signal so the second pulse is evident. At SGP, a 
daily window cleaning is part of the routine. During this time the double pulse can be 
seen. Without a sunshade or window cleaning it is impossible to say definitively if double 
pulse is present. 

The first system to produce a double pulse was MPL s/n 008 located at TWP – C2 Nauru in 
February 2005. The second system was MPL s/n 004 located at SGP in August 2005. The 
potential for the other systems to produce a double pulse is high. However, as of March 2006, 
the MPLs at NSA, TWP – C1, TWP – C3 and AMF have not recorded data with the double pulse 
artifact. 

The ARM program is in the process of replacing all the MPLs with a newer model. After the 
new MPLs are deployed the double pulse problem should not be an issue. The old systems 
will be relegated as spares. The double pulse problem will return if an old system is 
needed to replace a failed new system.

• detector_counts