TWP/MPL/C1 - Intermittent Double Pulse from laser

When Spectra Physics discontinued support for their laser diode power supply, an 
alternative was need 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.

• Attenuated backscatter(detector_counts)

TWP/MPL/C1 - Scope Temp is incorrect

Scope temperature is incorrect because the probe failed/was missing.

Ignore the scope temperatures and assume the true values are acceptable.

• Telescope temperature(scope_temp)

• Telescope temperature(scope_temp)

• Telescope temperature(scope_temp)

TWP/MPL/C1 - Energy monitor registers 0

When the instrument was returned to service following replacement of the scope temperature 
sensor, the power supply was not turned on; hence system was not operating, though the 
software was.

• Preliminary cloud base height(preliminary_cbh)
• Time offset of tweaks from base_time(time_offset)
• Polarization control enabled flag(polarization_control)
• Telescope temperature(scope_temp)
• Repetition Rate, or Trigger Frequency of the laser(pulse_rep)
• Distance from leading edge of first range bin to the center of each bin .(range_bins)
• base time(base_time)
• Array of heights for the range gates(height)
• lon(lon)
• Time in nanoseconds for each range bin of multichannel scalar card.(range_bin_time)
• Laser Temperature(laser_temp)
• Maximum altitude retrieved from multichannel scalar card.(max_altitude)
• range(range)
• Width of range-bins calculated from range_bin_time(range_bin_width)
• Attenuated backscatter(signal_return)
• Dummy altitude for Zeb(alt)
• time(time)
• Background Signal(background_signal)
• Polarization control voltage setting(polarization_control_voltage)
• Number of bins stored in data block(num_bins)
• effective range offset due to poor sync between laser firing and A/D trigger.(range_offset)
• Number of detector channels(num_channels)
• Background signal standard deviation(background_signal_std)
• Laser output energy per pulse(energy_monitor)
• Detector Temperature(detector_temp)
• lat(lat)
• Dead time correction flag(dead_time_corrected)
• Number of laser pulses summed during averaging interval(shots_per_avg)

• Laser output energy per pulse(energy_monitor)
• Attenuated backscatter, cross-pol(signal_return_cross_pol)
• Number of laser pulses summed during averaging interval(shots_per_sample)
• range(range)
• Number of detector channels(num_channels)
• Attenuated backscatter, co-pol(signal_return_co_pol)
• Maximum altitude retrieved from multichannel scalar card.(max_altitude)
• Dead time correction flag(dead_time_corrected)
• Preliminary cloud base height above ground level, co-pol(preliminary_cbh_co_pol)
• base time(base_time)
• lon(lon)
• Detector Temperature(detector_temp)
• Background signal standard deviation, co-pol(background_signal_std_co_pol)
• Array of heights for the range gates(height)
• *(aqc_signal_return_co_pol)
• Background signal standard deviation, cross-pol(background_signal_std_cross_pol)
• effective range offset due to poor sync between laser firing and A/D trigger.(range_offset)
• Number of samples used for cross-pol averages(nsamples_cross_pol_average)
• Background signal, cross-pol(background_signal_cross_pol)
• Time offset of tweaks from base_time(time_offset)
• Background signal, co-pol(background_signal_co_pol)
• Repetition Rate, or Trigger Frequency of the laser(pulse_rep)
• *(aqc_signal_return_cross_pol)
• Telescope temperature(scope_temp)
• Number of samples used for co-pol averages(nsamples_co_pol_average)
• Laser Temperature(laser_temp)
• Polarization control voltage, co-pol(polarization_control_voltage_co_pol)
• Preliminary cloud base height above ground level, cross-pol(preliminary_cbh_cross_pol)
• time(time)
• Number of bins stored in data block(num_bins)
• lat(lat)
• Dummy altitude for Zeb(alt)
• Distance from leading edge of first range bin to the center of each bin .(range_bins)
• Width of range-bins calculated from range_bin_time(range_bin_width)
• Polarization control voltage, cross-pol(polarization_control_voltage_cross_pol)

• Attenuated backscatter, co-pol(signal_return_co_pol)
• Attenuated backscatter, cross-pol(signal_return_cross_pol)
• Maximum altitude retrieved from multichannel scalar card.(max_altitude)
• base time(base_time)
• Width of range-bins calculated from range_bin_time(range_bin_width)
• Number of detector channels(num_channels)
• Laser Temperature(laser_temp)
• lon(lon)
• *(aqc_signal_return_cross_pol)
• Background signal, cross-pol(background_signal_cross_pol)
• effective range offset due to poor sync between laser firing and A/D trigger.(range_offset)
• Distance from leading edge of first range bin to the center of each bin .(range_bins)
• Preliminary cloud base height above ground level, cross-pol(preliminary_cbh_cross_pol)
• Number of laser pulses summed during averaging interval(shots_per_sample)
• Array of heights for the range gates(height)
• Background signal standard deviation, cross-pol(background_signal_std_cross_pol)
• lat(lat)
• Detector Temperature(detector_temp)
• Dead time correction flag(dead_time_corrected)
• Polarization control voltage, co-pol(polarization_control_voltage_co_pol)
• Background signal standard deviation, co-pol(background_signal_std_co_pol)
• Number of samples used for co-pol averages(nsamples_co_pol_average)
• Time offset of tweaks from base_time(time_offset)
• Polarization control voltage, cross-pol(polarization_control_voltage_cross_pol)
• Preliminary cloud base height above ground level, co-pol(preliminary_cbh_co_pol)
• Dummy altitude for Zeb(alt)
• Background signal, co-pol(background_signal_co_pol)
• Number of bins stored in data block(num_bins)
• *(aqc_signal_return_co_pol)
• Laser output energy per pulse(energy_monitor)
• Number of samples used for cross-pol averages(nsamples_cross_pol_average)
• Repetition Rate, or Trigger Frequency of the laser(pulse_rep)
• time(time)
• Telescope temperature(scope_temp)
• range(range)