TWP/MPL/C3 - Missing data due to instrument removal

The instrument was removed from this site to replace a failed instrument at TWP-C2 Nauru.

• null(Raw data stream - documentation not supported)

• Array of heights for the range gates(height)
• lon(lon)
• Time offset of tweaks from base_time(time_offset)
• Voltage level which operates the A/D card and the detector(voltage_05)
• Repetition Rate, or Trigger Frequency of the laser(pulse_rep)
• Background Signal(background_signal)
• time(time)
• Voltage level which operates the energy monitor(voltage_15)
• Distance from leading edge of first range bin to the center of each bin .(range_bins)
• range(range)
• Instrument Temperature(instrument_temp)
• Filter Temperature(filter_temp)
• effective range offset due to poor sync between laser firing and A/D trigger.(range_offset)
• Assumed zero-range bin number\(assumed_zero_range_bin)
• Voltage level which operates the thermistor(voltage_10)
• Width of range-bins calculated from range_bin_time(range_bin_width)
• Dummy altitude for Zeb(alt)
• Time in nanoseconds for each range bin of multichannel scalar card.(range_bin_time)
• Laser output energy per pulse(energy_monitor)
• Detector Temperature(detector_temp)
• base time(base_time)
• Number of laser shots recorded(shots_summed)
• Preliminary cloud base height(preliminary_cbh)
• Maximum altitude retrieved from multichannel scalar card.(max_altitude)
• Attenuated backscatter(detector_counts)
• Laser Temperature(laser_temp)
• lat(lat)

TWP/MPL/C3 - 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)