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Diode Laser Hygrometer (DLH) Data and Instrument Status

Diode Laser Hygrometer (DLH) Data and Instrument Status. Glenn Diskin, Tom Slate, Mario Rana NASA LaRC Jim Podolske NASA ARC ATTREX Science Team Meeting Boulder, Colorado October 23-25, 2013. Diode Laser Hygrometer (DLH) - description.

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Diode Laser Hygrometer (DLH) Data and Instrument Status

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  1. Diode Laser Hygrometer (DLH)Data and Instrument Status Glenn Diskin, Tom Slate, Mario Rana NASA LaRC Jim Podolske NASA ARC ATTREX Science Team Meeting Boulder, Colorado October 23-25, 2013

  2. Diode Laser Hygrometer (DLH) - description • Open-Path tunable diode laser spectrometer operating in the 1.4 µm spectral region • Uses commercial telecom fiber-coupled laser • Similar to previous versions of DLH, but built specifically for ATTREX, with new features • Wavelength modulation at ~2 kHz • Data analysis based on 2F, 4F demodulation, normalized by signal power • Line-locked to absorption line in low-pressure reference cell • Laser wavelength scanned periodically to determine laser-generated harmonic offsets • Uses one of three absorption lines, depending on conditions (primarily concentration), plus an altitude-dependent modulation • Double-pass external path configuration • “Mirror” is panel of retroreflecting roadsign material on wing-mounted fin • Sample volume is primarily outside of aircraft boundary layer • Internal optical path is pressurized and purged with dry air • No inlet effects, such as condensation, evaporation, hysteresis, etc. • Long path-length, combined with line-locked, multi-harmonic detection provideexcellent sensitivity and rapid time response • Normalization by return power allows measurements to be made within clouds • Cloud extinction can be assessed from return power signal (not done yet for ATTREX) • Real-time mixing ratio determination using onboard pressure, temperature • During ATTREX-2 (2013), internal correction applied to real-time data

  3. DLH Installation on Global Hawk External Path 12.22 m RT Internal Path 0.55 m 27 7 18U 65 1 16 14 12 46 24 61 3 Zone 25

  4. DLH Optical Housing - Internal Path Internal Path larger than on previous installations - Accommodates offset to Global Hawk lower fairing - Includes actuated folding mirror to allow for in-flight adjustment of beam path DLH Optical Housing Humidity Assessment - Internal Optical Path Measurement - Vaisala DMT152 New During ATTREX-2 / 2013 - Coupling to Global Hawk fairing rigid, leak-free - Optical Housing pressurized during flight * Purge/Refill cycling on demand * Reduced sensitivity to internal moisture - Mixing fans added to improve internal homogeneity and aid in moisture removal

  5. DLH Data Status – ATTREX-2 • R0 data in ATTREX archive for all six science flights, R1 processing underway • R1 data use improved modulation assessment based on data acquired in AIDA chamber during AquaVIT2 • Data from ATTREX-1 will be reprocessed as well • Data cover entire range of altitudes and concentrations encountered • Data coverage is generally complete, with short calibration events every 30 minutes used to determine laser-induced harmonic offsets • Instrument signal to noise was very good; signal to internal background was much improved from ATTREX-1, and effect of internal humidity is measured and largely corrected • For R0 and R1 data, direct subtraction is used; still see what looks like residual early in each flight • Will investigate correcting using 2F/4F sensitivity differences, as was done for MACPEX

  6. DLH Data Examples -- comparison with NOAA WV Science Flight 5 (2013/02/26) Science Flight 6 (2013/03/01) Thanks to NOAA WV group for pre-release copy of R1 data

  7. Performance Issues Found in ATTREX-2,and Plans for Mitigation • Effects of internal humidity are still seen, early in flights, although magnitude is much improved from ATTREX-1 • More aggressive pre-flight purging of Optical Housing • Including adding / repositioning stirring fans • Purging of Electronics / Laser Chassis • Utilization of in-fiber sensor to separate effects from the two volumes • Utilization of multiple harmonics to further reduce impact on signal • Expect moisture control to be even more important flying from Guam • Will need larger supply of UZ Air than in ATTREX-2 • Internal Path optical alignment degraded in flight, reducing signal to noise of that component • Redesigned mounting of breadboard to decouple from optical housing flexure • Insufficient time to implement redesign; will use simpler mounting strategy instead for ATTREX-3

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