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The Passive A-band Wind Sounder (PAWS) for Measurement of Tropospheric Winds Brian R. Johnson (CO- I), Shane Roark (PI), Pei Huang, Grzegorz Miecznik, Ron Schwiesow and Phil Slaymaker Ball Aerospace & Technologies Corp 1600 Commerce Street, Boulder, CO, USA e-mail address: [email protected] Page 1 Introduction PAWS is a passive optical technique for measuring winds in the troposphere and lower stratosphere (~0 to 20km) Interferometer concept based on WINDII approach ─ Doppler Michelson Interferometer (DMI) measurement of upper atmospheric winds Extending the DMI technique to measuring of tropospheric winds is challenging ─ Observing absorption feature in presence of large background flux reduces sensitivity of interferogram to wind signal (higher SNR is required) ─ Pressure dependence of line shape and position ─ Aerosols, clouds and gradients in horizontal winds further limit sensitivity in lowest altitudes near surface Page 2 PAWS measurement objectives Applications of PAWS winds measurements: ─ mid and upper tropospheric chemical transport studies ─ UT/LS exchange studies ─ Augment current wind measurements Advantages of an passive optical technique for winds: ─ Compact, less complex instrument than active system ─ Augment DWL coverage but perhaps with reduced precision and accuracy ─ Accommodates a range of spacecraft altitudes (e.g. 400-800 km) with out suffer inverse square law loss in SNR ─ Unnecessary to scan a large aperture to retrieval vertical distribution of winds Page 3 Heritage for Space-Based Passive Wind Measurements Upper Atmosphere Research Satellite (UARS) Wind Imaging Interferometer (WINDII) ― September 1991 to December 2005 High-Resolution Doppler Imager (HRDI) ― September 1991 to April 1995 WINDII HRDI PAWS 80 – 300 km 10 – 115 km 0 – 20 km Vertical Interval 2 km 2.5 km 1 km Horiz. Cell Size 140 km 500 km 250 km Spectral Signal Emission Absorption Absorption Target Species O and OH O2 B and γ Bands O2 A-Band Spectrometer Imaging Michelson Triple Fabry-Perot Imaging Michelson Meas. Approach Large OPD, scan across one period Gimbal telescope Angle/gap scan OPD scan mirror (WINDII) or tilted mirror ~ 5 to 10 m/s ~ 5 to 10 m/s ~ 5 to 10 m/s (goal) Vertical Coverage Accuracy Page 4 Measurement Goals Table 1. Tropospheric wind measurement goals. Measurement Characteristics Value Nominal spacecraft altitude 800 km Vertical sounding 0 – 20 km Vertical resolution 2 km Vertical sampling 1 km Horizontal resolution 250 km Horizontal sampling 500 km Wind speed uncertainty ± 5 m/s Page 5 PAWS Measurement Approach flight direction Measure Doppler shift of well isolated O2 absorption line with a Michelson interferometer Spacecraft position (view 2) 45° Vertical distribution obtained by imaging limb over a range of altitudes from surface to ~20km Limb view enables high (~1 km) vertical resolution However, resolving horizontal variations in winds on scales smaller than ~ 250km is difficult Forward FOV Spacecraft position (view 1) 45° Aft FOV Two orthogonal views to resolve horizontal wind vectors from LOS winds Page 6 Oxygen A-Band Spectrum Hays (1982) suggested using molecular oxygen for measuring winds O2 is uniformity mixed Lines in a clear region of the atmospheric spectrum Lines are sharp and well resolved Wide range of line strength is available to optimize SNR A-band wavelength region is compatible with technology for high spectral resolution Oxygen A-Band Transmission for Vertical Trajectory Toward Zenith P-branch 13000 cm-1 R-branch Page 7 Limb Scattering Geometry Limb scattering of sunlight Single-scattering RT model is adequate to simulate the Doppler z a) Scattering volume Solar flux b) c) observer h ground 0.05 Normalized Radiance 0.04 0.03 0km 2km 4km 6km 0.02 8km 10km 12km 15km 0.01 perturbations in the observed limb spectrum (Hays and Abreu, 1989) Light scattered by the atmosphere comes directly from incident sunlight or sunlight reflected from the ground Sunlight is absorbed by O2 along the incident and scattered direction Both molecular scattering and aerosol scattering must be considered 20km 25km 30km 0.00 13017 13019 13021 13023 13025 13027 Wavenumber (cm -1) Page 8 Vertical Weighting Functions 50 45 Geometric Tangent height (km) LOS wind determined for each vertical pixel represents a weighted average wind along the limb path The vertical distribution of LOS winds must then be recovered by accounting for the path weighted values An optimal estimation approach is being considered for recover vertical winds Ortland et al. have used sequential estimation for deriving HRDI winds 40 35 30 25 20 15 10 5 0 0.0 0.2 0.4 0.6 Vertical Weighting Function, dvlos/dvz Page 9 Doppler Michelson Interferometer Atmospheric Column 20 km telescope & collimator Fixed mirror 2a Tilted mirror altitude Light is collected by an optical telescope (M1), collimated (M2) and passed through a nearly fixed path Michelson interferometer. A narrow filter (B) combined with a Fabry Perot etalon (FP) are used to isolate a single absorption line. A small tilt in one of the interferometer mirrors produces a spatial distribution of interference The interference pattern for each altitude position along the atmospheric column is simultaneously imaged onto a 2-D detector array by a cylindrical lens M2 0 km M1 B FP Michelson interferometer L1 Detector array Tilted mirror produces a spatial distribution of interference which is imaged onto 2-D detector Page 10 Interferogram Interferogram 1 0.8 Interferogram for absorption line 0.6 Magnitude 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0 1 2 3 4 5 Small spectral shift can be measured using a Michelson interferometer by examining the phase shift in the nearly sinusoidal interferogram signal Only a small portion of the interferogram is recorded A large OPD improves sensitivity to phase Absorption line significantly reduces fringe contrast as compared with emission line High SNR required to resolve small shifts for low fringe visibility Limb Radiance OPD (cm) 1 Interferogram 0.9 0.8 optical filter 0.7 0.6 Intensity I ( x) f ( ) L( )d [1 U V ( z, x) cos[2 d ] o 0.5 Absorption line 0.4 o (1 v / c) 2 o (v / c) xo Spectral shift 0.3 0.2 0.1 Phase shift 0 769 769.1 769.2 769.3 Wavelength (nm) 769.4 769.5 Page 11 Technology Development Objective: Demonstrate an instrument concept for passive measurement of troposphere wind profiles from lowearth orbit Interferometer being developed under the NASA IIP Progress Breadboard built May 07: Atmospheric test complete Nov 07: Engineering model design complete May 08: Engineering model construction complete ─ Nov 08: Engineering model demonstration complete ─ ─ ─ ─ Ground based testing Airborne Demonstration Space Mission Airborne Demonstration of winds Page 12 Summary PAWS is a Doppler Michelson interferometer technique being developed to measure winds in the troposphere and lower stratosphere PAWS will provide wind data to address: ─ mid and upper tropospheric chemical transport studies including UT/LS exchange ─ Augment current wind measurements over data sparse regions (e.g. over oceans and southern hemisphere) Interferometer technology being developed under NASA IIP A ground-based demonstrate of measurement technique performed later this year Airborne demonstration in late 2008/early 2009. 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