Page 1 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 address:

Page 2 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 3 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 km) with out suffer inverse square law loss in SNR ─ Unnecessary to scan a large aperture to retrieval vertical distribution of winds

Page 4 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 WINDIIHRDIPAWS Vertical Coverage80 – 300 km10 – 115 km0 – 20 km Vertical Interval2 km2.5 km1 km Horiz. Cell Size140 km500 km250 km Spectral SignalEmissionAbsorption Target SpeciesO and OHO 2 B and γ BandsO 2 A-Band SpectrometerImaging MichelsonTriple Fabry-PerotImaging Michelson Meas. ApproachLarge OPD, scan across one period Gimbal telescope Angle/gap scan OPD scan mirror (WINDII) or tilted mirror Accuracy~ 5 to 10 m/s ~ 5 to 10 m/s (goal)

Page 5 Measurement Goals

Page 6 PAWS Measurement Approach  Measure Doppler shift of well isolated O 2 absorption line with a Michelson interferometer  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 flight direction 45° Aft FOV Spacecraft position (view 1) ~2000 km Spacecraft position (view 2) 45° Two orthogonal views to resolve horizontal wind vectors from LOS winds

Page 7 Oxygen A-Band Spectrum  Hays (1982) suggested using molecular oxygen for measuring winds  O 2 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 R-branch P-branch cm -1 Oxygen A-Band Transmission for Vertical Trajectory Toward Zenith

Page 8 Limb Scattering Geometry  Single-scattering RT model is adequate to simulate the Doppler 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 O 2 along the incident and scattered direction  Both molecular scattering and aerosol scattering must be considered a) b) observer c) ground Solar flux Scattering volume z h Limb scattering of sunlight

Page 9 Vertical Weighting Functions  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

Page 10 Doppler Michelson Interferometer  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 Atmospheric Column Detector array Tilted mirror 22 BFP L1 Michelson interferometer Fixed mirror telescope & collimator M1 M2 0 km 20 km altitude Tilted mirror produces a spatial distribution of interference which is imaged onto 2-D detector

Page 11 Interferogram  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 Interferogram for absorption line Interferogram Spectral shift Phase shift

Page 12 Technology Development  Objective: Demonstrate an instrument concept for passive measurement of troposphere wind profiles from low- earth 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  Airborne Demonstration of winds Airborne Demonstration Ground based testing Space Mission

Page 13 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.