The Passive A-band Wind Sounder (PAWS) for Measurement of Tropospheric Winds

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 e-mail address: brjohnso@ball.com

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

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. 400-800 km) with out suffer inverse square law loss in SNR • Unnecessary to scan a large aperture to retrieval vertical distribution of winds

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

5. Measurement Goals

6. flight direction Spacecraft position (view 2) 45° Forward FOV 45° Spacecraft position (view 1) Aft FOV ~2000 km PAWS Measurement Approach • Measure Doppler shift of well isolated O2 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 Two orthogonal views to resolve horizontal wind vectors from LOS winds

7. Oxygen A-Band Spectrum Oxygen A-Band Transmission for Vertical Trajectory Toward Zenith • 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 P-branch13000 cm-1 R-branch

8. z a) Scattering volume Solar flux b) c) observer h ground Limb Scattering Geometry Limb scattering of sunlight • 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 O2 along the incident and scattered direction • Both molecular scattering and aerosol scattering must be considered

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

10. telescope & collimator Fixed mirror 20 km 2a Tilted mirror altitude B FP M2 Michelson interferometer 0 km M1 L1 Detector array 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 Tilted mirror produces a spatial distribution of interference which is imaged onto 2-D detector

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

12. Technology Development Ground based testing • 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 Space Mission

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.