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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 Corp1600 Commerce Street, Boulder, CO, USA
e-mail address: [email protected]
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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
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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
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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
Vertical Coverage 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 telescopeAngle/gap scan
OPD scan mirror (WINDII) or tilted mirror
Accuracy ~ 5 to 10 m/s ~ 5 to 10 m/s ~ 5 to 10 m/s (goal)
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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
Horizo ntal resolution 250 km
Horizontal sampling 500 km
Wind speed uncertainty ± 5 m/s
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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
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
Two orthogonal views to resolve horizontal wind vectors from LOS winds
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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
R-branch
P-branch13000 cm-1
Oxygen A-Band Transmission for Vertical Trajectory Toward Zenith
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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 O2 along the incident and scattered direction
Both molecular scattering and aerosol scattering must be considered
a)
b)observer
c)
ground
Solar fluxScattering volume
z
h
Limb scattering of sunlight
0.00
0.01
0.02
0.03
0.04
0.05
13017 13019 13021 13023 13025 13027
Wavenumber (cm -1)
Normalized Radiance
0km
2km
4km
6km
8km
10km
12km
15km
20km
25km
30km
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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
0
5
10
15
20
25
30
35
40
45
50
0.0 0.2 0.4 0.6
Vertical Weighting Function, δvlos/δvz
( )Geometric Tangent height km
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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
B FP
L1
Michelson interferometer
Fixed mirror
telescope & collimator
M1M2
0 km
20 km
altit
ude
Tilted mirror produces a spatial distribution of
interference which is imaged onto 2-D detector
Tilted mirror produces a spatial distribution of
interference which is imaged onto 2-D detector
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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
0 1 2 3 4 5-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1Interferogram
OPD (cm)
Magnitude
Interferogram for absorption line
]2cos[),(1[)()()( φδπσσσσσ
+⋅+∫ ⋅=Δ oxzVUdLfxI
Interferogram
)1( / cvo +=σσ
oo xcv )/(2 σπφ =
769 769.1 769.2 769.3 769.4 769.50
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1Limb Radiance
Wavelength (nm)
Intensity
optical filter
Absorption line
Spectral shift
Phase shift
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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
completeAirborne Demonstration of winds
Airborne Demonstration
Ground based testing
Space Mission
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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.
