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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
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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
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
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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
Horizontal resolution
250 km
Horizontal sampling
500 km
Wind speed uncertainty
± 5 m/s
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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
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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
Oxygen A-Band Transmission for Vertical Trajectory Toward Zenith
P-branch
13000 cm-1
R-branch
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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)
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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
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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
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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
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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
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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.
Page 13