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Laser Sounder for Global Measurements of CO2
Concentrations in the Troposphere from Space
Presentation to 14th Coherent Laser Radar
Conference Snowmass Village, CO July 10,
2007 James B. Abshire, Haris Riris, Graham
Allan, Xiaoli Sun, Michael A. Krainak, Stephan
R. Kawa, Jian-Ping Mao, Mark Stephen, John F.
Burris NASA Goddard Space Flight Center -
Sigma Research, - RSI Inc. Support
from NASA ESTO IIP and GSFC IRAD
programs Contact james.abshire_at_gsfc.nasa.gov
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Outline

• Why measure CO2 from space with lasers ?
• Measurement Approach - considerations
• Measurement Approach from space
• Breadboard CO2 sensor measurements
• In lab, across horizontal path
• Oxygen channel - status
• Scaling to space - notional mission
• Approach for flux retrievals mission
  requirements
• Summary

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Atmospheric CO2 - Cycle, Sources, Sinks
Of anthropogenic CO2 emitted to date, 30
can not be accounted for - the unknown sink
Unknown sink may be Northern Hemisphere
forests. How are the sinks spatially
distributed ? What are their dynamics ?
Will natural sinks operate in the future? How
will CO2 fluxes in Arctic respond to warming ?
People in Industrialized Countries Burning Fossil
Fuels
?
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Passive Spectrometer CO2 Missions Benefits
Opportunities for follow-on Missions
Modeled CO2 Aug 1, 1999
Measurement Approach Measures entire
absorption band Optical path varies during
orbit Sun illumination ? observation path
Benefits Global column measurements
Orders of magnitude higher density coverage
than ground networks ! No lasers !
NOAA CMDL Sample Sites ()
Measure CO2 absorption in both paths Cloud
Aerosol scattering are prevalent Atmospheric
Scattering causes bias errors

• Limitations
• Sunlit areas only
• High latitudes only in local summers
• Glint areas over oceans are limited
• Scattering of sunlight from aerosols thin
  clouds cause biases
• Different illumination observation paths
  complicate retrievals
• Difficult to achieve surface weighting, to
  enhance sensitivity

1 day Tracks shown Nadir view SZA lt 65º
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Laser Sounder for Active CO2 Mission

• Why this approach ?
• Measures at night at all times of day
• Continuous glint measurements over oceans
• Measures at high latitudes
• Illumination path observation path
• Smaller measurement footprint
• Measures through broken clouds
• Can use overlapped footprints to mitigate errors
• from changes in topographic reflectivity
• Can make laser altimetry height aerosol
  profiles
• Measurements to tops of thick clouds
• Can weight column to lower troposphere
• Can use time gating to isolate the full column
  signal

Ascends Mission from US NRC Decadal Survey
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Laser Sounder Approach for Active CO2 Mission
3 simultaneous laser measurements 1. CO2 lower
tropospheric column One line near 1572 nm 2. O2
total column Measured between 2 lines near 765
nm 3. Altimetry atmospheric backscatter
profile Surface height and atmospheric
scattering profile at 1064 nm Pulsed EDFA
lasers KHZ pulse rates Photon counting receiver
100 km along track averaging Target 1ppmV

• CO2 O2 column measurements
• Pulsed (time gated) signals
• Isolate surface echoes
• Reduces noise from detector solar background

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Active CO2 MissionConsiderations
1. Biology carbon cycle - measurement needs
(orbits, time of day coverage, spatial scale,
etc) 2. Atmospheric gas transport (x,y,z) 3.
Earths laser measurement environment 4.
Spectroscopy of CO2 O2 5. Diode fiber laser
technology 6. Optical receiver detector
components 7. Error sources in laser
spectrometers 8. Space lidar error analysis 9.
Space lidar mission design
Are pursuing a spiral development approach
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CO2 Measurement Considerations

• Accurate estimates of N depend on
• ?? - line cross section
• z effective path length
• Pref Transmitted laser power (l)
• Tsys System transmission (l)
• Prec (high SNRs)
• Some error sources
• ?? - temp effects in line cross section
• z, from atmospheric scattering
• System changes - small l?dependences in
• Pref, Tsys ?( loff)/?(lon)
• Noise (signal background shot noise, detector
  noise) in detected echo signal
• Goal
• Minimize all error sources
• Maximize received SNR

General form of DIAL equation for uniform
horizontal path
Estimated CO2 number density
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Using laser tuned to sides of Absorption line
provides Column Measurement weighted to lower
trop., via CO2 Line Broadening
CO2 Band Line Measurement Approach
1570 nm CO2 Absorption Band from Space
(HITRAN) Extinction of lines vary with of CO2
molecules in column
1
6
Selected CO2 Line
2
5
Sounder measures energy (signal photon counts) at
6 ?s on a single line
Column density from line extinctions E2/E1,
and E5/E6 (ideally equal)
3
4
Column Altitude Weighting Function (pts 2 5)

• Lasers Provide
• Narrow measurement line width (MHz)
• Very stable frequencies (MHz)

• Energy Measurement Resolution
• Need 10001 SNR for online energies (E2, E5)
  
• With similar errors for O2 gas measurement,
  results in 1 ppm error in CO2 mixing ratio

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CO2 Line Selection - temperature sensitivity

• CO2 cross section (?) is sensitive to atmospheric
  temperature
• 1572.35 nm line is relatively temperature
  insensitive in lower troposphere.
• Temperature profile estimates will still be
  required
• Use atmospheric models

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Geoscience Laser Altimeter System (GLAS) on the
ICESat Mission
gt1.4 Billion laser measurements of Earths
surface atmosphere in 12 campaigns (so far)
ICESat/ GLAS measured shaded relief map
ICESat/GLAS measured shaded relief map
Atmospheric Backscatter Profiles (one orbit
sample) measured by the GLAS 532 nm photon
counting receiver (J. Spinhirne)
Profiles across surface above Lake Vostok show lt
2.4 cm height resolution
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An Atmospheric Profile measured by GLAS on ICESat
(an example from campaign L2a)

• Aspects which have guided this measurement
  approach
• Scattering structure is complex
• Most nadir-zenith paths have some scattering
  above surface
• Many instances of thin clouds aerosols
• Some thick clouds
• Target Depth with clouds 15 km gt 133 usec in
  travel time

Atmospheric Backscatter Profiles (one orbit
sample) measured by the GLAS 532 nm photon
counting receiver (J. Spinhirne)
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Space Measurement Approach for CO2(similar for
O2)
Monopulse Approach
Time Gated photon Counting Receiver (all channels)
Along track motion
Spot on surface (200 m diameter)
Overlapped spots in FF
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Laser transmitter approach Diode Seed Lasers
High peak power EDFA
Tunable diode seed lasers
Pump diodes
An example of EDFA technology (Lucent)

• Characteristics
• Closed laser cavity - no contamination risk
• Fiber architecture - no misalignment risk
• Leverages large investments from industry
• Components built to Telcordia standards
• Diode pump technology is very reliable (undersea
  fiber optic repeaters)
• Distributed thermal load
• Electrical efficiencies gt 10
• Ongoing work for use on satellites
• Wide availability of highly engineered parts
• Wavelength flexibility

Fiber laser diagram (Cross sections shown for
display purposes. The fiber laser has a
monolithic structure.)
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Breadboard Sensor for Open Path CO2
MeasurementsTest Range (206 405 m one-way)
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Breadboard Sensor Stability - measurements using
a 30 cm absorption cell show 0.05 absorption
stability for hours
Must maintain stability between calibrations.
For space , many hours between calibrations to
ground sensors Changing etalon fringes can
cause drifts in Laser spectrometers Goal keep
breadboard drifts to lt 10-3 expected
absorption. For 50 absorption gt drift errors
lt 0.05
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CO2 Breadboard Instrument Measurements
Laboratory breadboard, range and targets
Laboratory Breadboard
LICOR
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CO2 measured over 206 405 m open path with
breadboard instrument in-situ sensor
1 absorption change

• In-situ samples
• Single-point measurements (Licor) from
  air intake on B33 rooftop

Sunrise 545 am
Sunset 837 pm
24 hour history
25 hour history

• Earlier Summer Measurements over 206 m path
• (vegetation active)
• Breadboard measurements offset and scaled
• Show diurnal change in Co2 near surface
• Agreement to 1 500 in absorption over 1st 16
  hrs
• Close to performance needed for space mission
• Improvements later improved reproducibility

8 day time history
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CO2 Sounder Receiver - Photon Counting Detectors

• Now
• Hamamatsu H9170-75 PMT
• Detector used in breadboard receiver

• Turn-key operation
• QE 2-3 at 1570nm
• InP/InGaAs photocathode
• Photocathode 5 mm diameter
• Dark count rate 200 KHz at -80 C (TEC cooled)
• PMT power consumption 150mW

TO-8 PMT package with transmissive photocathode
HV supply and PMT housing
Under Development Adapting HgCdTe Photon
Counting Detectors Candidate (from new Hubble WF
cameras)
80 photon detection/accumulation efficiency at
1550 nm with 0.04 dark counts/sec at 150K
HgCdTe (Example shown optimized for 1.6 um)
Present read-out configured for photon
accumulation
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Scan of CO2 line in outdoor 400m path using PMT
detector
Co2 Absorption line measured over path to a
non-cooperative target (Tree Trunk) using a
cooled PMT detector
Measurement conditions Diffuse target
Tree trunk (or trees) Amplifier output power
400 mW Averaging time 60 sec Number of
Scans 30K scans Scanning frequency
500 Hz (2 msec/scan) Receiver
attenuation PMT iris mostly closed Solar
background varied with time Background
dark rate l00-200 KHz
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Field Measurement Plans

• CO2 tower - Wisconsin (Park Falls)
• Co2 measured at 6 different altitudes from
  tower.
• Ground based FTS for collaborative measurements.

• Co2 Tower - Colorado (Erie) - BAO Tower
• Co2 measured at 3 separate altitudes to 500
  meters.
• Many km line of site for path measurements to
  tower.
• Elevator allows measurements to different end
  heights.

WLEF tower- Park Falls, Wisconsin
2007 deployments in lidar van
2008 airborne experiments on aircraft (such as
NASA WFF P3 or Twin Otter) Initially at variable
altitudes, near CO2 tower site
jfb 8.2
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Oxygen (Reference) Channel Laser Transmitter
Oxygen A band Calculated atmospheric
transmission for 100 m path at STP
Some lab measurements of an O2 line Wavelength
(50 pm/div) At 763.3 nm center wavelength

• Key components
• Distributed Feed Back Diode seed laser
• EDFA fiber Amplifier (1530 - 1540 nm)
• Frequency doubling crystal - Periodically-Poled
  (KTP)

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Oxygen A-band Transmission Spectrum from
SpaceTemperature Sensitivity to 1K in PBL
Selected region
off
on

• 764.7 nm region has
• Minimal temperature sensitivity,
• Medium absorption,
• Separation for off- on-line wavelengths

• 764.7 nm (Point 5)
• Trough between two strong lines
• Insensitive to temperature wavelength shift
• Has about 50 absorption

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Moving to SpacePreliminary Space Mission Study
showed space mission concept practical (to be
updated)
Taurus 92 Shroud
Delta 2320-10 M Shroud
To be updated in 2007 with improved estimates for
measurement , orbit, and components
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Notional Mission for SpaceCO2 Measurement
Approaches SNR calculations
Monopulse 8 KHz pulse rate 1 usec pulse width
0.36 Duty Cycle per wavelength Total 3KW peak
power 20W average optical power 200W to
transmitter With 8 EDFAs Each EDFA
350-400W peak Lowest Ave Power Most efficient
Calculations for assumed 1 m diameter telescope
15 Wm2 lidar constant
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Power Scaling and Frequency Doubling via Large
Mode Area Fiber Amplifier

• Aculight demonstrated EDFA fiber amplifier
  using 25 um mode diameter fiber
• Measurements show
• 360 watts peak power at 1540 nm with no SBS
• Beam quality M2 lt 1.1
• Frequency doubling to 770 nm with 56 efficiency

(Aculight M. Stephen/GSFC)
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Plans - Science Simulations (CO2 OSSEs)

• Purpose fly simulated instrument through
  simulated atmosphere
• How well does it recover CO2 sources sinks ?
• Test sensitivity on recovered CO2 distributions
• Vary mission and instrument parameters
• What are best orbits, times of days, etc ?
• Determine essential measurement aspects gt key
  instrument specs.

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Summary

• A needed challenging space measurement
• Complex measurement environment
• Many potential error sources
• Atmospheric transport
• A great opportunity for our community !
• Our approach
• Pulsed direct detection, EDFA lasers
• Photon counting receiver, gated
• CO2 lower tropospheric column
• O2 total column (surface pressure)
• Altimetry atmos. backscatter profile

• Next steps
• Measurements at CO2 tower site (2007)
• 2nd space mission design study (2007)
• Simulation studies (OSSEs) (2007 2008)
• Airborne demonstrations (2008)