Orbiting Carbon Observatory (OCO)

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Orbiting Carbon Observatory (OCO)
Vijay Natraj
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Mission Overview

• CO2 is the principal atmospheric component of the
  global carbon cycle
• Processes governing carbon cycle not well
  understood
• OCO will measure column-averaged CO2 vmr (XCO2)
  with more complete global coverage to better
  understand geographic distribution of the sources
  and sinks

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Measurement Strategy

• High resolution spectra of reflected sunlight
  taken simultaneously in NIR CO2 (1.58 µm and 2.06
  µm) and O2 (0.76 µm) bands
• 3 grating spectrometers
• Data acquired in nadir, glint, target modes

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Retrieval Algorithm

• Radiative Transfer Model
• Instrument Response Model
• Inverse Method

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Radiative Transfer Model

• Spectrum-resolving, multi-stream,
  multiple-scattering model for scattering,
  absorbing atmospheres
• BDRF at the lower boundary to characterise
  surface reflectance
• Together with ILS, called the forward model

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Inverse Method

• Based on optimal estimation theory (Rodgers,
  2000)
• Minimises cost function involving computed and a
  priori states, measured and computed spectra, a
  priori and measurement error covariances
• Simultaneously retrieves several properties of
  surface and atmospheric state, such as XCO2,
  temperature, humidity, aerosol, surface pressure,
  albedo, etc.

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Retrieval of Oxygen A-Band Spectra from Airborne
Measurements

• High-precision, high-resolution O2 A-band spectra
  of sunlight reflected from the sea surface
  (OBrien et al., 1996)
• Sample spectrum retrieved to within an rms error
  of 1.5
• Wavelength scaled to match the calculated
  spectrum
• Continuum level, tilt and zero level offset fitted

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Retrieval of Oxygen A-Band Spectra (contd)

• Objective is to retrieve column O2 with
  precisions between 0.1-0.3 from this data set
• Next logical step in demonstrating the
  feasibility of retrieving XCO2 from space with
  precision near 0.3