CO2 Observation from Space from Two Complementary Spectrometers; Oco-2 and Gosat
Abstract:Accurate instrument calibration and retrieval algorithms are essential for estimating atmospheric CO2 or CH4 from reflected sunlight. Since the GOSAT launch in 2009, retrieval accuracy has been improved gradually mainly by: (1) Radiometric degradation correction, (2) Non-linearity correction in spectral radiance, (3) Aerosol scattering treatment, (4) CO2 and O2 line parameters by laboratory spectroscopy, and (5) simultaneous retrieval of chlorophyll fluorescence. Other small systematic biases have not yet been identified.
OCO-2 uses grating spectrometers while GOSAT uses a Fourier transform spectrometer to record spectra near 0.76, 1.6 and 2.0 μm with different observing strategies, integration times, and spatial and spectral resolutions. After the successful launch of OCO-2, measurements from the two independent instruments can be compared to distinguish common forward calculation errors such as line parameters, aerosol scattering, and ocean glint reflection from instrument specific errors. Multiplexed and large throughput signal from the FTS requires a larger dynamic range and has higher possibility of non-linearity. Grating spectrometers have more complicated instrument line shape functions and pixel-to-pixel non-uniformity. These characteristics have to be properly modeled in retrieval process.
The initial steps of this inter-comparison are (1) radiometric comparison from a common vicarious calibration site at Railroad Valley in Nevada, where surface reflectance and bi-directional reflection distribution functions have been characterized over 5 years and XCO2 data are also available from air-borne and ground-based instruments, (2) spectral comparisons from the Lunar calibration data, and (3) comparisons of XCO2 from spatially and temporally coincident observations. Once the two instruments are characterized, they can view common targets from different geometries. This capability provides additional constraints on aerosol scattering, which is the largest uncertainty for CO2 retrieval from space. With the recent launch of OCO-2 in July 2014, it is now possible to provide initial comparisons of these new space-based high spectral resolution SWIR observations to those measured from the well characterized GOSAT satellite.