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Remote Sens. 2017, 9(11), 1158; doi:10.3390/rs9111158

The Cross-Calibration of Spectral Radiances and Cross-Validation of CO2 Estimates from GOSAT and OCO-2

1
Remote Sensing Technology Center of Japan, 2-1-1 Sengen, Tsukuba 305-8505, Japan
2
Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
3
Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO 80523, USA
4
Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba 305-8505, Japan
5
Joint Institute for Regional Earth System Science and Engineering (JIFRESSE), University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
*
Author to whom correspondence should be addressed.
Received: 30 August 2017 / Revised: 23 October 2017 / Accepted: 8 November 2017 / Published: 11 November 2017
(This article belongs to the Special Issue Remote Sensing of Greenhouse Gases)
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Abstract

The Greenhouse gases Observing SATellite (GOSAT) launched in January 2009 has provided radiance spectra with a Fourier Transform Spectrometer for more than eight years. The Orbiting Carbon Observatory 2 (OCO-2) launched in July 2014, collects radiance spectra using an imaging grating spectrometer. Both sensors observe sunlight reflected from Earth’s surface and retrieve atmospheric carbon dioxide (CO2) concentrations, but use different spectrometer technologies, observing geometries, and ground track repeat cycles. To demonstrate the effectiveness of satellite remote sensing for CO2 monitoring, the GOSAT and OCO-2 teams have worked together pre- and post-launch to cross-calibrate the instruments and cross-validate their retrieval algorithms and products. In this work, we first compare observed radiance spectra within three narrow bands centered at 0.76, 1.60 and 2.06 µm, at temporally coincident and spatially collocated points from September 2014 to March 2017. We reconciled the differences in observation footprints size, viewing geometry and associated differences in surface bidirectional reflectance distribution function (BRDF). We conclude that the spectral radiances measured by the two instruments agree within 5% for all bands. Second, we estimated mean bias and standard deviation of column-averaged CO2 dry air mole fraction (XCO2) retrieved from GOSAT and OCO-2 from September 2014 to May 2016. GOSAT retrievals used Build 7.3 (V7.3) of the Atmospheric CO2 Observations from Space (ACOS) algorithm while OCO-2 retrievals used Version 7 of the OCO-2 retrieval algorithm. The mean biases and standard deviations are −0.57 ± 3.33 ppm over land with high gain, −0.17 ± 1.48 ppm over ocean with high gain and −0.19 ± 2.79 ppm over land with medium gain. Finally, our study is complemented with an analysis of error sources: retrieved surface pressure (Psurf), aerosol optical depth (AOD), BRDF and surface albedo inhomogeneity. We found no change in XCO2 bias or standard deviation with time, demonstrating that both instruments are well calibrated. View Full-Text
Keywords: greenhouse gas remote sensing; satellite validation and calibration; inter-comparison; carbon dioxide; GOSAT; OCO-2 greenhouse gas remote sensing; satellite validation and calibration; inter-comparison; carbon dioxide; GOSAT; OCO-2
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Kataoka, F.; Crisp, D.; Taylor, T.E.; O’Dell, C.W.; Kuze, A.; Shiomi, K.; Suto, H.; Bruegge, C.; Schwandner, F.M.; Rosenberg, R.; Chapsky, L.; Lee, R.A.M. The Cross-Calibration of Spectral Radiances and Cross-Validation of CO2 Estimates from GOSAT and OCO-2. Remote Sens. 2017, 9, 1158.

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