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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 10 | Copyright
Atmos. Meas. Tech., 11, 5865-5884, 2018
https://doi.org/10.5194/amt-11-5865-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 24 Oct 2018

Research article | 24 Oct 2018

Averaging bias correction for the future space-borne methane IPDA lidar mission MERLIN

Yoann Tellier1, Clémence Pierangelo2, Martin Wirth3, Fabien Gibert1, and Fabien Marnas4 Yoann Tellier et al.
  • 1Laboratoire de Météorologie Dynamique (LMD/IPSL), CNRS, Ecole Polytechnique, Palaiseau CEDEX, France
  • 2Centre National d'Etudes Spatiales (CNES), Toulouse CEDEX 9, France
  • 3Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Weßling, Germany
  • 4Capgemini Technology Services (for CNES), Toulouse, France

Abstract. The CNES (French Space Agency) and DLR (German Space Agency) project MERLIN is a future integrated path differential absorption (IPDA) lidar satellite mission that aims at measuring methane dry-air mixing ratio columns (XCH4) in order to improve surface flux estimates of this key greenhouse gas. To reach a 1% relative random error on XCH4 measurements, MERLIN signal processing performs an averaging of data over 50km along the satellite trajectory. This article discusses how to process this horizontal averaging in order to avoid the bias caused by the non-linearity of the measurement equation and measurements affected by random noise and horizontal geophysical variability. Three averaging schemes are presented: averaging of columns of XCH4, averaging of columns of differential absorption optical depth (DAOD) and averaging of signals. The three schemes are affected both by statistical and geophysical biases that are discussed and compared, and correction algorithms are developed for the three schemes. These algorithms are tested and their biases are compared on modelled scenes from real satellite data. To achieve the accuracy requirements that are limited to 0.2% relative systematic error (for a reference value of 1780ppb), we recommend performing the averaging of signals corrected from the statistical bias due to the measurement noise and from the geophysical bias mainly due to variations of methane optical depth and surface reflectivity along the averaging track. The proposed method is compliant with the mission relative systematic error requirements dedicated to averaging algorithms of 0.06% (±1ppb for XCH4 = 1780 ppb) for all tested scenes and all tested ground reflectivity values.

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The French and German space agencies (CNES, DLR) are currently developing MERLIN, a satellite that will measure atmospheric concentration of methane, a powerful greenhouse gas. To reach the desired precision, horizontally averaging the measurements along the satellite track is performed but leads to a processing bias due to non-linear equations. This article studies the processing biases for several averaging schemes and bias correction algorithms and recommends a best approach to limit biases.
The French and German space agencies (CNES, DLR) are currently developing MERLIN, a satellite...
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