Satellite measuring atmospheric methane with laser technology.

Decoding Methane: How Scientists are Fine-Tuning Satellite Measurements for Climate Insights

Nico Varela in Climate & Environment August 2025 • 4 min read.

"Averaging Bias Correction Unveiled for the MERLIN IPDA Lidar Mission"

Methane, a potent greenhouse gas, plays a significant role in driving climate change, second only to carbon dioxide. Accurately measuring methane concentrations on a global scale is critical to improving surface emission estimates and deepening our understanding of the global carbon cycle. Satellites offer a unique vantage point for this task, enabling comprehensive monitoring of methane levels across the planet.

The MERLIN mission, a collaborative effort between France and Germany, aims to achieve unprecedented accuracy in measuring methane dry-air mixing ratios (XCH4) from space. This mission utilizes Integrated Path Differential Absorption (IPDA) lidar technology, which involves measuring laser light scattered back from the Earth's surface to determine the concentration of methane along the laser's path. To achieve the targeted precision of 1% on XCH4 measurements, MERLIN signal processing relies on averaging data over 50 km, and that's where things get complicated.

The IPDA lidar equation, which links the backscattered signals to the methane column, is non-linear. This non-linearity, combined with the variability of the observed scene (surface elevation, reflectivity, meteorology) along the averaging window, introduces biases into the averaged data. These biases can compromise the accuracy of the measurements, making it essential to develop effective correction strategies.

The Challenge: Unpacking Averaging Bias in Methane Measurements

Satellite measuring atmospheric methane with laser technology.

The central challenge lies in reconciling the need for signal averaging to reduce random errors with the introduction of systematic biases due to the non-linear nature of the IPDA lidar equation. These biases arise from variations in surface characteristics and atmospheric conditions within the averaging window, impacting the accuracy of the retrieved methane concentrations.

Researchers have identified several key sources of averaging bias:

Noise-Induced Bias: Random noise in the on-line and off-line signal measurements can lead to systematic errors in the calculated XCH4 values.
Geophysical Variations: Changes in surface reflectivity, topography, and atmospheric conditions within the averaging window can disproportionately influence the averaged signal.
Averaging Concentrations vs. Molecules: Directly averaging XCH4 values is not equivalent to averaging the Differential Absorption Optical Depth (DAOD) and the Instrument Weighting Function (IWF) separately, leading to discrepancies in the results.

To address these challenges, scientists have explored various averaging schemes and developed correction algorithms to mitigate the impact of averaging bias. These algorithms aim to account for the non-linearities in the IPDA lidar equation and the variability of the observed scene, ultimately improving the accuracy of methane measurements from space.

The Future of Methane Monitoring: Towards More Accurate Climate Models

By carefully analyzing and correcting for averaging biases, scientists are paving the way for more accurate and reliable methane measurements from space-based instruments. The MERLIN mission, with its advanced IPDA lidar technology and sophisticated signal processing techniques, promises to provide valuable insights into the global methane cycle and its impact on climate change. These improved measurements will contribute to better surface flux estimates and ultimately enhance our understanding of the complex interactions that govern Earth's climate system, as well as the effectiveness of various mitigation strategies.

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This article is based on research published under:

DOI-LINK: 10.1051/epjconf/201817602020, Alternate LINK

Title: Averaging Bias Correction For Future Ipda Lidar Mission Merlin

Subject: General Medicine

Journal: EPJ Web of Conferences

Publisher: EDP Sciences

Authors: Yoann Tellier, Clémence Pierangelo, Martin Wirth, Fabien Gibert

Published: 2018-01-01

Everything You Need To Know

What is the MERLIN mission, and what specific technology does it employ to measure methane?

The MERLIN mission is a collaborative effort between France and Germany focused on achieving highly accurate measurements of methane dry-air mixing ratios (XCH4) from space. It uses Integrated Path Differential Absorption (IPDA) lidar technology, which measures laser light scattered back from Earth to determine methane concentration. The goal is to achieve a 1% precision in XCH4 measurements, contributing to a better understanding of the global carbon cycle.

What causes averaging bias in the context of methane measurements using IPDA lidar technology, and why is it a significant challenge?

Averaging bias in methane measurements arises due to the non-linear nature of the IPDA lidar equation. When data is averaged over the 50 km required for signal processing, variations in surface characteristics (like elevation and reflectivity) and atmospheric conditions within the averaging window introduce systematic errors. This is further complicated because directly averaging XCH4 values is not equivalent to averaging the Differential Absorption Optical Depth (DAOD) and the Instrument Weighting Function (IWF) separately.

What are the main contributing factors to averaging bias when measuring methane concentrations using satellites?

The key sources of averaging bias include noise-induced bias (random noise in signal measurements leading to errors), geophysical variations (changes in surface reflectivity, topography, and atmospheric conditions disproportionately influencing the averaged signal), and discrepancies from directly averaging XCH4 values versus averaging the Differential Absorption Optical Depth (DAOD) and the Instrument Weighting Function (IWF) separately. Addressing these sources is vital for accurate methane concentration retrieval.

How are scientists addressing the challenge of averaging bias in methane measurements obtained from space, specifically in the context of the MERLIN mission?

Researchers are developing and applying various averaging schemes and correction algorithms to mitigate the impact of averaging bias. These algorithms account for the non-linearities in the IPDA lidar equation and the variability of the observed scene. By carefully analyzing and correcting for these biases, scientists aim to obtain more accurate and reliable methane measurements from space-based instruments like MERLIN.

What are the broader implications of achieving more accurate methane measurements through missions like MERLIN, particularly for climate modeling and mitigation strategies?

By improving the accuracy of methane measurements through missions like MERLIN and advanced bias correction techniques, we can enhance our understanding of the global methane cycle and its influence on climate change. Accurate methane data contributes to improved surface emission estimates and better climate models, which are crucial for evaluating the effectiveness of mitigation strategies and predicting future climate scenarios. This enhanced monitoring directly informs policy and action towards reducing greenhouse gas emissions.