Two satellites launched to observe Earth's atmosphere

On July 26, 2025, the Vega-C rocket launched the MicroCarb satellite into orbit from the Kourou base. On August 12, it was Ariane 6's turn to launch Metop-SG. On board, several instruments will scrutinize the Earth's atmosphere and its composition, but with different scientific objectives.

IASI-NG: the details of the atmosphere

Metop-SG carries IASI-NG (New Generation Infrared Atmospheric Sounding Interferometer). It will observe the radiation emitted by the Earth's atmosphere and analyze its different wavelengths (the spectrum) in the infrared range, then deduce the essential variables for understanding the Earth's climate: temperature, humidity, cloud formation, aerosols, greenhouse gases, etc. The entire globe will be scanned twice a day.  
“The strength of this mission is that it meets the needs of three areas: weather forecasting, atmospheric chemistry, and climate study,” explains Cyril Crevoisier, CNRS research director at the LMD and co-principal investigator for the IASI-NG mission.  

Thanks to an ingenious system (a Mertz interferometer), this instrument will be able to collect more light—and therefore provide more accurate measurements—than its predecessor, IASI, which was carried aboard three weather satellites launched between 2006 and 2018. At an altitude of 800 km, IASI-NG will be able to detect variations of 0.1°C in the atmosphere and ensure the continuity of atmospheric studies over several decades. This is a critical issue in the era of climate change.

“IASI is currently the leading contributor to weather forecast reliability, and also sets the global standard for other instruments of its kind,” explains Cyril Crevoisier. The aim is for IASI-NG to replace it. Following this initial launch, two further models of the instrument will be sent into space in 2032 and 2039.

MicroCarb: Focus on CO2

The MicroCarb mission is also designed to measure atmospheric spectra, but using a different system from IASI-NG (a grating spectrometer rather than an interferometer, and shorter wavelengths in the infrared). Above all, MicroCarb focuses on one gas in particular: carbon dioxide (CO₂). CO₂ is the main greenhouse gas. Its emission from human activities is responsible for climate change.

But there is also a natural carbon cycle, particularly via vegetation. MicroCarb's first objective is to quantify this natural component as accurately as possible. The second objective is to focus on a specific region, such as a city. “The instrument will be able to take images measuring 13 kilometers by 40 kilometers with pixels of approximately 2 kilometers, each pixel representing a spectrum,” explains Laure Corazza, a doctoral student at the LMD. MicroCarb is one of the first satellites to take this type of spectral image (along with GoSat GW, launched by the Japanese space agency in June 2025). These images will be used to map emissions from human activities.

The downside is that the volume of data provided by these missions is increasing significantly compared to previous ones. In her thesis work at the LMD, Laure Corazza is developing new methods for processing this data, using neural networks (a type of artificial intelligence technique). “One of the biggest challenges is to process an image in less than a second while achieving the highest possible measurement accuracy,” the researcher points out.

MicroCarb also serves as preparation for the European CO2M mission, scheduled for launch in 2028, for which the algorithms developed at LMD will also be used.

Before beginning their observations, the two satellites will undergo a deployment and validation phase during which scientists will verify that the data meets expectations. The data will be compared with those obtained, for example, from atmospheric balloons and processed using a numerical code to calculate what the satellites should observe. The LMD is also involved in this phase, in particular through this numerical code (a radiative transfer code), which is recognized as a benchmark in the field of atmospheric research.

*LMD: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris, ENS, Sorbonne Université