For one to make the best use of the data collected by satellites, an effective retrieval algorithm is needed. A new dissertation work explores mathematical models at the core of such algorithms. Presented solutions have great potential to support both the development of new remote sensing technologies and the re-evaluation of already existing satellite observations.
Earth observing satellites are indispensable tools for gaining information about Earth system processes. With careful data collection and processing, satellites’ observations can be used to monitor and keep track of the changes in both natural and human-made environments. Nowadays, this kind of space-based information gathering supports a wide range of applications in such fields as environmental protection, energy sector, logistics, and urban planning.
While satellite instruments have been used in monitoring the Earth’s atmosphere for decades now, it became a truly widespread practice at the turn of the millennium. Since then, the focus has shifted from the monitoring of ozone holes towards investigating the global climate change. However, since satellites can only measure electromagnetic radiation, one needs to make do with observing different processes just indirectly. To obtain the desired information from radiation measurements, a retrieval algorithm is needed. And at the core of the retrieval algorithm resides a mathematical model for the movement of radiation in the Earth system, the radiative transfer equation.
On 18 June 2025, Doctoral Researcher Antti Mikkonen from the Finnish Meteorological Institute (FMI) defended his doctoral dissertation Computational approach to challenges in Radiative Transfer for Remote Sensing of Atmospheric Composition in the Faculty of Science at the University of Helsinki. Mikkonen’s work explored several aspects of radiative transfer, ultimately gaining new advancements with exciting potential for the future of Earth observation.
Senior Scientist, Ph.D. Piet Stammes from the Royal Netherlands Meteorological Institute served as the opponent, while the FAME Flagship’s Second Vice Director Samuli Siltanen from the University of Helsinki served as the custos.
In the summertime
Mikkonen started his academic studies at the University of Eastern Finland’s (UEF) Department of Applied Physics, where he specialised in Computational Physics. When the time finally came to write his Masters’ thesis, Mikkonen focused on quantitative photoacoustic tomography, a medical imaging technique that uses laser light and sound waves to reconstruct detailed images of tissues. On paper, there seemed to be little connection between Mikkonen’s thesis work and the field of remote satellite observations.
However, his supervisor – and the future Director of the FAME Flagship – Tanja Tarvainen raised FMI as a potential place for research work opportunities in the case that Mikkonen would be interested to pursue doctoral education. By lucky coincidence, a friend of Mikkonen was involved in organising the first Finnish Satellite Workshop – later to be known as Winter Satellite Workshop – in early 2018. Having participated in the event out of curiosity, Mikkonen was eventually introduced to Professors Johanna Tamminen and Hannakaisa Lindqvist of FMI, who would later become his dissertation supervisors. Not too long after that Mikkonen landed himself a summer job at FMI.
“It was an excellent motivator to finish my Masters’ thesis, in which I had struggled a bit”, Mikkonen describes his first summer at the Institute. “As I had experience in light path and radiation modelling, I was offered an opportunity to develop new radiation simulations for FMI. Having been greatly inspired by this, I finalised my thesis during autumn and was able to officially start my doctoral education in spring 2019.”
“Officially” being the appropriate word here, as the research work that culminated to Mikkonen’s public examination had at this point already taken its first significant steps.
Simulations for the future
As a summer employee at FMI, Mikkonen initially started working on radiative transfer models that could be used in algorithms to enhance satellite instruments’ processing capabilities. As it turned out, this would also mark the beginning of the design and programming work for new ARSCA and Raysca simulation software, later utilised and focused on more detail in the Paper I of Mikkonen’s dissertation. Likewise, the final research article, Paper IV, in which different transfer models are being systematically compared, can also trace it roots to the summer of 2018.
“It’s been really active research and programming work from pretty much ever since”, Mikkonen says. “One could very well describe my dissertation’s progress as organic, where each seemingly unconnected research topic has been built up from a shared starting point.”
With all his doctoral research papers now collected into a completed dissertation work, it is apparent that Mikkonen has spent his time at FMI well. From the creation of an easy-to-use software for faster and more accurate calculations to the development of new reflection model simulation for snow surfaces, Mikkonen’s work has the potential to support both the development of new remote sensing technologies and the re-evaluation of already existing satellite observations.
“Personally, I would argue that the Paper II, in which we introduced a completely novel scattering graph method to handle the scattering of radiation within the planetary atmosphere, is the most exciting in terms of further research and future utilisation”, Mikkonen states. “With this method we could analyse the satellite image in whole instead of pixel by pixel, and its results seem to agree acceptably with the previous state-of-the-art method. However, I don’t yet fully understand all properties of our method. There’s a lot of mysteries left to research.”
The work continues
Starting from autumn, Mikkonen will move to the Institute of Environmental Physics at the University of Bremen, Germany, for at least a year-long postdoctoral research position. While there, he will work on to enhance the European Space Agency’s (ESA) CO2M satellite mission’s retrieval algorithm. CO2M, or the Copernicus Anthropogenic Carbon Dioxide Monitoring, will be launched in 2027 to measure how much carbon dioxide is released into the atmosphere specifically because of human activity. As the first mission of its kind, CO2M will allow countries to track their situation in achieving the Paris Agreement goals.
“In an ideal situation we would receive measurements of carbon dioxide and methane levels from all over the world in real time”, Mikkonen ponders the desired future for his research field. “And while this might not necessarily come to fruition as such, hopefully we will be able to collect more accurate and unbiased information on CO2 sources and carbon sinks to achieve a better picture of the situation and best ways forward. And I will aim to help in this to the best of my abilities.”
Photo: Samuli Siltanen | From the left: Prof. Samuli Siltanen, Doctoral Researcher Antti Mikkonen, Prof. Johanna Tamminen, Prof. Hannakaisa Lindqvist, and Ph.D. Piet Stammes at the historical Senate Square next to the University of Helsinki’s Main Building in central Helsinki.
