Innovative spaceborne optical instrumentation for improving climate change monitoring

Climate change has a major effect on our planet, impacting human and animal lives by inducing extreme weather events, by threatening ecosystems and by worsening air and water quality.Climate action can therefore no longer be postponed. A first action is to enable accurate and adequate monitoring of global warming. Global warming is caused by increasing amounts of greenhouse gases in the atmosphere, inducing a stronger storage than emission of the incoming radiative energy. This results in an imbalance in the Earth’s energy budget, which in its turn leads to the warming up of our planet. State-of-the-art instruments fail to monitor this imbalance on a global scale in an accurate manner and with a short revisit time. Current methods being inconclusive, a multidisciplinary approach is required to tackle this challenge. This PhD thesis therefore aims at providing potential solutions to this issue.
The research performed in this doctoral thesis indeed intends to fill current gaps in the ability to monitor the Earth’s energy imbalance in an adequate manner, by targeting the development of four novel space-based wide-field-of-view instruments that enable observing the Earth from limb to limb: (1) a radiometer, (2) a visible to near-infrared camera, (3) a thermal infrared camera, and (4) a near-infrared imaging spectrometer. Our research investigates the scientific and technical requirements of these instruments, their designs, their performances, a comparison with the state-of-the-art, and for some, their demonstration in the laboratory. These instruments will constitute the payload of an innovative new space mission. Combining the measurement results jointly obtained by these four instruments will enable a better assessment of climate change, while contributing to the Sustainable Development Goals.
The first instrument is a radiometer monitoring the incoming radiant energy that the Earth receives from the Sun, as well as the outgoing radiant energy reflected and emitted by the Earth towards space. Our compact and innovative radiometer enables retrieving the radiation imbalance causing global warming with an unprecedented accuracy. It does so by measuring the small difference between the incoming and outgoing radiations in a differential way, improving the state-of-the-art measurements by one order of magnitude. Key improvements, compared to previous space-based Earth-observing radiometers, are the use of a shutter in order to avoid the thermal offset problem, and a dedicated wide-field-of-view design featuring a uniform angular sensitivity and maximizing light absorption, while minimizing thermal non-uniformity.
Once the solar radiation reaches the Earth, about 29% is reflected by the clouds and by the planetary surface, while the rest of this radiation is absorbed by our planet. The absorbed portion causes heating of our planet, which then re-emits thermal radiation towards space. To distinguish between the solar radiation reflected by our planet, and the thermal radiation emitted by our planet, our radiometer is supplemented by a visible to near-infrared camera and a thermal camera. These cameras also improve the spatial resolution of the radiometer measurements towards a ground spatial resolution of a few kilometers, allowing to discriminate cloud and aerosol radiative forcing. We designed and optimized the imaging optics based on aspheric optical components in view of achieving the required image quality, while minimizing the system dimensions. In addition, we carry out radiative transfer simulations indicating that the broadband radiation, both reflected and emitted, can be retrieved with a certainty exceeding 95%.
The fourth and final instrument addressed in this doctoral research is a near-infrared imaging spectrometer based on freeform optics, which is one of the latest technologies exploited in the field of optical design. Freeform optics allows achieving exceptional performance while going well beyond the state-of-the-art in several respects, including compactness, field-of-view, and spatial resolution. The spectrometer enables monitoring the presence of the three most prominent greenhouse gases in the atmosphere (water vapor, carbon dioxide, and methane). When combined with the previously discussed instruments, the spectrometer does not only allow to measure the greenhouse gases concentration, but also allows linking the evolution of these gases with the Earth’s radiation budget and explaining climate change.
The four developed optical instruments, described in this doctoral thesis, pave the way to-wards the definition of a future space mission, with the ambition to fulfill the societal need of a better understanding of climate change by improving the monitoring of the Earth’s radiation budget and the Earth’s energy imbalance, with improved absolute accuracy and temporal stability. In addition, by the use of innovative compact instruments, a dedicated small satellite constellation for an adequate sampling of the diurnal cycle can be envisaged.