Satellites form an indispensable tool to observe the earth’s atmosphere and study how the climate is changing. They provide a continuous and reliable source of independent measurements. The Dutch space instrument TROPOMI for example has identified many large methane leaks from oil and gas facilities, landfills and coal mines.
With its daily global coverage, it spots every large methane emission on the planet. However, a high spatial resolution is needed for accurate pinpointing of the source and the detection of smaller emissions. This is only possible with a smaller filed-of-view, meaning more satellites are needed for global coverage.
Current limitations
Earth observation instruments on satellites traditionally use bulky conventional optics such as lenses, mirrors and diffraction gratings. The consequence is a large physical volume and weight. This makes the manufacturing and deployment in space expensive and hinders the scaling to constellations with more satellites. Therefore new optical solutions are needed to reach a similar performance with smaller satellites.
Figure 1: Artist impression of an optical system with a metalens and a photonic crystal array on a detector as developed at SRON.
Nanostructured optical elements
Recent years have seen considerable advances in the manufacturing of nanostructured optical elements. For example, conventional lenses were replaced by flat optical elements, consisting of an array of tiny pillars made of materials such as silicon or silicon nitride. This is called a metalens. Similarly, periodic holes in silicon or silicon nitride can form two-dimensional photonic crystals to act as optical filters.
Figure 2: Examples of manufactured nanostructured optical elements. (a) photonic crystal array, (b) zoom-in with e-beam microscope on photonic crystal array, (c) metalens, (d) zoom-in with e-beam microscope on the metalens.
SRON is exploring if such nanostructured optical elements can reduce the size of satellite instruments for earth observation. For the photonic crystal filters, the key concept is to use several filters and to infer from the incoming signals on a detector a greenhouse gas concentration, either directly or via the polarization of the incident light. Theoretical analysis has shown that in this way we can achieve the same performance as with a conventional grating spectrometer, in a volume ten times smaller. SRON is now working together with national and international partners towards prototypes for applications such as methane or aerosol detection from space.
Figure 4: Greenhouse gas retrieval approach with the direct fitting of a radiative transfer model to the measured integrated signals from the detector with different filters.
Relevant missions
Our expert
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Read moreRalf Kohlhaas
Head of OptX section
