# A New Orbiting Deployable System for Small Satellite Observations for Ecology and Earth Observation
## 3 Study Cases
### Application 1: Composition of the Atmosphere  
The Earth's atmosphere composition is primarily N\({}_{2}\)O\({}_{2}\), Ar, and H\({}_{2}\)O, plus trace gases (in the order of ppm) (e.g., [89]). Despite their negligible amount, these trace molecules represent a key factor in controlling the radiation budget of the Earth. Compounds such as H\({}_{2}\)O, CO\({}_{2}\), CH\({}_{4}\), O\({}_{3}\), N\({}_{2}\)O, SO\({}_{2}\), and halocarbons are examples of greenhouse gases, which, trapping the incident solar radiation, determine the degree of heating at the Earth surface, and at the end influence the climate (e.g., [89; 90]).  
The effects of anthropogenic gases on the atmosphere can be evaluated by monitoring: (i) the ozone loss in the lower stratosphere, (ii) the source of the polluting gases, and (iii) how these gases are transported vertically and horizontally through the atmosphere [89]. For instance, the mapping of SO\({}_{2}\), which can have either a natural (e.g., from volcanic eruptions, [91]) or anthropogenic (e.g., air traffic and industrial processes, [92]) origin, is important, as it contributes to aerosols formation, which cause variations in the total radiative balance and in the air navigation visibility when injected in large quantities into the atmosphere [93]. At a spatial resolution \(<\)1 km, it would be possible to monitor SO\({}_{2}\) in volcanic plumes and from air and maritime navigation, through their emission trails.  
Spectrometers, operating from the near UV up to TIR, are used to study the atmospheric composition, e.g., the main IR absorption lines are associated with vibro-rotational transitions of trace gases [94], and concentration, by iterative inverse methods (c.f., [94; 95; 96]). As reported in [97; 98], differential absorption spectroscopy is commonly adopted to detect and quantify the narrow absorption lines in the UV-VIS spectral range characteristics of a specific trace gas. Micro- and nano-satellites have taken a leading role in the atmospheric characterization (e.g., HARP [99], PARASOL [100], and PICASSO [101]), coming up beside and replacing the large satellites (e.g., satellites such as ESA Sentinel-5P [102; 103],NASA/Aura [104], or programs such as EUMESAT MetOp [105; 106], or fleets such as the NOAA's Joint Polar Satellite System (JPSS) [107; 108]).  
SO\({}_{2}\) can be monitored by micro- and nano-satellites equipped with innovative interferometers, such as the Fabry-Perot interferometer (FPI) [97; 98]. FPI technologies have been developed and applied, from UV to TIR, to micro-spectrometers placed in CubeSat-type hyperspectral imagers [109; 110]. An operational version is aboard a CubeSat 2U form factor Nano satellite (a CubeSat multiple of the standard size 1U), which mounts a hyper-spectral infrared camera (0.9-1.4 \(\upmu\)m). The FPI interferometer can also be realized by means of the Micro-electromechanical system (MEMS) technologies, which can determine the concentration of multiple gases at a nanometer resolution (e.g., [111; 112; 113; 114; 115]). To perform a global SO\({}_{2}\) coverage, e.g., to monitor volcanic emissions, the revisiting time should be at least 24 h. For regional/local acquisitions, e.g., to monitor aerial or naval routes, or industrial and urban plumes, the revisiting time should be few hours, which is feasible only by means of a constellation of satellites. The SNR for the FPI technique can be a factor of six to nine with respect to what can be obtained with the differential optical absorption spectroscopy [97].