## 2 Materials and Methods
### FTIR Spectrometer Unit  
An FTIR spectroradiometer unit is integrated into the design of the small spacecraft developed for greenhouse gas remote sensing. Namely, to determine the oxygen and carbon dioxide concentrations in the air along the line of sight of the device, it measures the reflected sun radiation which passes the air column twice (Figure 2). Ground-based prototypes are described in papers [34; 35; 36]. Substance concentrations are determined by the depth of spectral lines according to the Beer-Lambert law [37; 38; 35]. Carbon dioxide molecules have fundamental absorption bands around 1.61 \(\upmu\)m and 2.05 \(\upmu\)m, which can be measured by analyzing reflected IR radiation from the Earth's surface. The concentration of molecular oxygen O\({}_{2}\) is constant, well known and uniformly distributed throughout the atmosphere [39]. The use of an additional channel on the fundamental oxygen absorption line (1.27 \(\upmu\)m or 0.765 \(\upmu\)m) [40] creates a reference channel to calibrate the accuracy of greenhouse gas concentration measurements by the developed system [41]. The device is based on a Michelson interferometer (Figure 3). Satellite and scientific instruments provide the target image and spectral information about the content of the target gases; O\({}_{2}\) is the first channel and CO\({}_{2}\) is the second channel. A distinctive feature compared to other missions is the combination of high spatial resolution and low spectral resolution in a compact volume. The operating principle of the FTIR spectrometer is as follows: Solar radiation reflected from the Earth's surface passes through the atmosphere and is collected by the telescopic system (Figure 3, positions 1 and 2) of the spectrometer. To reduce the dimensions and optimize the layout, the optical axis of the spectrometer system is deflected by mirror 3 (Figure 3, position 3). The radiation, collected by the telescope by falling on mirror 3, is reflected from it and enters the interferometer. The interferometer includes a beam splitter (Figure 3, position 4) consisting of a beam splitter plate, a compensator plate made of ZnSe and two corner reflectors (Figure 3, positions 5 and 6). We use corner reflectors in the receiving channel of the spectrometer to ensure the stability and reliability of its operation (Figure 3, positions 5 and 6) [36; 42] as mirrors with an aperture of 25 mm and a deviation of 1 angular second. Due to the longitudinal movement of the corner reflector ULPR-10 (PLX) (Figure 3, position 6) relative to the fixed corner reflector (Figure 3, position 5), a phase  
Figure 2: FTIR spectrometer principle of operation and unit design. difference occurs and an interferogram is formed at the output. We mounted the movable reflector on a spring parallelogram, which provides a stroke of 4 mm in one direction. The FTIR raw signal (interferogram) has about \(3\times 10^{4}\) points, which corresponds to a spectral resolution of 2 cm\({}^{-1}\)[43]. The radiation emerging from the interferometer falls on a splitter plate (dichroic filter, Figure 3, position 7) installed at an angle of 45 degrees. A dichroic filter is designed for spectral separation of the recorded radiation. The splitter reflects 98% of the radiation in wavelengths from 0.7 \(\upmu\)m to 1.2 \(\upmu\)m and transmits 95% in wavelengths from 1.2 \(\upmu\)m to 2.4 \(\upmu\)m. The transmitted radiation passes through a bandpass filter (Figure 3, position 8) and is collected by the focusing lens (Figure 3 position 9) on the photodetector (Figure 3 position 10). A signal channel is used for the registration of carbon dioxide and methane absorption. The signal is recorded by an InGaAs photodetector IG26X1000T9 (Laser components) (Figure 3 position 10) with an active area of 1 mm and a detectivity of \(D^{*}\sim 2\times 10^{12}\)cm \(\times\) Hz\({}^{1/2}\)/W.