## 3 Results and Discussion  
A spacecraft mockup was created (Figure 6a). External panels (including the installation of solar cells and antennas), frames, a mockup of the star sensor, a block of service systems (including the boards of systems and devices), a propulsion system and a camera were manufactured. The satellite's weight is about 23 kg, its dimensions are \(23\times 23\times 46\) cm and 43% of its mass falls on the payload. Designed with proliferation and manufacturability in mind, the satellite is optimized for greenhouse gas monitoring tasks by using readily available parts (e.g., industrial electronic components) and inexpensive production conditions. An FTIR spectrometer mockup (Figure 6b) for ground-based tests of greenhouse gas concentration measurements was created. The technical characteristics of the FTIR mockup are given in Table 3. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Spectral range & \(\mu\)m & 1.0–1.7 \\ Spectral resolution & cm\({}^{-1}\) & 10 \\ FOV & deg & 4 \\ Entrance aperture & mm & 100 \\ IR photodetector material & & InGaAs \\ IR active area size & mm & 2 \\ IR Photodetector NEP & W/Hz\({}^{1/2}\) & \(1.7\times 10^{-15}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The FTIR spectrometer mockup technical characteristics. Figure 6: The mockup design of a spacecraft (**a**) and FTIR spectrometer design (**b**): 1—input lens; 2—beam splitter; 3, 4—corner reflectors; 5—focusing lens; 6—signal IR photodetector IG17X200T9 (Laser Components); 7—632 nm laser; 8—reference photodetector PIN-040A (OSI Optoelectronics); 9—parallelogram; 10—power supply board; 11—linear drive control board. Experimental measurements were taken in September 2022 near the Baumanskaya district (Moscow, Russia). Solar radiation passes through the air masses of the atmosphere and is reflected off both inhomogeneities of the atmosphere and off topographic objects. The mockup registered reflected solar radiation. The line of sight direction did not change during the experiment. The overall time of measurements was 5 h. The ambient temperature was 12 \({}^{\circ}\)C and did not change significantly during the experiment. The humidity was 83%. The signal was recorded in cloudless weather. Figure 7 shows the scheme (Figure 7a) of solar reflection measurements in urban conditions and the appearance of the mockup (Figure 7b). The mockup of the spectrometer (Figure 7b, position 1) was installed outdoors on a tripod (Figure 7b, position 2), which allowed it to be oriented in the desired direction. The mockup was powered by two Rigol DP832 laboratory power sources (Figure 7b, position 3), which were located next to it. The recorded signal from spectrometer was transmitted to a laptop (Figure 7b, position 4) for processing. The FTIR mockup was oriented towards the open terrace at about 20 degrees to the horizon. The OCO retrieval algorithm [53] uses the fitting of the O\({}_{2}\) A-band at 0.76 \(\upmu\)m and the CO\({}_{2}\) bands at 1.61 \(\upmu\)m and 2.06 \(\upmu\)m, and it can be applied to many different retrieval problems. We used a type of OCO algorithm for the absorption bands of O\({}_{2}\) at a wave number of 7880 cm\({}^{-1}\) that is appropriate for the CO\({}_{2}\) and CH\({}_{4}\) bands at 6250-6350 cm\({}^{-1}\) and 6024 cm\({}^{-1}\), respectively. The spectral transmittance of the atmosphere obtained with an FTIR spectrometer is shown in Figure 8. The spectrum was obtained by averaging 15 interferograms with a total recording time of 1 min. The experimental spectrum includes oxygen absorption lines of O\({}_{2}\) at the wave number of 7880 cm\({}^{-1}\), CO\({}_{2}\) at the wave numbers of 6250 cm\({}^{-1}\) and 6350 cm\({}^{-1}\) and CH\({}_{4}\) at 6024 cm\({}^{-1}\). For a single measurement, the SNR in the spectrum is equal to 1220.