## 3 Results of Studying Atmospheric Propagation of subTHz Radiation
At the same time, it was proved that it would be much more practical to install a relatively small (15-21 m) high-quality (RMS < 50 \(\mu\)m) mirror, which would ensure efficient operation in the 1.3 and 0.8 mm transparency windows on the slope of the neighboring Aktashtau mountain (3200 m) (Figure 6a) to add it to the main 70 m dish of the Sufffa observatory built on the plateau at an altitude of 2400 m. Exploitation of RT-70 at waves shorter than the 3 mm window is quite expensive, requires too much time, and was demonstrated as not effective due to the astroclimate problems at 2400 m. This suggestion was included in the current plans for updating the Sufffa project. The selection of a specific site for a smaller mirror requires more detailed studies planned for 2022. Based on the results of direct measurements, some corrections of the efficiency indicators [3] of the Sufffa Observatory were proposed as an element of the EHT-derived analysis based on the satellite data. Sufffa is much better than the predicted satellite data. Figure 5: Elbrus IAP RAS expedition August 2021: night measurements at 5630 m made by MIAP-2. There is \(\sim\)0.25 \(\pm\) 0.25 Np of absorption of 3 mm waves. The first results of 2021 from Aktashtau generally confirmed the results previously obtained near Mount Pastukhova (Figure 7) and predicted theoretically based on the atmosphere model. A difference of about 0.01 Np was also observed. However, more thorough and longer-time measurements are required, including the year-round cycle of observations, which has already been completed at three sites (Siberia, Caucasus, and Svalbard) [32]. The Suffa project, as mentioned above, is now viewed as part of the further EHT community [3]. However, predictions of its performance parameters are not quite so good for an element of the EHT, as our estimations have shown. Paper [3] shows that Suffa is a reasonable candidate among the new candidates for EHT elements from the viewpoint of water absorption and a rather suitable one to add to the coverage of the VLBI system. However, Suffa is even better than paper [3] predicts. First of all, contrary to an altitude of 2000 m, indicated in [3], of RT-70 of the Suffa Observatory, the height is almost 2500 m. Hence, at least 0.01 Np of zenith losses can be omitted from the calculation. Another and more important fact is that the second antenna of the Suffa Observatory will be installed not so far from Suffa, at an altitude of \(\sim\)3000 m, and the gain in the altitude should be taken into account with a decrease of more than 0.1 Np. As of today, in the western part of the North Caucasus (Karachay-Cherkessia, Kabardino-Balkaria), no good places have been identified as promising for the installation of 1.3 and 0.8 mm instruments [39]. At the same time, based on the preliminary studies and the  
Figure 6: Suffa Plateau (2400 m) and Aktashtau mountains (3200 m): views (**a**) from Aktashtau to RT-70 in clouds and (**b**) Aktashtau from the RT-70 location (the left one). Figure 7: (**a**) Difference of \(\tau\) (Np) between two points near Mount Pastukhova differing in altitude by 700 m [25]. Measurements of \(\tau\) were performed over \(\sim\)3.5 h at 12:15–15:45 (top picture) at relatively low temperature (red, bottom) and humidity (blue dots) variations. (**b**) Measurements at Akdashy slope (2900 m) June 2021 by MIAP-2 confirmed possibility to install a small (15–21 m) antenna of the Suffa observatory. study of satellite and meteorological data, there are specific prospects at the sites located to the east: North Ossetia, Chechnya, and Dagestan, where the integral air humidity and precipitation level give substantial grounds to find promising sites for subTHz instruments on the slopes of some mountains at a height of about 3000 m. The meteorap of the Caucas with a total annual rainfall level indicates the difference in annual rainfalls by more than an order (from 200 mm to 2000 mm), and there are lots of dry places.