# Propospheric Attenuation in GeoSurf Satellite Constellations
## 6 Final Remarks and Conclusions  
In GeoSurf satellite constellations, any transmitter/receiver, wherever it is located, can be linked to a satellite with zenith paths. In general, GeoSurf satellite constellations can have most of the advantages of the current GEO, MEO, and LEO satellite constellations without having most of their drawbacks [1]. In the present paper, we have studied one of the many feasibility issues that should be addressed before any industrial design is started, which is the tropospheric attenuation expected in the GeoSurf links, compared to other LEO satellite constellations and to GEO satellites. To be specific in predicting the annual average probability distributions of tropospheric attenuation, we have estimated both rain and clear-sky attenuation at 39.6 GHz. For link budget, once the outage probability is fixed, the total tropospheric attenuation exceeded at this probability sets the minimum signal-to-noise power ratio that must be provided to any link. Clear-sky and rain attenuation depend mainly on the frequency, site, and elevation angle under which the satellite is seen from the ground station. For the purpose of illustration, we have studied the tropospheric attenuation predicted for some reference sites (Canberra, Holmdel, Pasadena, Robledo, and Spino d'Adda), which also set the meridian on which we have considered sites with latitudes ranging between 60\({}^{\circ}\) N and 60\({}^{\circ}\) S. The choice of this particular latitude range is due to the fact that for larger latitudes, the GEO paths would be too low on the horizon to be practical and useful, whereas LEO and GeoSurf satellites can always be used at any latitude. We have found that at high probabilities for system design (around 1% of an average year), in the latitude between 30\({}^{\circ}\) N and 30\({}^{\circ}\) S, there are no significant differences between GEO slant paths and GeoSurf zenith paths. On the contrary, significant differences are found for latitudes greater than 30\({}^{\circ}\) N or 30\({}^{\circ}\) S and lower probabilities. For example, along the meridian of Spino d'Adda (9.5\({}^{\circ}\) E), at 60\({}^{\circ}\) N, the average difference \(A_{GEO}-A_{Z}\) predicted is about 5 dB at a 1% probability, 11 dB at 0.1%, and 20 dB at 0.01%. In the low latitude range of \(\sim\)20\({}^{\circ}\) N to \(\sim\)20\({}^{\circ}\) S (GEO elevation angle along the meridian is larger than about 67\({}^{\circ}\)), the difference \(A_{GEO}-A_{Z}\) is quite small, and even negative at a probability of 0.01%, as most physical models (Global SST and Excell) retain some spatial features concerning small high-intensity rain cells. For comparing the tropospheric attenuation in GeoSurf paths (zenith) with that expected in highly variable LEO slant paths, we have considered, as an example, a LEO  
Figure 15: The difference \(A_{LEO}-A_{Z}\), calculated, at equal probability, in the probability range 1% to 10% between the clear-sky attenuation exceeded in the LEO path, \(A_{LEO}\), and that exceeded in the GeoSurf path, \(A_{Z}\), in the reference sites. satellite orbiting at 817 km as seen from Spino d'Adda and have supposed that it is also seen in the same way in the other reference sites. From the results of our simulations using the Global SST, it is evident that GeoSurf zenith paths \"gain\" several dBs compared to LEO slant paths. For example, at a probability of 0.1%, the minimum difference \(A_{LEO}-A_{Z}\approx 5\) dB is found at Pasadena and Canberra, and the maximum difference \(A_{LEO}-A_{Z}\approx 9\) dB is found at Holmdel. We have also studied the more static total clear-sky attenuation (due to water vapor, oxygen, and clouds), which depends on the site and annual probability considered for link design. As expected, for any path, drier sites (Pasadena) are less affected than wetter sites (Holmdel). Moreover, both GEO and LEO slant paths show larger attenuation than GeoSurf (zenith) paths.