# Geometric Spherical Surfaces Emulating the Geostationary Orbit at Any Latitude with Zenith Links
## 4 Discussion and Conclusions  
At this stage of developing GeoSurf constellations, we can draw only some general remarks, summarized in Table 1.  
One of the main advantages of the GeoSurf design is having zenith paths worldwide. With the GeoSurf constellation, we do not need steering antennas, the tropospheric propagation fading is minimized both for the stationary fading due to water vapor and oxygen, and for rain attenuation, because at zenith the path length in the troposphere is shortest [16]. The footprint of a single satellite is always the same, therefore making the design easier. However, several footprints can overlap with possible frequency interference, to be avoided by a careful frequency plan and coding. The Doppler phenomena are largely minimized because the connected satellite would be always viewed almost at the local zenith. The extra free-space loss, due to the fixed pointing of all antennas, is 6 dBs when \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & **GEO** & **MEO** & **LEO** & **GeoSSurf** \\  & **(38,876 km)** & **(23,000 km)** & **(1500 km)** & **05,876 to 1500 km, or \\ \hline Number of satellites & few & several & many, up to 12,000 & many, up to 2592 \\ \hline \multirow{2}{*}{Free–space attenuation} & \multirow{2}{*}{large} & \multirow{2}{*}{medium} & \multirow{2}{*}{low} & minimum (zenith path): 6 dBs extra loss when the satellite enters or leaves the local -3 dB service area \\ \hline Antenna steering & fixed & moderate & large & fixed (zenith) \\ \hline Minimum (zerith path for all) two–way latency due to propagation (ms) & \multirow{2}{*}{238.7} & \multirow{2}{*}{133.4} & \multirow{2}{*}{10.1} & \multirow{2}{*}{from 238.7 to 10.1} \\ \hline Link elevation angle & fixed & variable & largely variable & fixed 90\({}^{\circ}\) (zenith) \\ \hline High latitude service & no service & service & service & equal service for any latitude \\ \hline Satellite footprint & fixed & almost fixed & changes very much & fixed, large moderate to \\  & & & & lattices \\ \hline \multirow{2}{*}{Tropospheric attenuation (no rain)} & stationary, easy to estimate. & can be smaller than GEO, & can be much larger than & minimum (zenith path), very \\  & Fade countermeasures & easy to estimate. Fade & GEO, difficult to estimate. & easy to estimate. Fade \\  & “easy” to implement. & countermeasures retrieved & Fade countermeasures very & countermeasures”easy” to \\ \hline \multirow{4}{*}{Scintillation} & moderate or large, depends on elevation angle and antenna size, easy to estimate & can be smaller than GEO, & can be much larger than & minimum (zenith path), very \\  & & easy to estimate & GEO, difficult to estimate & easy to estimate \\ \hline \multirow{2}{*}{Rain attenuation} & \multirow{2}{*}{can be large, depends on site, easy to estimate} & can be smaller than GEO, & can be much larger than & minimum (zenith path), very \\  & & easy to estimate & easy to estimate & GEO, difficult to estimate & easy to estimate \\ \hline Switchovers in a given time interval & none or very few & few & many & few to many \\ \hline Inter-satellite links & none or very few & few & many & few to many \\ \hline Doppler phenomena & negligible & small & large or very large & negligible to very small \\ \hline Frequency plan and & moderate & moderate & large to very large & from moderate to very large \\  & complexity & & & \\ \hline Interference with GEO & small & moderate & large & only for sites very close to \\  & satellites & & & the equator \\ \hline Orbit capacity & moderate & moderate & large & from moderate to large \\ \hline Launching sites for & close to equator & medium latitudes & medium \\  & & & latitudes & any latitude \\ \hline Control and maintenance & minimum & moderate & large & from moderate to large \\ \hline \hline \end{tabular}
\end{table} 
Table 1: General characteristics (advantages and disadvantages) of the most popular constellations (identified by their altitude above the Earth surface) compared to the GeoSurf constellation.