# Worldwide Coverage Mobile Systems for
## 2 Satellites Constellations  
Current geostationary satellites, which orbit along the equator, have difficulties in providing service coverage at the highest latitudes (polar regions) due to the elevation angle needed to transmit/receive the signal guaranteeing it will reach the receiver, as shown in Table 1.  
The most important factors in establishing reliable links using LEO satellites are, among others:  
* Ability to adjust the frequency of the receiver to compensate and track the frequency offset of the received signal band due to the Doppler effect during the satellite's run over the entire path. * Ability to track the satellite, not only across the sky (azimuth or compass indication and elevation angle over the horizon), but also very important, keeping the polarization angle aligned with the polarization of the satellite. * Sufficient gain in the receiving antenna system, as described in Table 1, to guarantee proper performance. GEO satellites constellations are not useful for deploying a global coverage in Smart Cities and wider areas due to the low angle required for transmission/reception, as shown in Table 1. Using LEO constellation for global coverage shows some drawbacks. One of them is that possible orbits are limited and can be saturated. Furthermore, there exist some issues in some regions of the space due to the possibility of fostering a large number of satellites in a small area of the space, given that each constellation has many satellites. Nevertheless, at the short term, it is not much likely such situation given, despite it is produced when orbiting at low altitude, situated in a region called Van Allen belt which is affected by strong radiation, it is at a distance about 900 km away and a large number of orbits are available and 60.000 LEO satellites can operate without mutual disturbance. Table 2 summarizes the advantages and drawbacks that satellite communications show for the various types of constellations, i.e., satellites' orbit height and relative speed. It is worthy to note that LEO is the constellation which shows the lowest delay. This is due to the orbit height, the lowest, making the link path distance earth-to-satellite the shortest. In consequence, it is quite appropriate for live transmission (video, audio, etc.) and low latency services (video-conference, gaming, e-health, etc.). Furthermore, due to its shortest link, the signal suffers a lower attenuation and the required antennas gain is lower and the transmit power as well. Regarding the drawbacks, the main design constraint is due to the large Doppler effect due to the relative speed of LEO satellites. Moreover, satellite tracking is required to make the antenna pointing to the satellite located in the earth mobile station is linked to, due to the relative movement \begin{table}
\begin{tabular}{l l c c c} \hline \hline  & **Name** & **Number** & **Altitude (km)** & **Angle (\({}^{\circ}\))** \\ \hline \multirow{4}{*}{LEO} & ISS & 1 & 360–400 & 51.64 \\  & Orbcomm & 29 & 785 & 45 \\  & GlobalStar & 48 & 1414 & 52 \\  & Iridium & 66 & 765 & 86 \\ \hline \multirow{2}{*}{MEO} & GPS & 24 & 20,200 & 55 \\  & Glonass & 24 & 19,132 & 64,8 \\ \hline \multirow{2}{*}{HEO} & Molniya & 4 & 504–39.863 & 63,4 \\ \hline \multirow{4}{*}{GEO} & Astra & 16 & 35,788 & 0 \\  & Hispasat & 7 & 35,788 & 0 \\ \cline{1-1}  & Goes & 4 & 35,788 & 0 \\ \cline{1-1}  & Meteosat & 3 & 35,788 & 0 \\ \cline{1-1}  & Intelsat & 63 & 35,788 & 0 \\ \cline{1-1}  & Immarsat & 12 & 35,788 & 0 \\ \cline{1-1}  & Thuraya & 3 & 35,788 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Satellites constellations at different orbits. of the satellites. This is because omnidirectional antennas are preferred, which requires less accurate satellite tracking or even no tracking. The features described above makes LEO constellations the most appropriate for global coverage mobile telephony and multimedia services. The development and innovation in technologies for satellites are in constant and fast progress.