models Figure 11.11 Iridium LEO satellite constellation 3 An Iridium satellite phone. Photograph taken by Mark Pellegrini in the Steven F. Udvar-Hazy Center (Smithsonian Air and Space Museum extension in Dulles, VA, USA). Modified by ivob. Satellite Communications Table 11.2 Details of the main mobile and broadband satellite systems System Number of Orbital Frequency Downlink Uplink Transmission satellites height (km) spectrum spectrum rate down/up Iridium L/K band 1616-1626.5 MHz 28.8 kbps Globalstar S/L band 1610-1618.75 2483.5-2500 9.6 kbps (telephony) Teledesic K band 28.6-29.1 GHz 720/100 (Mbps) K band 17.7-20.2 0 GHz 27.5-30.0 24 Mbps have since been restructured to serve specific niches such as mining operators and military applications: indeed the conflicts in the Middle East have been a boon to the satellite com- munications companies in both the GEO and LEO fields. Another system, Other 3 billion (O3b) is being launched and is expected to provide global connectivity to the other half of the human population who do not have access to the internet. Note that the O3b orbital height of 8000 km classifies it as a MEO satellite. Details of various mobile and broadband satellite systems are shown in Table 11.2. Example 11.4: Iridium System Capacity The Iridium system uses the frequency spectrum between 1616 MHz and 1626.5 MHz. This 10.5 MHz spectrum is divided into 240 channels of 43.75 kHz, and an effective channel of 41.67 kHz with guard band as illustrated in Figure 11.12. Each satellite uses 48 spot beams within its coverage area, with frequency reuse factor of 12 as shown in Figure 11.13. Because of this frequency reuse each cell contains 240/12 = 20 frequency channels, which are time division multiple accessed by 4 users. Therefore, each satellite can support 20 * 4 = 80 telephone links/spot beam. Overall, each satellite can carry 48 * 20 = 960 links. As the Iridium system is made of 66 satellites, a total of 3168 spot beams are available. However, because of uninhabited regions around the world, only 21