to use a sophisticated modulation scheme to increase the data rate over an ordinary 1.25-MHz CDMA chan- nel. The problem with 3G systems in the United States is that a much smaller frequency band is available for 3G service than in areas following European frequency division. 5.4.7 Mobile Satellite Systems One application of satellite communications is for point-to-point transmis- sion, as presented in Section 4.7. Satellites also provide mobile communica- tions services to ships and aircraft, and they are used in desert areas where other communications services are not available. In the systems that use geo- stationary satellites, mobile stations are expensive and the cost of service is quite high. Plans were made to implement lower cost satellite services that could be used with handy MSs. The multimode MSs could use satellite or, if available, lower cost PLMN, such as GSM or CDMA. Examples of these sys- tems are Iridium and Globalstar. These systems use a number of satellites that are in orbit at a 700- to 10,000-km distance from the Earth instead of in a geostationary orbit, which is at a distance o 36,000 km. The satellites are circling the Earth in such a way that some of them are visible all the time from any point on the Earth's surface (Figure 5.8). Each of the satellites performs base station functions and takes care of the large cell below it. These systems need and use functions similar to those of cellular net- works. Examples are the mobility management and handover functions, which are used to manage the movement of satellites (BSs) instead of sub- scribers. Earth stations of a satellite system control the operation of satellites and behave as connection points to the public land networks. Introduction to Telecommunications Network Engineering Mobile satellite systems use many low or medium orbit satellites that move around the Earth Multimode terminals use satellite service if land mobile network is not available Earth 10,000 km station A satellite that operates as a base station of a public land 