the required total capacity/transmission rates are significantly smaller than that of GEO satellites for most services offered. Low Earth Orbit Satellites The LEO satellites orbit the Earth at altitudes of less than 2000 km. Because of this their orbit- ing time is significantly shorter. For example, a LEO satellite orbiting at 800 km experiences an orbital time of around 100 min. Such a satellite would not be very suitable for a real-time telecommunications system since the receiver on the ground can only view the satellite for a short period: the satellite rises from one horizon and sets in the opposite horizon on average in less than 50 min. As a result, continuous connectivity to one satellite is clearly impossible. However, a constellation of such satellites can be designed to maintain constant connectivity. Such a constellation helps SO when one satellite sets, a new one has already risen to maintain Broadband Telecommunications Technologies and Management connectivity. In this way, a call is relayed through a network of several satellites to a ground station. In LEO systems the communications distance is short and therefore the path loss is much lower compared with GEO satellites. For example a LEO satellite operating at the 4 GHz band orbiting at 800 km altitude experiences the following path loss to the Earth: L =32.4+20*log(800)+20*log(4000)=162.5dB = The link budget is improved by 33 dB, or 2000 times, compared with GEO satel- lites which means smaller antennas can be used. In practice such an antenna can be mounted on a mobile phone (pictured). Several LEO satellite systems are presently in operation or on the design board. Iridium is one such system. It consists of a constellation of 66 satellites in 6 orbital planes as shown in Figure 11.11. The system is designed SO an Iridium satellite is always visible from any point on the Earth, including the poles. Iridium and another LEO system, Globalstar, were unsuccessful commercially at their launch in the late 1990s (see Case Study 11.1). Their business 