 42 2164 - 6378 = 35786 km from the surface of the Earth. Orbits higher than GEO are collectively referred to as high Earth orbits (HEOs). Orbits below GEO are called medium Earth orbits (MEOs) or low Earth orbits (LEOs) if they are lower than 2000 km, as illustrated in Figure 11.7. Examples of satellite systems at these orbits include GEO long-distance communica- tions satellites from operators such as Intelsat; a number of LEO mobile satellite systems orbiting in altitudes of 700-800 km and the Global Positioning System (GPS) operating in the MEO of 20200 km. Broadband Satellite Telecommunications Systems A number of satellite telecommunications of interest to broadband services exist, and these may operate in a range of orbital altitudes. Directional antennas with strong gain can be used to communicate with a GEO satellite as these are 'stationary'. However, due to the long dis- tance between the satellites and the Earth, path loss is very high and in the order of 200 dB or more. This means that systems with smaller antennas cannot provide high capacity transmis- sion rates. Moreover, signal delays can be excessive for many applications as end-to-end con- versational delay is in the order of 0.5 S. Yet another limitation is the maximum number satellites that can be placed in the GEO altitude and plane: for example if a 2-3° separation is needed the maximum number of satellites will be 120-180. GEO Satellite Systems As stated in the Historical Note, the first GEO satellite system was operated by Intelsat, an intergovernmental organisation under the auspices of the United Nations. It soon launched satellites to cover the entire Earth and operated as a long distance voice and TV Superscript(2)A sidereal day is the time it takes for the Earth to once rotate fully around its axis, and is slightly shorter than a 24-h 'solar' day. This is because the earth travels approximately 1/365th, or almost 1°, of its solar orbit a day. For the same point to directly face the sun, the Earth needs to rotate around its axis sligh