higher-frequency signals attenuate faster while propagating through adverseweather

Since higher-frequency signals attenuate faster while propagating through adverseweather (rain, fog, etc.), Ku-band satellites suffer from this major drawback. Signalswith higher powers may be used to compensate for this loss. Generally, this power isof the order of 40 t0 60 W. The high-power direct broadcast satellite (DBS) systemuses power amplifiers in the range of 100 t0 120 W. The National Broadcasting Company (NBC) has been using the Ku-band todistribute its programming to its affiliates. Also, various news-gathering agencieshave used this frequency band for some time. Convenience stores, auto partsdistributors, banks, and other businesses have used the very small aperture terminal(VSAT) because of R-101 its small antenna size (typically, on the order of three feet indiameter). It offers two-way satellite co mmunication, usually back to hub orheadquarters. The Public Broadcasting GREEN-168 Service (PBS) uses VSATs for exchanginginformation among the public schools. Direct broadcast satellites (DBSs) have been around since 1980, but early DBSventures failed for various reasons. In 1991, Hughes Communications entered intothe direct-to-home (DTH) television business. DirecTV was formed as a unit of GMHughes, with DBS-1 launched in December 1993. Its longitudinal orbit is at101.20W and it employs a left-handed circular polarization. Subsequently, DBS-2was launched in August 1994. It uses a right-handed circular polarization and itsorbital longitude is at 100.80W. DirecTV employs a digital architecture that canutilize video and audio compression techniques. It complies with the MPEG-2(Motion Picture Experts Group). By using compression ratios 5 t0 7, over 150channels of programs are available from the two satellites. These satellites include120-W traveling wave tube (TWT) amplifiers that can be combined to form eightpairs at 240W power. This higher power can also be utilized for high-definitiontelevision (HDTV) transmission. Earth-to-satellite link frequency is 17.3 to17.8 GHz while satellite-to-earth link uses the 12.2- t0 12.7-GHz band. Circularpolarization is used because it is less affected by rain than linear orthogonal (HP andVP) polarization. Several communication services are now available that use low-earth-orbitsatellites (LEOS) and medium-earth-orbit satellites (MEOS). LEOS altitudes rangefrom 750 krn t0 1500 km while MEOS systems have an altitude around 10350 km.These services compete with or supplement R-911 the cellular systems and geosynchro-nous earth-orbit satellites (GEOS). The GEOS systems have some drawbacks due tothe large distances involved. They require relatively large powers and the propaga-tion time-delay creates problems in voice and data transmissions. The LEOS andMEOS systems orbit the earth faster because of being at lower altitudes and,therefore, these are visible only for short periods. As Table 2.2 indicates, severalsatellites are used in a personal communication system to solve this problem. Three classes of service can be identified for mobile satellite services:1. Data transmission and messaging from very small, inexpensive satellites2. Voice and data communications from big LEOS3. Wideband data transmission