Mobile Satcoms For The Future

Nearly all recent proposals for satellite communications sys- tems to provide a service to mobile stations have common features. They include the use of geostationary satellites, operating at radio frequencies around 900 MHz or 1.5 GHz; either low-gain omnidirectional antennas or higher-gain steer- able directional antennas for the mobile terminals; and com- munication on narrow fre- quency bands which permits only a single channel to be car- ried on each allocated fre- quency (known as single channel per carrier, or SCPC, access techniques, which means restriction of data trans- mission rates to the low figure of some 16 kilobit s-1). Communication via geostation- ary satellites gives global coverage from a threesatellite constellation, which is ideal for most maritime and aeronautical applications, but it suffers from somewhat severe propagation problems when the line-of-sight path from the ground station to the satellite is at a moderate angle of elevation. This is es- pecially...