relations of only several erlangs. DAMA may also be used for overflow traffic. It operates something like a telephone switch with its pool of available circuits. Calls are directed to an earth 9.3 SATELLITE COMMUNICATIONS station, where through telephone number analysis, that call will be routed on a DAMA circuit. With centralized DAMA control, the earth station requests a DAMA channel from the master station. The channel is assigned and connectivity is effected. When the call terminates (i.e., there is an on-hook condition), the circuit is taken down and the DAMA channel is returned to the pool of available channels. Typical DAMA systems have something under 500 voice channels available in the pool. They will occupy one 36-MHz satellite transponder. 9.3.6 Earth Station Link Engineering 9.3.6.1 Introduction. Up to this point we have discussed basic satellite communication topics such as access and coverage. This section covers satellite link engineering with emphasis on the earth station. The approach is used to introduce the reader to essential path engineering. It expands on the basic principles introduced in Section 9.2 dealing with LOS microwave. As we saw in Section 9.3.4, an earth station is a distant RF repeater. By international agreement the satellite transponder's EIRP is limited because nearly all bands are shared by terrestrial services, principally LOS microwave. This is one reason we call satellite communication downlink-limited. 9.3.6.2 Satellite Communications Receiving System Figure of Merit, G/T. The figure of merit of a satellite communications receiving system, G/T, has been introduced into the technology to describe the capability of an earth station or a satellite to receive a signal. It is also a convenient tool in the link budget analysis. 14 A link budget is used by the system engineer to size components of earth stations and satellites, such as RF output power, antenna gain and directivity, and receiver front-end characteristics. G/T can be written as a mathematical identity: G/T=GdB-10 