The Evaluation of Satellite Link Availability

The need to predict overall link availability in a satellite communications system has motivated efforts to devise theoretical and empirical models to predict fade statistics for line of site propagation between an earth station and a satellite. Investigators have largely concentrated on measuring and analyzing the key propagation phenomena, especially attenuation and depolarization due to rainfall, and deriving simple parametric models which relate rain drop geometry, frequency, and available local meteorological data to probability distributions of attenuation and depolarization. However, in allocating margins for carrier-to-noise ratios, the designers of operational satellite systems still rely mainly on operational experience, and where limited experience is available margins tend to be conservative. While this approach may be the most practical with limited experimental and theoretical information, the sufficient variability of earth station to earth station link behavior suggests that without more detailed knowledge of the underlying mechanisms, system inefficiencies and unexpected problems may arise. Furthermore, an accurate link model is essential in the proper assessment of new systems, especially those systems using the 14/12 GHz, or higher, allocated satellite frequency bands. This paper derives the long term probability distribution of C/N over a complete satellite link that includes the effects of signal attenuation due to transmitting earth station pointing error, up-path propagation, geostationary satellite position error, down-path propagation, and receiving earth station pointing error. Non-tracking and tracking antennas subject to wind loading are considered, and the effects of the non-linear Satellite power transfer characteristic and the correlation of up- and down-link pointing losses due to satellite position error are accounted for. Results presented for 6/4 GHz links show that the satellite non-linear transponder, the proportion of noise in the up- and down-links, and satellite station keeping all affect the net fade distribution. It is therefore demonstrated that system parameters as well as uppath and down-path propagation statistics determine fade margin requirements for a satellite link.