Recent developments in propagation counter-measures for VSAT services

The need to provide open VSAT networks mandates that links meet agreed international standards. These standards fall into two categories: performance and availability. Performance objectives define the requirements for a connection over the bulk of the period it is in use. For satellite systems, this equates to clear sky conditions. Availability objectives refer to signal thresholds below which an outage is declared. A `degraded performance´ region exists between these objectives where the link can still be maintained, but at less than the designed quality. The margin between clear sky and degraded performance is small for VSATs and it is often difficult to separate performance and availability issues as they relate to propagation impairments. These aspects are considered below and some new results supporting impairment counter-measure investigations are given. The possible direction for counter-measures in VSAT systems is discussed