Satellite Systems

The Satellite Constellation

Each satellite is a node in the fast-packet-switch network and has intersatellite communication links with other satellites in the same and adjacent orbital planes. This interconnection arrangement forms a

robust non-hierarchical mesh, or "geodesic," network that is tolerant to faults and local congestion. The network combines the advantages of a circuit-switched network (low delay "digital pipes"), and a packet-switched network (efficient handling of multi-rate and bursty data).

From a network viewpoint, a large constellation of interlinked switch nodes offers a number of advantages in terms of service quality, reliability and capacity. The richly interconnected mesh network is a robust, fault-tolerant design that automatically adapts to topology changes and to congested or faulty nodes and links. To achieve high system capacity and channel density, each satellite is able to concentrate a large amount of capacity in its relatively small coverage area. Overlapping coverage areas plus the use of on-orbit spares permit the rapid repair of the network whenever a satellite failure results in a coverage gap. In essence, the system reliability is built into the constellation as a whole rather than being vulnerable to the failure of a single satellite.

The lowest frequency band with sufficient spectrum to meet Teledesic's broadband service, quality and capacity objectives is the Ka band. The terminal-satellite communication links operate within the portion of the Ka frequency band that has been identified internationally for non-geostationary fixed satellite service and in the US, licensed for use by Teledesic. Downlinks operate between 18.8GHz and 19.3 GHz, and uplinks operate between 28.6 GHz and 29.1 GHz. Communication links at these frequencies are degraded by rain and blocked by obstacles in the line-of-sight. To avoid obstacles and limit the portion of the path exposed to rain requires that the satellite serving a terminal be at a high elevation angle above the horizon. The Teledesic constellation assures a minimum elevation angle (mask angle) of 40º within its entire service area. Using this design, the Teledesic Network is able to achieve availability of 99.9% or greater.