18.1.1 Geostationary satellites

Geostationary satellites orbit the Earth at an altitude of 22,300 miles above the equator. The position of the satellites is specified in terms of the longitude of the nadir point (subsatellite point on the Earth's surface). In practice, geostationary satellites can drift from their designated location due to gravitational perturbations- these effects are not modelled in ns.

Two kinds of geostationary satellites can be modelled. Traditional ``bent-pipe'' geostationary satellites are merely repeaters in orbit- all packets received by such satellites on an uplink channel are piped through at RF frequencies to a corresponding downlink, and the satellite node is not visible to routing protocols. Newer satellites will increasingly use baseband processing, both to regenerate the digital signal and to perform fast packet switching on-board the spacecraft. In the simulations, these satellites can be modelled more like traditional ns nodes with classifiers and routing agents.

Previously, users could simulate geostationary satellite links by simply simulating a long delay link using traditional ns links and nodes. The key enhancement of these satellite extensions with respect to geostationary satellites is the capability to simulate MAC protocols. Users can now define many terminals at different locations on the Earth's surface and connect them to the same satellite uplink and downlink channels, and the propagation delays in the system (which are slightly different for each user) are accurately modelled. In addition, the uplink and downlink channels can be defined differently (perhaps with different bandwidths or error models).