SaVi - satellite constellation visualization. Simulate Iridium, Globalstar, Sirius, GPS, Teledesic and other systems, modify them, or design your own. Uses Tcl/Tk and Unix libraries; 3D viewing option requires Geomview running on X Window. BSD license.
Looks like an interesting book. The online version has lots of annoying advertisements, automatic(?) links (e.g. Octave points to the Wikipedia article about the frequency ratio, whereas the text refers to the free Matlab-like language) and "tool tips". A hardcopy version is announced for March.