I want to explore some of the basics of transmitting digital signal over a satellite link, both for general interest and also as a way of getting these concepts more solidly integrated into my own understanding. For the most part, in my work supporting a couple of over-the-air IP networks, the mechanics of the space link are often a sort of amorphous set of givens, much like the “cloud” graphic used to depict the Internet in network diagrams. My goal here is to remove as much of the fuzziness as I can without getting too lost in details of things like link budgets and Reed-Solomon codes.

Communicating digitally over a satellite connection can often feel a lot like using an old-school modem: your sleek, digital broadband datastream gets snarled in a slow, often balky analog link. Indeed, the mechanics are the same – data is modulated, passed over a long (in this case, very long) distance, where it is demodulated and passed along to its destination. When you begin to get a sense of all the things that can (and often do) happen to an electromagnetic signal on the way to and from a satellite 35,000 kilometres above the Earth, you may be struck, as I was, at what a miracle it is that the data can pass at all, let alone at a decent speed and error rate! Read the rest of this entry »