In order to start a series of articles about the American signal intelligence satellites, written with guest author Rob1, I thought it could be interesting to give some background on what those satellites listen to. So here is a quick overview of the various types of signal intelligence targets, with an emphasis on the Cold War period.
The discovery of radio waves revolutionized communications. Instead of having to transport messages by horse, train or plane, and instead of having to build long telegraph lines, it became possible to transmit information instantaneously between two points without any infrastructure in-between.
The advantage was obvious, especially for military applications. Remote outposts, ships at sea, and planes, could easily receive their orders and report their status. Conversely, being able to intercept those communications became equally critical. During World War II for instance, the Allied forces put a lot of resources in intercepting and decrypting German and Japanese communications. After the war, the political situation changed, and for the Americans the USSR became the focus of their intelligence effort. The closed nature of the Soviet government and society made it a tough target to crack. US diplomatic presence, and US spies in the Eastern bloc, brought some light on the Soviet activities, but much of it remained inaccessible.
To gather more information, the US turned to signals intelligence (SIGINT) – the collection and analysis of electronic emissions – in order to answer the most pressing political and military questions. Because SIGINT relies on collecting signals from targets, different questions will result in collection against different targets. A few of those targets are listed below, with a bias towards installations targeted by the USA in the Soviet Union.