Signals intelligence operational platforms are employed by nations to collect signals intelligence, which is intelligence-gathering by interception of signals, whether between people (i.e., COMINT or communications intelligence) or between machines (i.e., ELINT or electronic intelligence), or mixtures of the two. As sensitive information is often encrypted, signals intelligence often involves the use of cryptanalysis. However, traffic analysis—the study of who is signalling whom and in what quantity—can often produce valuable information, even when the messages themselves cannot be decrypted.
It can be difficult to draw the line between a ground-based SIGINT receiving station, and facilities that have control, coordination, and processing functions in the "bigger picture" of signals intelligence. Many stations, for the countries with stations in many parts of the world, do have both aspects. There are also some that are clearly intercept only.
The first signals intelligence platforms were listening stations on the ground. Early tactical stations were in use as early as World War I, but permanent strategic signals intelligence stations were established as world tensions grew before WWII.