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.
Ground platforms
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.
Arguably, one combined intercept and jamming technique of WWI was the use of shotguns against carrier pigeons, followed by reading the message attached to the bird.
While pigeons can probably be safe, other collection techniques may enjoy a resurgence. One specialized technique, originally used in the First World War but again in the Korean War, was interception using ground return from wired telephones. In mountainous terrain, it might again have applications, such as Afghan caves where wire might be run without the danger of free-space interception.
Satellite communications generally must be intercepted by large parabolic antennas on the ground, although there are possibilities that aircraft, intelligence satellites, and ships might also intercept. "To receive satellite signals ... only parabolic antennas are used. If the parabolic antennas are standing on an open site, it is possible to calculate on the basis of their position, their elevation and their compass (azimuth) angle which satellite is being received. This is possible, for example, in Morwenstow (UK), Yakima (USA) or Sugar Grove (USA)."
Australia: Ground platforms
The current (2019) Joint Military Communications Ground Station (JMCGS) is a SIGnals INTelligence intercept facility located at Kojarena appx 15 Mi east north east of Geraldton, Western Australia. The facility has been built in the early 1990s and originally manned by AUS defence personnel, later augmented by UK personnel previously assigned to Hong Kong. In 2007 the AUS and US authorities approved a Memorandum Of Understanding (MOU) stipulating the expansion and future rules of cooperation for the JMCGS. The new facility, becoming ops in 2010, is equipped with 4 large 25m radomes covering dish antennas intended to monitor and intercept voice and metadata SATellite COMmunications. The other antennas, a 15m radome and seven uncovered smaller dishes, are intended for the fully automated US naval Mobile User Objective System, a new 'next generation' narrowband networked satellite constellation for Ultra-High-Frequency satellite communications enabling secure all-weather and all-terrain 3 and 4G mobile telecommunications. The JMCGS is controlled by the Australian Signals Directorate (ASD) and is being operated under the UKUSA agreement meaning that all data obtained is being shared with the NSA.