SIGINT

SIGINT is a blend of two words Signal and Intelligence. In this use I refer to information gained from intercepted radio signals, including broad traffic analysis. Since first days that data could be transmitted, opposing parties were trying to intercept it. A detailed history of SIGINT would take many thousands of pages. Thankfully I'm only interested in it's radio applications. But before I try to find a narrative here, let me first roll out a bit of military jargon:

COMINT - (Communications Intelligence) communications between people.
ELINT - (Electronic intelligence) communications not containing speech or text.
OpELINT- Operational ELINT
MASINT - (Measurement and Signature Intelligence)
FISINT - (Foreign Instrumentation Signals Intelligence)
LEO COMINT  - Space based communications with orbiting satellites below 2000 km
GEO COMINT - Space based communications with geosynchronous satellites

These are all programs at US security agencies. SIGINT includes radio, but is not limited to radio. It's best summarized broadly as intelligence-gathering by the interception of signals. This even includes conversations  (COMINT.)  It also, traffic analysis—not just traffic but signal on the radio band as well. It's an examination of who sends signals to who, and how often. That's regardless of whether the signals themselves can be decrypted and/or understood. This is what spooks do. They are not James Bond, they are peeping toms. By and large most people are OK with that. More here.

Obviously these programs are much older than the agencies that oversee them now. Eavesdropping is surely as old as our species, Encryption is at least as old as the Romans, and probably cracked in the same era.  But ELINT is only as old as radio. Some of the first intercepted signals were transmitted in 1900 during the Boer war.  The British went on to monitor Russian marine wireless in the Russo-Japanese War in 1904.  This all became more of a international game in WWI. The British cut German undersea cables, forcing them to use radio so they could intercept the signals. The British built Y-stations on land and afloat  for interception and direction finding.

US was late to the game. Our communications monitoring of naval signals started around 1918. The Navy built 52 direction finding sites on the East Coast. What little we built in WWI was let to rot in the interval. We only ramped up again in the lead up to WWI in order to provide some direction finding on Japanese signals in the Pacific. Real COMINT programs started in the late 1930s with ships intercepting signals from Germany and Italy. When japan again joined the fray we ramped up on the west coast as well. Following WWII the military began approaching permanent monitoring installations, a direction remains unchanged today.