Signals Intelligence: Catching Secret Messages in the Air

During World War II, German navy ships tapped out secret orders using a machine called Enigma, which scrambled every message into gibberish. The Germans thought no one could ever read them. But a team of codebreakers in England, working in secret at a place called Bletchley Park, listened to those radio signals and cracked the code. Reading the enemy's own messages helped end the war years sooner. That kind of work is called signals intelligence.

1. What signals intelligence, or SIGINT, means. 2. How radio waves carry messages through the air. 3. How codebreakers turn scrambled messages back into words. 4. How direction finding locates where a signal comes from.

When you talk into a walkie-talkie, your voice becomes a radio wave that zooms invisibly through the air to another walkie-talkie. Radios, phones, and ships all send messages this way. Here is the tricky part: radio waves spread out in every direction. Anyone with the right antenna can catch them, even people the message was not meant for. Signals intelligence is the job of catching those signals and figuring out what they say.

Enemies scramble their messages so a stolen signal still looks like nonsense, such as XQPLM instead of ATTACK. Codebreakers hunt for patterns to unscramble it. For example, the letter E is the most common letter in English, so whatever scrambled letter shows up the most is probably standing in for E. At Bletchley Park, a mathematician named Alan Turing built an electric machine called the Bombe that tested thousands of possible code settings much faster than any person could. His ideas about machines that solve problems later helped invent the computer you use today.

Even without breaking the code, you can learn a lot just by finding where a signal starts. Listeners point special antennas at a radio signal to sense the direction it is strongest. If two stations far apart each point their antenna at the same signal, the spot where their two lines cross on a map is where the sender must be. This trick is called direction finding, and it once helped hunt for hidden submarines in the ocean.