A submarine is quiet, but it is not silent. Its machinery makes faint, low-frequency sound that carries a long way through water. Hearing that sound across an ocean, and telling it apart from waves, whales, and merchant ships, is a signal-processing problem. The Cold War system that solved it, SOSUS, was a proving ground for the frequency-domain analysis at the heart of modern signal processing.
The idea
The information is not in how loud a sound is but in its frequency content. Decompose a noisy recording into its component frequencies, and a submarine’s machinery reveals itself as telltale spectral lines that a broadband listener would never pick out.
Passive listening at scale
SOSUS, the Sound Surveillance System, was a passive sonar system built to track Soviet submarines. Passive means it only listened; it sent out no ping that could give it away. It consisted of bottom-mounted hydrophone arrays connected by underwater cables to facilities ashore, where the listening and analysis happened.
LOFAR, the spectral analyzer
The analysis was done by equipment called the Low Frequency Analyzer and Recorder, or LOFAR, based on AT&T’s sound spectrograph and modified to analyze low-frequency underwater sound. A spectrograph takes a stretch of sound and shows how its energy is distributed across frequencies over time. That is spectral analysis: turning a wall of noise into a picture in which structure becomes visible.
Detection in noise
Underneath the hardware is one of the core problems of signal processing and statistics: detection, the decision of whether a faint signal is actually present in a sea of noise, or whether you are fooling yourself. SOSUS made that decision continuously, at continental scale, for decades, before being officially declassified in 1991.
Related Notes
- Cryptography, Codebreaking, and the NSA, the other great signals-analysis discipline
- Hypothesis Testing, the statistics of deciding signal from noise
- Bayesian Inference, updating belief as faint evidence arrives
- Computing and the U.S. Military, the cluster index
Sources
- “SOSUS,” Wikipedia. https://en.wikipedia.org/wiki/SOSUS . Supports SOSUS as a U.S. Navy passive-sonar submarine-detection system, its bottom-mounted hydrophone arrays connected by underwater cables to facilities ashore, the Low Frequency Analyzer and Recorder (LOFAR) based on AT&T’s sound spectrograph for low-frequency spectral analysis, and its official declassification in 1991.