How do you detect enemy aircraft before radar?

A four-horn acoustic locator was used during World War I to detect enemy airplanes. The low speed of aircraft of the time, combined with their telltale engine noise, made acoustic detection a relatively effective early warning system before the advent of radar.



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Some high profile aircraft, such as Air Force One are not displayed. Most other aircraft subject to restriction are shown as anonymized by aircraft type.

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Why do military planes not show up on flight radar? The main reason is that they want to protect their security and avoid detection by potential enemies. Military planes often fly on classified missions or sensitive areas that require stealth and secrecy.

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Websites such as ADSBExchange.com, FlightRadar24.com and FlightAware.com aggregate flight data in the United States and abroad using a mix of commercial and citizen-owned sensors that capture the movements of commercial, civilian and military aircraft in real time, 24 hours a day.

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