A team of scientists from the National Key Laboratory of Microwave Imaging of the Chinese Academy of Sciences has achieved a breakthrough in submarine detection by using high-energy microwave synthesis technology to create a radio-emitting source in the sky.

According to its developers, the virtual signal source — also called a ghost radar — emits extremely low-frequency (ELF) electromagnetic waves continuously while traveling at nearly the speed of light. The ELF electromagnetic waves can reportedly penetrate seawater, thereby enabling the detection of submarines concealed hundreds of meters below the surface.

The scientists observed that when nuclear submarines in seawater are exposed to signals with frequencies as low as 100 Hz, their radar cross-section (RCS) can reach up to 947 square feet. This enables the detection of underwater targets using “common magnetic detectors.” They propose that equipping drones with these compact detectors could allow for “gradient detection of targets across the entire field.”

ELF signals, with wavelengths exceeding 328 feet, typically require significant spacing between antenna units. Historically, generating such low-frequency signals has relied on massive antenna installations — for instance, the ELF facility in central China, which features antennas spanning over 62 miles.

Conversely, the team of scientists reduced the length of the emitting array to roughly 328 feet, which reportedly makes it simple to install these antennas on Chinese naval ships. High-frequency, high-power electromagnetic waves emitted by these antennas converge in the sky to form a virtual radio-emitting source. As one source fades, another is immediately generated, maintaining a seamless stream of low-frequency signals.

The team explained that they used an array structure to simulate high-speed Doppler signals in space step by step, effectively enabling near-light-speed motion. This approach allows for a substantial reduction in signal frequency and an increase in signal pulse width.

The Doppler effect occurs when the frequency of a wave perceived by an observer differs from the source frequency due to their relative motion. When the source and observer move closer, the observed frequency rises; when they move apart, the observed frequency decreases.

The team of scientists calculate that the technology has potential applications for communication between surface ships and submarines, with a range of up to 3,700 miles.

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