ELF signals, with wavelengths longer than 100 meters (328 feet), typically require large distances between antenna units. Traditionally, generating low-frequency signals needed massive antennas, like the ELF facility in central China, which has antennas over 100 km (62 miles) long.
In contrast, Li’s team has reduced the length of the emitting array to just about 100 meters (328 feet), making it possible to easily install these antennas on Chinese naval ships. The high-frequency, high-power electromagnetic waves emitted by these antennas converge in the sky to create a virtual radio-emitting source. As one source dissipates, another is instantly generated, ensuring a continuous flow of low-frequency signals.
as @D61@hexbear.net pointed out, the virtual source is what's moving at near the speed of light. the virtual source is the sky. the actual source is on a ship. it sounds like they're generating an interference pattern and reflecting that off the atmosphere to produce extremely low frequency electromagnetic waves capable of penetrating the water column. it's the group velocity of the wave packet that's moving at close to the speed of light, or something like that.
this article has a better explanation
https://interestingengineering.com/military/submarine-detection-at-light-speed-china
as @D61@hexbear.net pointed out, the virtual source is what's moving at near the speed of light. the virtual source is the sky. the actual source is on a ship. it sounds like they're generating an interference pattern and reflecting that off the atmosphere to produce extremely low frequency electromagnetic waves capable of penetrating the water column. it's the group velocity of the wave packet that's moving at close to the speed of light, or something like that.