These days, the media is more interested in the latest Moscow sex tape than the Secret Lives of Drones. The Chinese know what the drone is for, and the U.S. military know what the drone is for. So you may as well know.
Although the U.S. conducts patrols in the South China Sea for the purpose of asserting freedom of navigation, a change of doctrine occurred with the deployment of China’s DF-21 “carrier killer” missile. To deploy a carrier in the South China Sea, under conditions of actual hostility, now incurs excessive risk. Quoting the United States Naval Institute via Wikipedia,
United States Naval Institute in 2009 stated that such a warhead would be large enough to destroy an aircraft carrier in one hit and that there was “currently … no defense against it” if it worked as theorized.
To retain some capability in the South China Sea under wartime conditions, the doctrine has switched to the submarine as the primary weapon system. You’ve undoubtedly heard of SONAR, which uses sound to determine the position of a target. The similarity of the acronym with RADAR suggests that it is the underwater equivalent of RADAR. It is not. Radar relies on radio waves, which (almost, though not exactly) travel in a straight line at a known speed.
Perhaps, from old movies, you remember a submarine captain using a periscope to locate a target visually, with light the physical equivalent of radio waves. Light travels in a straight line. Allowing for the course of the target, where the periscope points is roughly the direction the torpedo will be sent (a discussion of torpedo data computers is beyond the scope of this.) But the modern submarine engages targets beyond visual range.
Under water, sound does not travel in anything remotely like a straight line. Properties of the water, temperature and salinity, confine sound to “ducts”, which twist and turn with location, depth, and season. Sophisticated techniques exist to work these ducts to the advantage of the submarine. But they have to be mapped in three dimensions.
The South China Sea has been extensively mapped. The topography of the bottom has received particular attention, because the sea is very shallow, only about 65 feet on average. Mapping the bottom is done with towed sonar arrays. The hazard of inaccurate maps was shown by the U.S.S. San Francisco, when the ship collided with an unmapped undersea mountain in 2005.
It would have been hard for the Chinese to make off with a towed array. But the use of an autonomous drone to map temperature and salinity avoids turbulence that would affect the measurements.
You might be wondering why the Chinese are holding onto a commercially available drone. They are taking it apart. They want to know if it has any nonstandard instrumentation. The drone has a data logger, with a flash drive they’ll unload. Maybe they’ll give it a shine before they hand it back.
Or perhaps you are completely bored. I’m waiting for the next Moscow sex tape.