Private satellite imagery from intelligence firm ImageSat International first released the image of Mischief Reef in the Spratly Islands showing the aerostat on Nov. 24, 2019. One of the company's satellites had grabbed this image of the island six days earlier. The reef is one of the largest of China's controversial man-made outcroppings in the South China Sea, which the country has been expanding since 2014.

Satellite imagery from last month shows that a tethered aerostat, a type of unmanned airship, floating in the air above one of China's man-made islands in the South China Sea . This lighter-than-air craft is almost certainly carrying a sensor system of some kind, such as a radar. A network of these platforms could help provide additional, but relatively low-cost early warning capabilities, especially against low-flying cruise missiles, and improved general situational awareness throughout the disputed region.

China has reportedly been actively developing aerostats for military use for at least two decades, according to Jane's 360 , which said it could not immediately identify the type of airship seen in the imagery. The image does show it hovering to the eastern side of the port facility on the island. It makes sense to keep it away from the large runway on the western side of the island, as it could pose a hazard to aircraft.

"For the first time, #China's aerostat, probably for #military #intelligence-gathering purposes, seen by #ISI at #Mischief Reef," a Tweet from iSi on Nov. 24 read. "The use of #aerostat allows China a continuous situational awareness in this resource-rich region."

A map showing the location of Chinese man-made outposts in the Spratly Islands, including Mischief Reef, which had initially been classified as two seperate outposts.

It is also impossible to tell what sensor or sensors it might be carrying from the imagery. One of the most likely options would be a radar with air and surface search and potentially acquisition and fire control capabilities.

A radar-carrying aerostat would provide a very capable, but also a relatively cheap option for monitoring various activity around Mischief Reef, as well as potentially cueing surface-to-air and anti-ship missiles to engage potential threats.

The elevated position would help increase the overall range of the system substantially, which would be especially valuable for spotting low-flying threats, such as cruise missiles or swarms of small unmanned aircraft as they crest over the horizon. Though China is steadily improving the defenses on its man-made islands, low-flying cruise missiles certainly represent a continuing threat to its outposts.

Aerostat-mounted radar systems can also remain aloft for long periods and, depending on their exact capabilities, in many types of weather, making them far more cost-effective and easier to maintain than manned aerial sensor platforms. They can also fly much higher, and therefore have far longer line-of-sight to the horizon, than even large mast-mounted ground-based radars.

The aerostat flying over Mischief Reef may also have electro-optical and infrared camera packages, which could provide more immediate surveillance coverage around the island. It can also act as a radio relay so that line-of-sight communications are more robust with vessels and other islands nearby. Other payloads, such as those that provide signals intelligence, are another possibility. Again, the tethered aerostat platform offers a low-cost, but persistent means of providing this kind of sensor coverage around fixed locations.

China would hardly be the first country to employ aerostats in this way. The U.S. military infamously poured billions of dollars into the abortive Joint Land Attack Cruise Missile Defense Elevated Netted Sensor System, or JLENS, between 1996 and 2017. Prototypes of this tethered aerostat surveillance system carried either a VHF-band surveillance radar or an X-band fire control radar.