China has carried out major upgrades to its electronic warfare and surveillance systems across its major bases in the Spratly Islands to strengthen its intelligence capabilities in the South China Sea, according to an American think tank.

Satellite imagery analyzed by the Asia Maritime Transparency Initiative in a report last week showed new and enhanced radar facilities, antenna fields and reinforced electronic warfare installations at the “big-three” Chinese outposts—Mischief, Subi and Fiery Cross reefs.

“These upgrades underscore the fact that a major function of China’s bases is to provide unparalleled ISR (intelligence, surveillance, and reconnaissance) coverage of the South China Sea, supporting the operations of China’s coast guard and navy in peacetime and allowing Beijing to contest the use of the electromagnetic spectrum by others in the event of a conflict,” the report said. The study detailed the most significant improvements in the past two years.

According to AMTI, China has deployed fixed monopole antennas and vehicle-mounted units equipped with jamming and sensing systems at the three bases. “Identification of specific systems is difficult even in aerial photography, but the grouping resembles several Chinese mobile EW systems where each vehicle carries different jamming equipment for targeting specific bands of the electromagnetic spectrum,” the think tank said.

Although most of China’s recent work focuses on electromagnetic capabilities, “Beijing is also investing in support facilities for equipment with more kinetic potential,” it added.

Reinforced artillery positions built along the western and northern parts of Mischief Reef—many of them still empty—could eventually host mobile rocket launchers or other strike weapons.

AMTI said the Chinese buildup comes as Vietnam accelerates its own construction in the Spratly Islands, and noted it will be important to watch whether Hanoi’s activities follow China’s trajectory.

China claims almost the entire South China Sea, despite overlapping claims from several countries, including the Philippines and Vietnam, turning the area into a flashpoint that heightens tensions and undermines regional security. A 2016 international arbitral tribunal ruling found that Beijing’s expansive claims have no legal basis under international law.

Over the weekend, Chinese forces fired three flares from Subi Reef toward a Philippine aircraft carrying out a maritime domain awareness flight in the South China Sea. “The Bureau of Fisheries and Aquatic Resources aircraft recorded video footage of three flares fired from the reef toward the aircraft during its lawful overflight,” the Philippine Coast Guard said, which joined the flight with the fisheries agency.

“These flights aim to monitor the marine environment, assess the status of fisheries resources, and ensure the safety and welfare of Filipino fishermen in the West Philippine Sea,” the PCG said, referring to Manila’s exclusive economic zone in the South China Sea.

Last month, the United States temporarily deployed MQ-9A Reaper medium-altitude, long-endurance unmanned aerial vehicles to the Philippines to bolster the country’s maritime surveillance and intelligence-gathering capacity.