In fact, something far more dangerous was happening, and the Manukai's captain was unaware of it. Although the American ship's GPS signals initially seemed to have just been jammed, both it and its neighbor had also been spoofed—their true position and speed replaced by false coordinates broadcast from the ground.
Analysts noticed that the attacks had actually started the previous summer, increasing as the months rolled on. The most intense interference was recorded on the very day in July that the Manukai's captain reported difficulties, when a total of nearly 300 vessels had their locations spoofed.
The spoofing could be China testing a new electronic weapon. Or it could be sand pirates trying to sneak through the area:
Chinese builders call it "soft gold." Sand dredged from Yangtze River, which has the ideal consistency and composition for cement, helped fuel Shanghai's construction boom in the 1980s and 1990s. By the turn of the millennium, reckless sand extraction had undermined bridges, trashed ecosystems, and caused long stretches of the riverbank to collapse. In 2000, Chinese authorities banned sand mining on the Yangtze completely.
The trade continued illicitly, however, expanding to include the illegal dredging of sand and gravel from the Yangtze estuary and the open seas near Shanghai. By day, such ships look innocuous. By night, they lower pipes to the riverbed to suck up thousands of tons of sand in a single session.
Sand pirates were blamed for almost two dozen wrecks in 2018, and authorities have seized 305 sand-mining vessels in 2019.
Read on at Technology Review for a primer on how GPS works, and the strange effects being observed near the Port of Shanghai.