On a sultry summer night in July 2018, the MV Manukai was arriving at the port of Shanghai, near the mouth of the Huangpu River. This busy tributary of the Yangtze winds through the city and includes the Bund, a historic waterfront area and tourist hot spot. Shanghai would be the American container ship’s last stop in China before making its long homeward journey to Long Beach, California.
As the crew carefully maneuvered the 700-foot ship through the world’s busiest port, its captain watched his navigation screens closely. By international law, all but the smallest commercial ships have to install automatic identification system (AIS) transponders. Every few seconds, these devices broadcast their identity, position, course, and speed and display AIS data from other ships in the area, helping to keep crowded waterways safe. The position data for those transponders comes from GPS satellites.
According to the Manukai’s screens, another ship was steaming up the same channel at about seven knots (eight miles per hour). Suddenly, the other ship disappeared from the AIS display. A few minutes later, the screen showed the other ship back at the dock. Then it was in the channel and moving again, then back at the dock, then gone once more.