FOREIGN ships passing through Philippine territory should keep their transponders on so authorities can identify them, a Supreme Court magistrate said after reports that Chinese vessels have sneaked into Philippine seas.

“They should not be passing secretly because if you turn off your automatic identification system (AIS), that means you don’t want to be seen,” Associate Justice Antonio T. Carpio told reporters yesterday. “I think it’s very stealthy if you do that.”

The AIS is a tracking system that uses transponders on ships and is used by vessel traffic services.

The magistrate said he agrees with Defense Secretary Delfin N. Lorenzana, who said ships that pass through the country’s territorial seas should not turn off their transponders.

“So if you turn it off, you are not acting innocently,” Mr. Carpio said.

While transponders are not required, the country should notify foreigners and announce a policy requiring the same, he added.

Mr. Lorenzana on Tuesday said he told Chinese Ambassador Zhao Jianhua not to turn off their AIS following reports last month of a Chinese aircraft carrier passing through Sibutu Strait without notifying authorities.

China has been building artificial islands in the disputed Spratlys and setting up installations that include runways. It claims sovereignty over more than 80 percent of the South China Sea based on its so-called nine-dash line drawn on a 1940s map.

Mr. Duterte has sought closer investment and trade ties with Beijing, including over resources in the disputed sea, since he assumed office in 2016.

His predecessor, Benigno S. Aquino III, sued China before an international arbitration tribunal over its territorial claims, and won. He also strengthened Philippine alliance with the US to try to check China’s expansion in the main waterway.

Aside from China and the Philippines, other claimants to the main waterway are Brunei, Indonesia, Malaysia, Taiwan and Vietnam. — VMMV