PETALING JAYA: A US Air Force aircraft used a transponder code to electronically impersonate a Malaysian plane while spying on China’s military bases, the US-based Popular Mechanics has reported, quoting a Chinese think tank.

The RC-135W Rivet Joint reconnaissance aircraft flew off China’s Hainan island on Tuesday (Sept 8), coming within 90km miles of the Chinese mainland.

The South China Sea Probing Initiative, a think tank operated by the Chinese government, shared a pair of screenshots that showed a RC-135W take off from Kadena Air Base, a US Air Force base on the island of Okinawa.

The plane flew southwest, following the Ryukyu islands chain, past Taiwan, to loiter off the coast of Hainan Island, Popular Mechanics said, adding that enough details were provided on Twitter for independent verification.

“The plane’s International Civil Aviation Organisation (ICAO) Mode-S number, a 24-bit identifier assigned to all aircraft and broadcast by onboard transponder, was AE01CE.

“At some point, the plane’s Mode-S number suddenly changed, from AE01CE to 750548.

“That’s the ICAO number for an unknown Malaysian aircraft. The RC-135W, call sign RAINY51, then flew a racetrack pattern between Hainan Island and the Paracel Islands, an archipelago in the South China Sea that China claims, but whose ownership is in dispute, ” added the US publication.

It also noted that ICAO numbers are not supposed to be changed, as that degrades overall confidence in the system.

The RC-135W Rivet Joint is a converted Boeing 707 jetliner designed to collect electronic intelligence for later analysis.

It’s also not clear why the RC-135W engaged in the deception, reported Popular Mechanics.

“If the reconnaissance is happening outside sovereign airspace, there is no pressing need to engage in that sort of deception, ” it quoted Steffan Watkins, a Canadian open source intelligence researcher, as saying.

“It’s perfectly legal, and done in plain sight off the coast of Russia, Syria, and Crimea all the time, ” added Watkins.

The South Sea Probing Initiative is a China-based think tank whose advisory board includes serving officers of the People’s Liberation Army and Chinese Navy.

Its announcement on Twitter was “likely a warning to the Pentagon that the Chinese military sees through the deception, and that it’s watching the watchers, ” said Popular Mechanics.