The three most modern radar spy ships in the People's Liberation Army Navy (PLAN) -- all belonging to the Yuanwang class -- left Chinese ports one after the other from March 27 to April 10 to allegedly keep track of the launch of a new Chinese spacecraft on April 20.

Yuanwang-5, Yuanwang-6 and Yuanwang-7 are currently steaming towards undisclosed locations in the Pacific Ocean and will use their modern radars to ostensibly track the progress of the Tianzhou 1 unmanned cargo spacecraft after this spacecraft's launch on April 20.

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The Yuanwangs are officially classified as maritime tracking ships, each equipped with large dish antennae for tracking Chinese launch vehicles and satellites. The radars aboard these ships, however, are used mainly to track the path of intercontinental ballistic missiles (ICBMs), such as those operated by the United States.

These ships are dual-use platforms serving military and scientific ends. One western analyst described the Yuanwangs as "spy ships" and estimated that up to 70 percent of the work done by the Yuanwang fleet consists in gathering signals intelligence, which is one of the more sophisticated forms of spying, from targets such as the U.S.

Signals intelligence or SIGINT is intelligence-gathering by interception of communications signals between people or from electronic signals.

Chinese state-run revealed the Yuanwang fleet, which consists of five operational ships, will carry out 19 maritime "space monitoring" missions this year.

It said that in 2016, the Yuanwang fleet completed 14 major scientific research and experiment tasks. The ships monitored the maiden flights of the Long March-7 and the Long March-5 launch vehicles, and tracked the orbiting Tiangong-2 space lab and the Shenzhou-11 manned spacecraft.

Yuanwang-7, the newest ship, was commissioned in July 2016 and can operate for up to 100 days at sea. It will be used for maritime tracking of the Shenzhou 11 manned space mission and the Tiangong 2 space laboratory. Its three large dish antennae measure some 10-12 meters in diameter.

Yuanwang-7 has performed scientific research and experiment-related tasks, including tracking missions for the maiden flight of the Long March-5; the space rendezvous and docking of Shenzhou-11 and Tiangong-2.

Yuanwang-7 is 220 meters long, 40 meters high and displaces nearly 30,000 tons.