QINGDAO:- China has achieved real-time transmission of deep-sea data at 6,000-meter depth through its self-developed BeiDou satellites for the first time, a move essential to more secure, independent and reliable deep-sea data transmission. China's most sophisticated research vessel Kexue (Science) returned to the eastern port city of Qingdao on Thursday after wrapping up a 74-day, 12,000-nautical mile expedition. During the trip, Chinese scientists maintained and upgraded the country's scientific observation network in the West Pacific, according to the Institute of Oceanology under the Chinese Academy of Sciences (CAS).