The change of sea surface height can be measured by mm; the height of a rushing wave can be calculated by meters. If you ask how high is the orbital module of "Shenzhou IV" spacecraft from the ground, the answer might be 331.25631 km. All these accurate results should be attributed to breakthroughs made in China's space positioning technology, according to experts with the Shanghai Observatory under the Chinese Academy of Sciences (CAS).
Measurement results gained from last week's spacecraft were of accuracy unit of 1cm, a thousand times higher than previous standards, because a laser reflector, with a diameter of only 20 cm and a weight near 0.8 kg was for the first time installed at the bottom of the spacecraft. The successful precise measurement, experts said, marks a milestone not only in China's space positioning technology, but also in the nation's oceanic studies.
How high a wave can reach? How about the details of ocean circulation? These figures couldn't be completely mastered in the past. But now, with a CAS-made microwave altimeter on board, the distance between a wave and the spacecraft can be precisely measured, provided that the accurate position of the craft itself and its track are taken earlier. Before launching of the "Shenzhou IV", the measurement accuracy of the distance can only be up to somewhere within ten meters.
The height of sea surface and direction of ocean circulation are of great influence to climate, expert said, while to know more about the ocean will help find out the reasons of some catastrophic climates. For example, how the EL Nino phenomena came into being? Will the rise of sea surface threaten coastal cities such as Shanghai? The remarkable lift up of positioning accuracy enables China to draw more precise maps on wave, ocean circulation and sea surface, and raise the level of ocean studies and exploitation.
The position and track of "Shenzhou IV" was worked out by calculating the flying time of laser pulse between the craft reflector and ground, under the help of GPS and other measurement systems on board.
The laser position experiment, which lasted three days on end, turned out a full success. China's four laser observatory stations, including the Shanghai one, received more than 2000 measurement values, which are now undergoing further analyses.
(People?s Daily January 14, 2003)
