Chinese scientists and engineers have been working hard over the past three days, calculating data from its manned spaceship to ensure the precision needed for the country's first manned space docking mission, the program's control officials said on Monday.
"The frequency of guidance control from a long distance and its requirement on precision is unprecedented," said Ma Yongping, deputy head of the Beijing Aerospace Control Center (BACC), where every single command for the space docking mission is issued.
Shenzhou-9, with its crew of three astronauts, successfully docked with the Tiangong-1 lab module on Monday afternoon after repeated orbit corrections and a long journey catching up with its target.
The space lab received its first visitors after the three Chinese astronauts entered it to conduct experiments, exercise and rest.
"Only with precision can we avoid risks, ensure safety and achieve success," explained Qian Yongping, chief designer of the tracking and communication system of the manned space program. "Our mission is to be as precise as possible."
For the first step of the docking, Shenzhou-9 had to be guided to the rendezvous point about 52 km behind Tiangong-1.
The space rendezvous and docking requires a precise match of the orbital velocities of Shenzhou-9 and Tiangong-1, allowing them to remain at a constant distance through orbital station-keeping.
Both the spacecraft and the lab module are flying at a speed of 7.8 km per second, making it very difficult to maintain their relative position and height under the command of the BACC.
According to the original plan, the control center had to conduct guidance control and changed the orbit for the spaceship five times before the docking, using the tracking and control system on the ground."But we did it after only four orbit corrections," said Tang Geshi, a researcher at the BACC.
Tang noted the center canceled one orbit modification for Shenzhou-9 when it was orbiting Earth for the 13th time.
The revision of the original plan came as a result of calculations, in which researchers found they could use the fixed deviations from the orbit changes when the spacecraft was orbiting Earth for the fifth and 19th time to substitute a correction at the 13th orbiting.
"The simplified flight maneuvering procedure boosted safety and reliability for our astronauts and the craft's flight rather than undermining the precision," according to Tang.
Starting from the rendezvous point, Shenzhou-9 collected data with its microwave and laser radars about the relative position and state of the space lab and approached the target via a flying course and speeds according to preset procedures.
As the aerospace tracking and control network on the ground is restricted by factors such as distance and location, the spacecraft can only rely on its automatic guidance when it gets closer to the lab module, explained Qian.
The chief designer of the spacecraft tracking and control system noted the spacecraft has been installed with modern distance measuring devices, the accuracy of which can be kept within centimeters.
To leave room for adjustment, engineers set up four berth points for the spaceship on the same orbit five km, 400 meters, 140 meters and 30 meters away from the orbiting lab.
Shenzhou-9 kept an unchanged relative motion from the lab at each berth point for several minutes before the ground control center gave the go-ahead command for further moves.
During the process, should any unexpected emergency happen, astronauts will step in and start manual maneuvering, Qian explained.
The astronauts will stay inside the coupled complex for six days before the two machines separate for a manual docking attempt, which will again test the precision of astronauts' maneuvering.
