Once the modifications were made, however, it became possible to add medium Earth orbits and inclined geosynchronous orbits to their launch scopes, she added.
Cen Zheng, project manager of the Long March 3 family at the China Academy of Launch Vehicle Technology, said the frequent Beidou missions that will be launched in the coming years will prompt designers and engineers to improve the rockets' reliability and safety.
They have conducted more than 400 technical modifications, which means each Long March 3 has undergone an average of 28 modifications, he added.
In all, scientists and engineers, plus technicians and related staff members at numerous institutes and factories, including the Xichang Satellite Launch Center, have made great contributions to the Beidou program.
Guo Xinyu, a technician at the China Academy of Space Technology who is in charge of fueling the satellites, said almost all the technicians involved have respiratory problems because undertaking such hazardous work for long periods inevitably results in exposure to toxic propellants.
However, no one in the team has expressed reluctance or complained, he added.
He noted that the two most experienced technicians, Bai Kunshun and Zhao Zhenping, still take part in fueling operations and spend several months every year at launch centers, even though they are both in their 60s.
Wu Yong, head of a team of assembly technicians at the China Academy of Launch Vehicle Technology, said all the members of his crew work many hours of overtime every year to meet the tight production schedules, and it is common for them to spend weekends or vacation time at work.
"But many have told me they are proud of their work because what they do helps the country to launch Beidou satellites, which benefits the people," he said.
Civilian users
Since 2000, when the first Beidou satellite was placed in orbit, 33 satellites have been launched for the network.
In December 2012, the system began providing positioning, navigation, timing and messaging services to civilian users in China and parts of the Asia-Pacific region.
China plans to place 18 third-generation Beidou satellites in space before the end of the year.
They will work with earlier-generation models to provide services for all the nations involved in the Belt and Road Initiative, the navigation office said.
The initiative is a far-reaching infrastructure plan proposed by President Xi Jinping in 2013.
It comprises the Silk Road Economic Belt, which will link China with Europe through Central and Western Asia, and the 21st Century Maritime Silk Road, which will stretch from southern China to Southeast Asia and Africa.
The initiative will benefit about 4.4 billion people in 65 nations, according to government estimates.
At present, the Beidou system covers about 30 countries participating in the initiative, including Pakistan, Egypt and Indonesia, the navigation office noted.
Next year, and in 2020, China will place six third-generation Beidou satellites in medium Earth orbits, along with three in inclined geosynchronous satellite orbits and two to geostationary orbits.
According to plans drawn up by the China Satellite Navigation Office, the network will comprise 35 satellites before the end of 2020 - several currently in orbit will be decommissioned by then - allowing Beidou to provide global coverage.
Once the network is completed, it will provide civilian users around the world with free navigation and positioning services to an accuracy of about 10 meters.
However, paying customers will be provided with services that will be accurate to about a centimeter.
