On July 25, 2015, the Chinese conducted successful launch of Long March 3B launch vehicle with Yuan Zheng-1 (YZ-1) upper stage, carrying two BeiDou-M new-generation navigation satellites, into a medium Earth orbit (MEO) from Xichang Satellite Launch Center in the southwest China's Sichuan Province.
This was the third space mission for China in 2015, following a 'surprising launch' of Gaofen-8 on Long March 4B rocket on June 26, and a launch of the first new-generation satellite of BeiDou navigation system on Long March 3C booster on March 30.
The two satellites, designated as BeiDou M1-S and BeiDou M2-S, are the 2nd and 3rd spacecraft of the Phase-III of BeiDou Navigation Satellite System (BDS), which is a collective name for three generations of 'sub-systems'. The first is BeiDou Satellite Navigation Experimental System, or BeiDou-1. It consists of three satellites and offers limited coverage and applications. It has been offering navigation services, mainly for customers in China and neighboring regions, since 2000.
The second generation of the system, also known as COMPASS or BeiDou-2, has been developed since 2007. In December 2012, it began providing positioning, navigation, timing and short message services to civilian users in China and parts of the Asia-Pacific with 10 satellites operating in high orbits. The system has been gradually introduced into transportation, weather forecasts, marine fishing, forestry and telecommunications.
"Saturday's launch of 'twin sats' - the 18th and 19th of the whole Beidou project - is a big part of that project... We have deployed a new type of signal format, which enable us to double data exchange between satellites within the same period of time. That means faster and better service for BDS clients... 98% of 'twin sats' components, including their 'heart' - rubidium atomic clocks - were domestically made." Xie Jun, Chief Engineer of the Beidou project
Construction of the global network (Phase-III) officially began in 2015. It is planned that BeiDou satellite constellation will consist by 2020 of 35 spacecraft – 5 at geostationary orbit, 27 at MEO, and the remaining 3 at an inclined geosynchronous orbit. As a comparison, US GPS and Russian Glonass navigation satellites are only MEO-orbiting.According to SinoDefence, BeiDou-M ('M' standing for MEO) satellites are to be deployed in orbits with 21,500-24,100 km altitude and 55° inclination. An experimental BeiDou-M satellite based on the DFH-3B bus was launched in April 2007 to test an on-board payload. Operational BeiDou-M spacecraft, though, based on a dedicated navigation satellite bus. Satellites has a gross mass of 850 kg and a payload of 300 kg. They are 3-axis stabilized, and have a power output of 1,500kW. Their designed operational lifespan is 10 years.
Image credits: Xinhua (News.cn), SinoDefence, ChinaSpaceflight.
Three-staged Long March 3B rocket, which first launched in 1996, is the most powerful launch vehicle in China's current inventory, capable of delivering 5,200-5,500 kg of payload to the geostationary transfer orbit. The launcher used in this mission was added with a Yuan Zheng-1 (YZ-1) upper stage, allowing the rocket to place two satellites into their orbits. This upper stage was first tested during the BeiDou mission on March 30.