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Shentong, officially known as Zhongxing 20 or ChinaSat 20, is described as a “strategic communications” satellite and was launched in November 2003. It is reportedly the first Chinese satellite to use mul-tiple steerable Ku-band spot-beam antenna technology, which enables ground users to communicate at high data rates while on the move, and the first to use secured uplink transmission for satellite antenna con-trol; it reportedly has China’s most powerful onboard data-processing capability (“Feng Huo [Zhongxing 2X/ShenTong Series],” 2010).
China launched its first data-relay satellite, Tianlian 1, in April 2008, putting it into GEO at 77 degrees east. Tianlian is based on a DFH-3 bus. It was launched in preparation for the Shenzhou 7 manned mission, which occurred in September 2008, and is reportedly the first in a new network that will augment or replace China’s current ground-based space-tracking and telemetry stations and space-track-ing ships. Tianlian is referred to as a “space station” for relaying data (“China Sets Up First Space Station for Spacecraft Data Relay,” 2008). China’s space-tracking ships and ground stations were able to cover only 12 percent of Shenzhou 7’s orbit, according to Zhang Jianqi, a top Chinese space official quoted by Xinhua (“China Blasts Off First Data Relay Satellite,” 2008).
In addition to providing communications with manned space-craft, data-relay satellites can also provide relay services for other sat-ellites. Because of the curvature of the earth, a satellite in LEO at an altitude of 600 km, such as China’s Yaogan series of military reconnais-sance satellites, can communicate only with ground sites within about 2,800 km of its location. If there are no ground stations within that distance at the time the satellite collects an image, it must store the data until it comes within range of a ground station. For example, a Chinese reconnaissance satellite passing over Guam would have to store any images it collected until it passed within range of a Chinese ground station. A satellite in GEO, however, can communicate with a ground station or other satellite virtually anywhere on about one-third of the earth’s surface. Thus, a reconnaissance satellite in LEO could transmit images it collects up to a satellite in GEO, which could then imme-diately relay them back down to a ground station in China, where they could be processed (if the reconnaissance satellite was far away from China, more than one relay satellite might be needed). Tianlian 1 is located off the southern tip of India and thus is not ideally posi-tioned to act as a relay for Chinese satellites passing over the western Pacific region, but in the future, China will likely deploy additional relay satellites.
Dongfanghong 4
Dongfanghong 4 is a high-capacity communications satellite, also built by CAST. This design has experienced significant technical prob-lems. The first example, SinoSat 2, was built for Sino Satellite Commu-nications, a Chinese state-owned company now absorbed into China Satcom. Sinosat 2 was launched in October 2006, but its solar panels failed to deploy, and the satellite was a total loss. The second example, NigcomSat, was built for the Nigerian government and launched in May 2007. The drive on one of its solar arrays failed, and the satel-lite is apparently no longer operational. The third example, Venesat 1, was built for the Venezuelan government. It was launched in October 2008 and is apparently operating normally. The fourth and most recent example, ChinaSat 6A (also called Sinosat 6), was launched on Sep-tember 4, 2010, to act as a backup for ChinaSat 5C (described above). However, ChinaSat 6A suffered a leak in its helium-pressurization system that will likely shorten its operational life from a design goal of 15 years to at most 10 years (Covault, 2006; “NigComSat,” 2010; “UCS Satellite Database”; “Venesat (Simon Bolivar),” 2010; “Chinasat/ Zhongxing/STTW Series,” 2010; de Selding, 2010).
　
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