Tianlian — Wikipedia

Tianlian (in Chinese Celestial link ) is a family of telecommunications satellites of the People’s Republic of China. These satellites, placed in geostationary orbit, ensure the relay between the spaceships in low orbit and the land stations. They are more particularly designed to ensure voice and data links between the ground and the space vehicles placed in orbit for the habid space program of China.

The first two satellites launched the April 25, 2008 and the July 11, 2011 make it possible to ensure coverage of 85%. Three other satellites were launched respectively in 2012, 2016 and 2021, and the fourth is launched the November 22, 2016 . A second generation using a more modern platform is deployed from 2019.

When a vessel is in low orbit, due to the attitude and therefore the proximity of the horizon, the contact with a terrestrial station can only be very brief. It is necessary to have a very large number of terrestrial stations to ensure continuous coverage which is on the one hand expensive and in practice unrealizable above the most important oceans. The need for a continuous radio link is particularly important for inhabited space missions. Also the nations, which have such a program, have deployed a network of specialized telecommunications satellites located in geostationary orbit which constantly ensure the connection between the vessels and the soil: the radio program is issued by the vessel towards the geostationary satellite which sends it back to the ground (and vice versa). Three satellites of this type are enough to ensure coverage of 100% (excluding polar regions). NASA has deployed the family of TDRSS satellites, the Soviet Union redeployed its Loutch satellites and Europe despite more limited needs place in orbit useful EDRS loads. China began to deploy Tianlian satellites in 2008 to meet the growing needs of its own inhabited space program.

First generation [ modifier | Modifier and code ]

First generation TL-1 Tianlian satellites use the DFH-3 platform developed in 1994 and used by several dozen Chinese telecommunications satellites. These satellites are built by the company Aerospatiale Cast de Shanghai. It should be replaced by a second generation using a larger DFH4 platform and longevity ^{[ first ] , [ 2 ]} .

TL-1 Tianlian satellite has a mass of 2.1 tonnes. The platform has the shape of a hexahhedron of 2.2 x 1.7 x 2 m. It is stabilized 3 axes and its solar panels provide 1,700 watts. Its lifespan is at least 8 years. Its payload covers a large number of types of services: fixed, international telecommunications, inter -regional links, large -band communications, communications between mobiles, secure military communications and relays between satellites and ground stations (especially for the Chinese habited space program) ^{[ 3 ] , [ 2 ]} .

Second generation [ modifier | Modifier and code ]

The second generation of Tianlan satellites, whose deployment started in 2019, uses a DF-4 platform which significantly increases communication capacities. With a mass of 3.2 tonnes, the satellite has a lifespan of 15 years. Its solar panels providing 10.5 kW increases communication capacities. The links are made in strip C, KU, the and L. The precision of the orbital position is improved (0.05 °) as well as the score accuracy (0.1 °) ^{[ 4 ] , [ 5 ]} .

The satellites of the Tianlian program play a central role in the inhabited space program of China by allowing a continuous connection between the vessels and the space stations on the one hand and the ground stations on the other. The realization of a continuous connection with only ground stations would require a very dense network of stations at sea and on land given the reduced visibility of each station (10 minutes of continuous link per station) ^{[ 6 ]} .

Update March 4, 2023

Five copies of the first generation of these satellites were deployed in orbit respectively in 2009, 2011, 2012, 2016 and 2021 and are operational on this date. They were all placed in geostationary orbit by a 3C long walking rocket from the Xichang launch base ^{[ first ]} . Three copies of the second generation of Tialan were placed in orbit in 2019, 2021 and 2022 (situation in February 2023) ^{[ 4 ]} .

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