Relay satellites: making manned space flight and deep space exploration more convenient

On July 13, 2022, my country's Long March 3B carrier rocket sent the Tianlian-2-03 satellite into the predetermined orbit, marking the successful networking and operation of my country's second-generation geosynchronous orbit data relay satellite. It can meet the global coverage of medium and low orbit spacecraft and provide 24-hour uninterrupted communication.

Schematic diagram of the second-generation satellite system network (Source: China Aerospace Science and Technology Corporation Fifth Academy)

What is a relay satellite?

Tracking and data relay satellite, also known as relay satellite, is a type of satellite communications satellite whose main task is to provide relay communication services for spacecraft.

The reason for the emergence of such a satellite is mainly due to the demand for the development of manned space flight. As we all know, manned spacecraft and space stations operate in low-Earth orbit, so no matter how densely the ground stations are deployed, it is difficult to ensure 24-hour uninterrupted communication connection. Manned flight is a risky business. Even a few minutes of communication interruption can scare the ground control personnel, and the lack of such a communication connection will also result in incomplete telemetry data. The main reason for manned flight communication interruption is that the earth itself blocks the line of sight between the ground station and the spacecraft. So, if a batch of satellites are deployed in geostationary orbit, allowing the spacecraft to communicate with these satellites and then transfer the information to the control center on the ground, the phenomenon of communication interruption can basically be eliminated.

The U.S. relay satellite system

The world's first tracking and data relay satellite was NASA's Tracking and Data Relay Satellite System (TDRSS). NASA created the Spacecraft Tracking and Data Acquisition Network (STADAN) in the early 1960s, which consisted of antennas and telephone switching equipment distributed around the world. The orbital period of low-orbit spacecraft is about 90 minutes, and this network can provide 15 minutes of ground-to-space communications services. In order to achieve full coverage of manned space flight, NASA established a Manned Space Flight Network (MSFN) to serve the Apollo mission together with the Deep Space Network (DSN).

After the establishment of the space shuttle program in the 1970s, NASA found that the above networks had overlapping functions, so it merged MSFN and STADAN into the Spacecraft Tracking and Data Network (STDN). However, STDN also has great disadvantages, especially the network is composed of ground stations all over the world, which is easily affected by political changes in the host country. In order to maintain high reliability and higher data transmission speeds, NASA began a study to use space-based communication nodes to enhance system capabilities. This is the origin of TDRSS.

In the 1980s and 1990s, the United States deployed a large number of TDRSS satellites using space shuttles and launch vehicles. Currently, the United States has more than 10 TDRSS satellites in orbit.

Relay satellites are different from traditional communication satellites. Although the ground station is fixed, the spacecraft in the medium and low orbits is moving at high speed. It is necessary to solve the problem of accurate capture and tracking between the spacecraft, the space station and the synchronous orbit satellite. At the same time, in order to ensure sufficient communication bandwidth, the relay satellite also needs to use a large-aperture tracking antenna to realize the inter-satellite link, which puts extremely high demands on the capture and tracking capabilities. Such a large tracking antenna needs to shake constantly, which also poses a great challenge to the satellite's attitude maintenance.

China's Tianlian satellite system

China began researching relay satellites in the early 1980s. In 2003, China began developing the Tianlian-1 relay satellite, which used the Dongfanghong-3 satellite platform. In April 2008, Tianlian-1-01 was successfully launched. This satellite immediately played a significant role, and the tracking and control coverage of the Shenzhou-7 spacecraft increased from 18% of the previous six spacecraft to 50%.

With the successful launch of Tianlian-1 02 in July 2011, the orbital coverage of the spacecraft reached 85%. In July 2012, Tianlian-1 03 was successfully launched, and three Tianlian-1 satellites were deployed over Africa, the Indian Ocean, and the Pacific Ocean, achieving nearly 100% coverage of the spacecraft orbit. In December 2016, Tianlian-1 04 was successfully launched to replace 01. In July 2021, Tianlian-1 05 was successfully launched. The launch of this batch of satellites makes China the second country after the United States to have a relay satellite system with global coverage capabilities. Tianlian-1 provides data relay and measurement and control services for manned spacecraft such as the Shenzhou spacecraft, space laboratory, and space station, especially supporting space rendezvous and docking missions, playing a huge role.

In 2010, China launched the development of the Tianlian-2 satellite. Tianlian-2 uses the large-sized Dongfanghong-4 platform, which greatly improves the payload capacity and power generation capacity. As a result, Tianlian-2 can use more advanced payload technology and multiple new antennas, and the transmission rate has increased by 100%. Tianlian-2-01 was launched in March 2019, and Tianlian-2-02 was successfully launched in December 2021.

The Tianlian satellite system has served the Shenzhou VII to Shenzhou XIV manned spacecraft, the Tianzhou I to IV cargo spacecraft, the Tiangong I target spacecraft and the Tiangong II space laboratory. The construction and operation of the space station are also inseparable from the support of the Tianlian series of satellites. The conversations between the central leaders and the astronauts and the "space lessons" in the space station are also carried out with the support of the Tianlian series of satellites.

Simulation image of Tianlian-1 05 satellite (Source: Fifth Academy of China Aerospace Science and Technology Corporation)

Data relay satellites can also support various other space activities, such as launch control services for carrier rockets, on-orbit control and data transmission services for various remote sensing satellites, etc. For example, ChinaSat 6D, launched in April 2022, is China's first commercial satellite to use relay control technology. Its control information is transmitted back to the ground station via the Tianlian satellite, reducing its dependence on overseas control sites and measurement ships. At present, Tianlian satellites also provide services for medium and low orbit remote sensing, mapping, meteorological and other satellites, and are very busy every day.

Relay satellites in deep space exploration

China also has another type of data relay satellite, the "Queqiao" satellite deployed in lunar orbit. The Chang'e-4 probe will land on the back of the moon, and the moon blocks the line of sight between the landing point and the earth. If the data relay method is not used, the ground will not know what is happening to Chang'e-4. The Queqiao satellite is deployed at the Earth-Moon L2 point, where it performs periodic motion and maintains the stability of its orbit through regular orbit control. Ground measurement and control commands are transmitted to Chang'e-4 through Queqiao, and the videos and collected data shot by Chang'e-4 and Yutu-2 are first transmitted to the Queqiao satellite and then sent back to the earth.

In May 2018, the Queqiao satellite was launched, achieving a breakthrough in the history of human spaceflight. On January 3, 2019, the Queqiao relay satellite successfully supported the Chang'e-4 lunar landing and separation mission, which shocked the world. NASA once proposed whether Queqiao could provide support for the United States' lunar landing activities. However, due to the legal restrictions of the U.S. Congress, this could not be realized.

There are also a number of data relay satellites active in the orbit of Mars. However, these satellites are not dedicated, but carry data relay payloads on the orbiting probes, including China's Tianwen-1 and Europe's Trace Gas Detector, which have such functions. They assume the data bridge function between the lander and the Earth. In future interstellar exploration, it will become a normal practice for orbiters to take into account the data relay function.