What does a data relay satellite do? How did it develop?

The development of data relay satellites requires mastering many unique key technologies, which is extremely difficult, so its development is relatively slow. It took the United States about 20 years from proposing the relay satellite concept to launching the first generation of relay satellites, and another 17 years to launch the first satellite of the second generation constellation.

In general, the development path of data relay satellite technology is to adopt more advanced satellite platforms, upgrade antennas, adopt new radio frequency band technologies and link modulation systems, etc., to enhance the comprehensive capabilities of satellites and provide users with more types of services with stronger functions. At the same time, different application scenarios such as near-Earth space missions, deep space exploration missions, near-space, low-altitude ultra-high-speed flights, etc. have put forward different service requirements for relay satellites, promoting the development of relay satellites from traditional full-function types to professional types. Through innovative system architectures and the adoption of new network technologies and constellation technologies, major space powers continue to promote the construction of a new generation of data relay satellite systems, complete the relay satellite system, and meet the needs of future space missions.

Relay satellites serve space stations and other spacecraft

Driving satellite performance improvement through technological development

The main payload of data relay satellites is antenna arrays. The development of traditional full-function relay satellite technology is mainly reflected in the improvement of single-address channel performance, antenna configuration, data transmission rate, etc.

The payload of the first generation of relay satellites in the United States includes an S-band multi-access link phased array antenna composed of 30 helical antenna units and two 4.9-meter-diameter rotatable S and Ku dual-band single-access link parabolic antennas. The intersatellite link works in the S and Ku dual-bands, and the satellite-to-ground link works in the Ku band. The S-band multi-access forward and return rates are 300 kilobits per second, the single-access maximum transmission rate is 10 megabits per second, and the Ku-band single-access forward rate is 25 megabits per second and the return rate is 150 megabits per second.

After the transition from the second-generation system, the performance of the United States' third-generation relay satellite has been greatly improved. It adopts Boeing's BSS-601HP platform, and is also equipped with two single-address antennas and one multiple-address phased array antenna. The single-address antenna provides Ku, Ka and S band communications. The phased array antenna adopts the new S-band multiple-access antenna technology, and introduces new modulation forms such as low-density parity-check code, Turbo product code, and 8PSK modulation. The Ka-band transmission speed for single-address users reaches 800 megabits per second.

Laser communication technology can greatly increase the data transmission rate, realize the miniaturization, lightness and low power consumption of communication payloads, improve receiving sensitivity, and has the advantages of good confidentiality, strong anti-interference and anti-interception capabilities. It is an important direction for the development of relay satellite technology. Due to the great influence of atmospheric turbulence, laser communication is mainly used in intersatellite links and has entered the practical stage. The fourth-generation relay satellite development stage of the United States has clearly added laser links, and its laser terminal transmission rate can reach 72 megabits per second to 2.88 gigabits per second. When applied to relay satellites in the future, the rate is expected to exceed 10 gigabits per second.

Relay satellites use lasers to transmit data

ESA is a world leader in the field of relay satellite laser link technology. The first technical experimental relay satellite launched in 2001 was equipped with a laser communication payload. Since April 2003, it has provided high-speed data transmission services for the French SPOT-4 optical satellite and the ESA Envisat radar satellite. The SPOT-4 satellite uses a laser relay link.

Since 2016, ESA has launched and deployed the second generation of relay satellites. Among them, the EDRS-A satellite is equipped with two single-address antennas, which provide laser and Ka-band intersatellite communication links respectively, with a laser return data transmission rate of up to 1.8 Gbit/s and a Ka-band return data rate of up to 300 Mbit/s; the EDRS-C satellite only has a laser communication terminal, and its indicators are the same as those of the EDRS-A satellite.

Japan's satellite laser communication technology has developed very rapidly. In 2002, Japan launched the first generation of data relay satellites, equipped with S and Ka band intersatellite links and a transmission rate of 240 megabits per second. In 2020, Japan launched the second data relay satellite, directly entering the optical communication stage, using a combination of optical communication and radio frequency communication, jointly using lasers and S and Ka bands, and jointly establishing relay links with low-orbit reconnaissance satellites such as the "Advanced Optical Satellite", with a laser communication rate of 1.8 gigabits per second.

As cognitive radio and software-defined radio technologies mature and the capabilities of onboard devices improve, relay satellite links will be adaptively adjusted in real time based on interference conditions, atmospheric environmental conditions, etc., to achieve the best match between link parameters and environmental conditions, increase data transmission capacity, and upgrade and improve link modulation systems at any time through software loading according to system usage.

Building a new data relay satellite system architecture

Let’s first take a brief look at the current status of relay satellite systems in various countries.

The United States is currently developing a third-generation relay satellite system. Its first-generation relay satellites TDRS-1 and TDRS-4 have been deorbited and scrapped, while the remaining four satellites are still in orbit. The second-generation and third-generation satellites, each with three satellites, are operating normally. A total of ten relay satellites are in orbit, building the world's most complete and largest-scale relay satellite system, achieving global coverage, with user spacecraft accessing the system nearly a thousand times a day.

Russia's relay satellite system currently has three second-generation "Ray" satellites in orbit, stationed at three nodes in the east, middle and west (167 degrees east longitude, 95 degrees east longitude, 16 degrees west longitude), achieving quasi-global coverage, with user spacecraft accessing the system nearly 100 times a day on average.

ESA's relay satellite system consists of two geosynchronous satellites and a ground system. The satellites are stationed near 9 degrees east longitude and 31 degrees east longitude to achieve regional coverage. User spacecraft access the system dozens of times a day.

Japan has one relay satellite in orbit, achieving regional coverage, with user spacecraft accessing the system dozens of times a day. In December 2021, China's Tianlian-2-02 satellite was launched into space, working in conjunction with the Tianlian-1 constellation and Tianlian-2-01 satellite. As a result, China has launched seven relay satellites, building the world's second relay satellite system with global coverage.

It must be recognized that due to differences in application requirements among countries, the scale and architecture of relay satellite systems vary greatly.

Relay satellite fully deployed

The United States operates the world's largest space and ground-based aerospace infrastructure. To support space missions operated by the country and international partners, the huge demand has spawned the largest space-based relay system, providing measurement, control and communication services to various users, especially to meet the full-time measurement, control and communication support needs of manned spaceflight. In order to achieve the integration of ground-to-space measurement and control and keep consistent with the measurement and control frequency band of the ground measurement and control network, the United States focuses on the development of S-band measurement and control technology.

During the Soviet era, a vast space infrastructure was built, manned spaceflight was vigorously developed, and a large-scale data relay satellite system was established. However, after 2000, due to funding constraints, Russia's space activities shrank, and the number of relay satellites also decreased significantly.

Compared with the United States and the Soviet Union in the past, ESA has fewer space infrastructure facilities and cannot independently carry out manned space missions. It develops relay satellites mainly to provide high-speed downlink data links to satellites and low-speed downlink services for satellite, manned spacecraft and launch vehicle tracking. The development focus is on the application of microwave and optical communication technologies in intersatellite links.

On the other hand, with the development of satellite technology, in addition to traditional full-function types, data relay satellites are also developing in the direction of distributed and specialized types.

Neither ESA's EDRS-A nor EDRS-C are independent data relay satellites: EDRS-A is a functionally independent relay payload carried on a communications satellite; EDRS-C is both a relay satellite and a communications payload with other functions, sharing the same platform. The combination of ESA's dedicated satellite and multifunctional payload makes the relay satellite system more flexible and diverse, and improves system flexibility.

With the development of small satellite technology and the gradual maturity of the distributed space-based system construction ideas, the United States proposed that the next generation of data relay satellites will not integrate all new technologies into one satellite, but will implement different services on multiple spacecraft under a new architecture. This "separation" approach can independently supplement the existing satellite service capabilities and provide new services based on demand and technology maturity.

Therefore, the next generation of relay satellites may develop in multiple professional directions such as full-function, relay and mobile, measurement, control and navigation, high-speed data relay and communication, etc., and provide data transmission, relay and other services in the form of constellation networking. Expanding interplanetary communication relays In deep space exploration activities, due to the extremely long distances, the weight and power of the probe are greatly limited. The development of dedicated relay satellites is a necessary way to improve the detection capability. In addition, in some specific detection areas and time periods, the probe cannot directly contact the earth. For example, the back of the moon is always facing away from the earth, and the South Pole of the moon is invisible from the earth for about half of the time; the orbital planes of planets such as Mars and Jupiter are close to the earth and operate with different revolution periods. There will be a period of solar obstruction and interference for several consecutive months, and the probe will lose contact with the earth for a long time. Launching dedicated relay satellites is the key to solving these problems.

The Chang'e-4 mission achieved the first soft landing of the probe on the far side of the moon. The prerequisite for success was the launch of the Queqiao relay satellite. In May 2018, the Queqiao relay satellite was launched and entered the Halo orbit around the Earth-Moon L2 point. This orbit is located on the Earth-Moon extension line, more than 400,000 kilometers from the Earth and 65,000 kilometers from the Moon. The gravity and centrifugal force of the two celestial bodies of the Earth and the Moon are ingeniously balanced here. The Queqiao satellite can maintain stability with only a small amount of fuel, while "seeing" the far side of the moon and the Earth at the same time.

On January 3, 2019, with the support of Queqiao, the Chang'e-4 probe successfully landed in the designated area on the far side of the moon, and transmitted the close-up view of the far side of the moon and the working images of the Yutu-2 lunar rover back to the ground. During the entire mission, Queqiao built a "lifeline" for the earth-moon communication of Chang'e-4, ensuring the smooth communication and data transmission links, ensuring the smooth startup of the lander and rover payloads, carrying out scientific experimental projects, and transmitting a large amount of scientific exploration data back to Earth.

In China's Mars exploration mission, the Tianwen-1 probe's orbiter is equipped with a 2.5-meter-diameter high-gain antenna. After separating from the lander and rover, it adjusted to enter the relay communication orbit and carried out a relay communication mission lasting approximately three months, establishing a communication link between the lander and the Earth's tracking and control station, and transmitting images of the Martian surface back to the ground.

In November 2021, the China-Europe Mars rover conducted an on-orbit relay communication test, with the Zhurong Mars rover sending test data to the European Space Agency's Mars Express orbiter, which then forwarded the data to the European Space Agency's deep space tracking and control station and then to the Beijing Aerospace Flight Control Center. The test was a complete success. The Tianwen-1 probe orbiter played an indispensable role during this test.

ESA's Mars Express serves as a communications relay satellite between Earth and Mars

Human beings are gradually entering the era of "deep space exploration", and exploration of the sun, moon, Mars, Jupiter, and the edge of the solar system is in full swing. Interplanetary measurement, control, communication, and navigation are important issues facing mankind in exploring the starry sky.

Through the construction of space communications and navigation networks, the United States has acquired comprehensive space communications and tracking capabilities and achieved all-day deep space communications capabilities. By continuously launching relay satellites to serve missions such as the moon and Mars, it will gradually cover the solar system and ensure communication connectivity between the Earth and the Moon, and the Earth and Mars at any time and any location.

China is developing and launching a universal interstellar relay communications satellite constellation to create a global deep space exploration communications infrastructure and pave the way for humanity's "interstellar Internet." In the future, it is expected to provide seamless commercial measurement, control, communication and navigation services for all types of deep space vehicles from Venus to the asteroid belt and even the orbit of Jupiter.

In the future, the interstellar communication relay network will become the basic support for humans to travel through the starry sky.