How is live broadcast from space achieved? What builds space WIFI?

On July 6, 2021, my country successfully launched a satellite named "Tianlian-1 05". Compared with the "everyone witnessed" landings of the Shenzhou XII spacecraft, Tianwen, and Zhurong on Mars, this launch mission is particularly low-key. However, the launch of this inconspicuous small satellite is closely related to the "live broadcast from space" of the first two. Let's take a look at how "Tianlian" brings us closer to "Zhurong" and "Tianwen"!

How do space and the ground communicate? Why do we use satellites to build WiFi?

Each launch and flight mission of a spacecraft requires tracking, measuring and controlling the launch vehicle and spacecraft in flight. Millions of data will be collected by the space flight command and control center, which will be processed and transformed into instructions. Among them, there are often ocean-going measurement ships installed on the ground or drifting on the sea to "bridge" the spacecraft and the ground command and control center.

However, due to the influence of the earth's curvature, the orbital coverage of low- and medium-orbit spacecraft on the ground is very limited. Except for some geosynchronous orbit satellites, other spacecraft, especially low- and medium-orbit spacecraft operating at an altitude of 400KM, need to have hundreds of tracking and control stations evenly arranged on the surface of the earth in order to "keep everything under control". Even the United States and the Soviet Union during the Cold War could not have done it. The deployment location of maritime tracking and control ships is relatively flexible, but it is difficult to cover everything due to the influence of ship preparation, sea conditions and meteorological conditions.

The time a spacecraft spends passing over China is very short, and the observation stations and communication stations within the country have limited time to maintain effective communication. To get rid of the influence of the earth's curvature, the higher the tracking and control points are, the farther they can see. Therefore, both the United States and the Soviet Union in the past and China today have set their sights on deep space and placed tracking and control stations in geosynchronous orbits tens of thousands of kilometers high.

There are two important factors for choosing this orbit: one is synchronization. The revolution period of the satellite in this orbit is the same as the rotation period of the earth. Therefore, the geosynchronous orbit satellite can communicate with fixed satellite ground receiving stations around the clock, which is very convenient. The second is that it is high enough. Most low-orbit satellites, manned spacecraft, and space stations are distributed at an altitude of 100-400km. A small number of navigation satellites and lightning orbit satellites are at an altitude of 20,000km, and the altitude of the geosynchronous orbit is as high as 36,000km. Except for a few spacecraft in the Earth-Moon transfer orbit that require extra care, all other spacecraft are under the nose of the geosynchronous orbit.

In the 1980s, the United States took the lead in launching the world's first data relay satellite; and in 2008, my country successfully launched its first data relay satellite "Tianlian-1", filling the gap in my country's data relay and space-based measurement and control.

On April 25, 2008, my country's first data relay satellite "Tianlian-1 01" was successfully launched at the Xichang Satellite Launch Center.

Source: Xinhua News Agency/Photo by Li Gang

Available WLAN 'Tianlian': Let China have a space data "router"

In 2003, in order to fully support the measurement and control needs of manned space flight, my country established and launched the Tianlian-1 relay satellite system project.

Tianlian-1 was developed by China Aerospace Science and Technology Corporation based on the mature Dongfanghong-3 general platform: as the first generation of synchronous satellites using three-axis stabilization technology, the overall design of the satellite common platform, communication channel antenna, full three-axis attitude stabilization technology, dual-component unified propulsion system and other fields were at the advanced level of similar international satellites (medium capacity) at that time. Since its first launch in 1997, by the time the development of Tianlian satellites started in 2003, 6 satellites had been successfully launched, undertaking China's public network business, VAST business and certain TV signal transmission tasks. As a mature and advanced geostationary orbit satellite, it naturally became the preferred platform for the development of Tianlian-1.

On April 25, 2008, the system's 01 satellite was successfully launched and positioned over the Indian Ocean, and then participated in and successfully completed the data relay service for the "Shenzhou 7". In 2011 and 2012, the Tianlian-1 02 and 03 satellites were successfully launched and positioned over the Western Pacific and West Africa respectively. They operate simultaneously and form a tracking and data relay satellite system with ground application systems and relay terminals. China has thus become the third country in the world to establish and is currently the only country with a global coverage relay satellite system for medium and low orbit spacecraft.

Since the design life of the Tianlian-1 satellite platform is 8 years and the design life of its subsystem is 6 years, the original Tianlian-1 01~03 satellites have been in orbit for many years and are in urgent need of replacement. Therefore, in 2016 and this year, my country launched Tianlian-1 04 and 05 satellites to replace the relay satellites that have reached the end of their service life, and participated in my country's lunar landing and Mars exploration missions.

The Tianlian-2 relay satellite is based on the Dongfanghong-4 satellite platform, and its carrying capacity and life span are greatly improved compared to the Tianlian-1.

In addition, in order to bridge the data for the first human exploration of the far side of the moon, my country launched the Queqiao relay satellite, my country's and the world's first dedicated relay communication satellite outside the Earth orbit, in December 2018. The picture shows the Chang'e-4 probe and the Yuetu rover successfully connected to the space "WALN" of China's space tracking and control. The service of Queqiao and Tianlian-2 has ushered in the "2.0" era of my country's relay satellites.

The "Road to Heaven" is busy but not congested: How to make the "Internet speed" in space faster?

In the early days of my country's aerospace telemetry, the "unified S-band measurement and control system" was used for spacecraft communication, which is also the origin of the "USB tracking is normal" that we often hear in live broadcasts. However, the performance improvement of the equipment on board each spacecraft has also made the access data larger and larger, and the original S-band bandwidth has become increasingly insufficient, so engineers have set their sights on higher-frequency telemetry systems. In terms of the characteristics of electromagnetic waves, the shorter the wavelength and the narrower the beam, the higher the frequency, the wider the bandwidth, and the faster the transmission speed. The Ka-band, which has been used in traditional satellite communications, has naturally become the first choice for space-based measurement and control.

Tianlian-1 was designed with an S/ka-band dual-feed parabolic antenna to support intersatellite links in both the S and Ka bands. As it enters the millimeter wave band, its bandwidth has skyrocketed to more than two orders of magnitude higher than the S band, and the transmission rate has also been upgraded. The downlink rate of a link in the Tianlian satellite system communication can reach 1.2Gbps. The relay satellite ground station receives space data in real time, and then transmits the data to the Beijing Flight Control Center, and then automatically distributes it according to different identifiers, with a delay of only seconds. This is also the reason why the transmission screen of Shenzhou 10 was upgraded from the "PPT" of Shenzhou 5, which was stuck and only a few minutes long, to a relatively smooth "live teaching".

Due to the requirement of high-speed transmission, the beam of the relevant antenna is very narrow. The shorter the working wavelength, the higher the shape accuracy of the antenna reflector surface is required. The antenna must have a large enough electrical size (antenna diameter/working wavelength) to achieve high gain. Tianlian-1 uses an aluminum alloy flexible mesh antenna to ensure that a large enough electrical size is obtained with a limited antenna volume. It once became the largest communication antenna carried by my country's satellites, and cooperated with the onboard closed-loop capture and tracking technology to capture and track high-speed moving user spacecraft.

The antenna of the Queqiao relay satellite is being debugged. Source: China Aerospace Science and Technology Corporation official website

The Tianlian-2 and Queqiao relay satellites use the technology of twisting extremely fine gold-plated molybdenum wire fibers with high strength, low thermal expansion coefficient and high reflectivity, which further increases the size of the antenna while reducing the weight by nearly 90%.

As my country's "Tianwen-1" heads to Mars to open the era of deep space exploration in my country, the measurement and control distance needs to extend to 800,000 to 1 billion kilometers of space. The large-capacity and ultra-long-distance measurement and control transmission capabilities required will also require a 70-meter-class antenna to support it. It is almost impossible to build an antenna of this size on the satellite. Therefore, during the period when the Queqiao relay satellite supported the Chang'e-4 lunar landing mission, my country also experimented with ultra-long-distance intersatellite laser link communication technology. With the lightweight and small laser communication terminal carried, the laser is several orders of magnitude wider than microwaves, providing a huge information exchange capacity, achieving 1Gbps-level data transmission at a rate of thousands of miles in deep space, so that my country's probes traveling on exoplanets in various places in the future can also enjoy "WIFI services".

With multiple spacecraft online at the same time, how does Tianlian enable a “shared hotspot”?

Relay satellites are high up and can see far, so they naturally need to keep an eye on more spacecraft than ground tracking and control stations. Especially with the reduction in spacecraft launch costs in recent years, the number of spacecraft in orbit will explode in the foreseeable future, which puts higher demands on the multiple access capabilities of relay satellite tracking and control networks. However, the communication bandwidth resources of relay satellites are limited, and different spacecraft have different data rates, modulation methods, bandwidths, and Doppler shifts. Therefore, how to enable multiple users to efficiently share link resources is a key issue that needs to be solved for the tracking and control network to support services for multiple spacecraft.

Schematic diagram of simulation of different users' aircraft sharing the measurement and control network

In response to the problem of limited number and capacity of relay satellite payloads, a more efficient on-demand access system for the space-based tracking and control network was designed for Tianlian. A hybrid free/on-demand allocation strategy (CFDAMA) was used to allocate tracking and control resources. When the channel load is low, time slot resources are mainly obtained through free allocation. When the channel load is high, time slots are mainly allocated by sending resource reservation requests on demand.

Normally, the relay satellite transparently forwards the data packet to the corresponding relay satellite ground station. The relay satellite ground station directly forwards the data packet to the ground resource dispatch center through the ground network, and searches for the available communication time period between the user aircraft and the current relay satellite according to the connection sequence between the user aircraft and the relay satellite, generates the corresponding data transmission service instruction, triggers the timer at the scheduled time point and sends the data transmission service instruction to the user aircraft. After receiving the instruction, the user aircraft performs the data transmission service backhaul. Through this set of operation methods, the utilization rate of the "Tianlian" channel can be maximized, and in theory, it can help 256 user aircraft to efficiently share link resources.

The launch of "Tianlian" and the cooperation with ground tracking and control stations at home and abroad and the Yuanwang survey ship further expanded my country's space tracking and control network, and also let us understand that human space activities have been upgraded from the simplest "short message broadcast" to "video live broadcast", making the distance between space and us no longer far away. With the explosive increase in the number of satellites, the initial formation of space station construction, and the continuous acceleration of deep space exploration, the demand for the communication and service capabilities of relay satellite systems will also increase day by day. In the future, my country will have more and more powerful "Tianlian" networks to provide spacecraft with improved, faster and farther communication services.