1.3.1The current situation and development trend of space-based cooperative transmission system
With the development of aerospace technology, the types and functions of space platforms using satellites as backbone networks are becoming more and more perfect. Different satellite systems separately used for information acquisition, transmission, and processing in space are connected organically by space-based transmission technology, and thus, a satellite-based space information network has been established. The characteristics of flexible networking, wide coverage, fast network construction, and no geographical restrictions make satellite networks have significant advantages in longdistance wireless communications. Many countries have carried out related research projects. For example, NASA and the US Air Force’s Advanced Extremely High Frequency (AEHF) military communications satellite and Transformational Satellite Communications System (TSAT) projects19 can achieve rapid information acquisition on a global scale. In addition, the German Aerospace Center proposed the TanDEM-X program,20 the French Space Agency proposed the CartWheel program,21 Italy proposed the BISSAT program,22 and Canada proposed the RadarSat-2/3 program.23
The AEHF satellite system24, 25 is a project of the US Department of Defense. It aims to provide a new generation of strategic and tactical communications satellites and ground-matching systems that can be used for military conflicts at all levels with global precision, high confidentiality, high communication capacity, and high survivability for the United States and its allies. In addition to the existing techniques of spread spectrum, frequency modulation, inter-satellite link, and on-board processing on the “military satellite”, the AEHF space segment satellite also uses phased-array antenna technology and beamforming technology.26 Phased-array antenna technology can change the direction of the RF beam by electronic means to make the beam between users jump instantaneously, thereby improving the transmission efficiency and flexibility. The beamforming network can use the automatic zeroing method to suppress the interference signal while providing services for legitimate users.
On June 21, 2010, the successful launch of the German radar satellite TanDEM-X symbolized the beginning of a new era in the global digital elevation model (WorldDEM). TanDEM-X and TerraSAR-X together form a high-precision radar interferometer that can acquire basic data for homologous WorldDEM. The two satellites form a unique satellite formation, flying in a tightly controlled spiral formation. They are very close with a minimum relative distance of only a few hundred meters. The main task is to produce a WorldDEM with good quality, high precision, and wide coverage. The accuracy of the WorldDEM will be higher than any existing satellite-based WorldDEM and has the following unique advantages: (1) relative vertical accuracy of 2 m and absolute vertical accuracy of 10m, (2) 12m × 12m scanning raster, (3) global homology, (4) no ground control information is required. The TerraSAR-X and TanDEM-X binary star systems developed by the German Aerospace Center also form flexible beam pointing through active phased-array antenna technology to provide array gain. Although it can improve the signal power and transmission efficiency to a certain extent, it does not significantly increase the channel capacity and is largely subject to the load and power of the satellite.27
The TSAT program28 was proposed by the US Air Force. Its core mission is using the virtual radar array established by cooperative communication of formation satellites to perform tasks such as passive radio radiation measurement, navigation, and communication (mobile tactical communication), thereby verifying that the formation satellites have the ability to achieve effective multitasking through cooperative communication.29 The satellite can achieve high-capacity global communications and can deliver highly mobile, over-the-horizon, and protected communications to thousands of users by the use of new technologies such as laser links and Internet Protocol (IP). It provides tactical users with medium-rate communication capacity and also provides greater capacity of connectivity to the onboard intelligence, surveillance, and reconnaissance platforms. Due to funding and other reasons, TSAT program was on hold in 2009, but its concept of global networking and building spatial information network has not disappeared. The technology of space routers accumulated in the early stage of research continues to develop.
In the same period with TSAT program, the Applied Physics Laboratory at Johns Hopkins University and the US National Security Space Office proposed the idea of performing military missions in the geostationary orbit (GEO) with separation modules, namely the space-based group, to overcome the shortcomings of large mass, complicated technology, high cost, long developing cycle, and difficult maintenance of the complex large satellites loaded with multiple payloads.30 The space-based group uses a main satellite to provide core services such as the space–ground link for the group and uses other low-cost, low-tech, task-specific sub-satellites and a main satellite to form a satellite group to perform tasks such as communication and remote sensing. And the group also includes on-orbit service satellites to provide support for satellite life extension and system reconstruction. Its key technologies include a high-speed low-power wireless network technology. Due to the working distance of only a few kilometers, the size, quality, and power consumption of wireless network equipment are exponentially lower than traditional satellite–ground links. It also combines IP routing technology with wireless networking and is equipped with a plug-and-play interface, which will be verified on-orbit. In terms of on-orbit service technology, the US Department of Defense Advanced Research Projects Agency (DARPA) has used the “Orbital Express” project to conduct tests such as fuel filling and equipment replacement. Recently, DARPA launched a research project codenamed “Phoenix”, also known as the “Zombie Satellite” program. The project aims to recycle the scrapped satellites that become space debris and integrate the components especially antennas of space debris to form an antenna array, which will eventually become a low-cost “communication center”. It can provide information services for the ground US military, realize the reuse of space resources, and reduce the cost of space development. The program will first launch a geosynchronous satellite (GEO) and then launch a series of small satellites that will work as controllers for the specified moving position. A robotic manipulator carried by the GEO will mount the recovered antennas on the launched small satellites. Finally, recyclable parts removed from space debris can form a “zombie antenna array”.
In order to build a future-oriented, flexible, and efficient spacecraft architecture, the DARPA proposed the F6 program.31–33 It is based on the idea of decomposing the traditional spacecraft into multiple combinable separation modules of different tasks and functions. These separate spacecraft modules can be mass-produced and independently launched on the ground. When operating normally in satellite orbit, they work together by the means of formation flight, wireless data transmission, and wireless energy transmission to combine the discrete modules into one complete unit. This “space-based group” transmission technology based on separation modules provides a new idea for the development of satellite communication systems.
Through the analysis above, the characteristics of the development trend of foreign space-based transmission systems can be summarized as follows:
(1)from single satellite to spatial information network;
(2)from completing the complex function by a single satellite to completing the complex function by a cooperative satellite group consisting of satellites with multiple functions;
(3)from adopting phased-array antennas to improve the received signal-to-noise ratio to adopting active antenna arrays to improve the channel capacity and transmission efficiency and achieve spatial multiplexing gain.
1.3.2The basic principle of space-based cooperative transmission system
We will introduce the basic principles of spatial multiplexing gain obtained by the space-based cooperative transmission system. Based on the active antenna array, we take the downlink between a ground receiver consisting of ME