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Introduction

1.1 Connotation of Inter-Satellite Link

With the rapid development of aerospace technology, national interests are gradually expanding beyond the traditional territory, territorial waters, and airspace, expanding and extending to the ocean, space, and electromagnetic space. With the rapid development of space technology, space has increasingly become a new source of international strategic competition commanding heights. Space technology embodies the political strength, economic strength, and scientific and technological strength of a country, and has become a strategic means for countries to demonstrate the progress of their national strength and defend their international status. In order to safeguard our maritime rights and interests, defend our space rights and interests, and ensure the expansion of national interests, China must focus on a global perspective and develop satellite navigation, communication, and other systems on a global scale. To this end, it is necessary to use space-based networking methods to break through the limitations of land and solve problems such as satellite full-orbit operation management, constellation autonomous operation, and rapid response to complex tasks, in order to ensure the development of space systems [1].

Satellite technology is the primary breakthrough for occupying space resources. After going through two stages of single-satellite application and constellation application, the satellite field has gradually moved toward networking. The networking between satellites must first require that the information and data between satellites can be interconnected. Considering factors such as security and national strategy, the number of satellite ground stations that can be established is very limited, and most of them are limited to the country. When the ground station is strictly limited, the establishment of an inter-satellite link has become one of the most important necessary conditions for inter-satellite networking [2, 3].

Once a communication network is established between satellites through inter-satellite links, the satellites are no longer isolated individuals, but a whole with a considerable scale, and satellites can only complete a small number of tasks to complete a general a lot of work. Under a good inter-satellite link network, the satellite not only has more powerful functions, but also its robustness and anti-interference have also been greatly enhanced.

The constellation network is a very complex space network. This is because in the inter-satellite link network, there are not only a large number of space nodes, but also many ground nodes are also part of the network, the composition is complex and changeable, the access is flexible and irregular, with typical flat and centerless features. These features require a high degree of flexibility and adaptability to build inter-satellite networks. In addition, the inter-satellite link is not a single, pure communication link. While carrying a certain rate of communication tasks, it may also need to complete high-precision inter-satellite measurement functions at the same time. In the navigation system, the inter-satellite link needs to meet the core requirements of precise orbit determination and time synchronization of the system, autonomous navigation applications, etc., and support the use of satellite-ground joint orbit determination and the transmission capability of measurement data for autonomous navigation. The construction put forward higher requirements.

Inter-satellite link (or crosslink) refers to the link between satellites and can also be extended to the link between spacecraft. The inter-satellite link can perform functions such as inter-satellite communication, data transmission, inter-satellite ranging, and inter-satellite measurement and control. Different space systems have different functions of inter-satellite links. The inter-satellite link of the communication satellite constellation can reduce the number of satellite-ground hops and communication delay; the inter-satellite link of the reconnaissance formation system can increase the aperture of the virtual camera and improve the resolution; the inter-satellite link of the navigation satellite constellation can support autonomous operation and improve the resolution. Positioning accuracy, the inter-satellite link of the relay satellite system can increase the measurement and control arc of the user satellite. In addition, there are some inter-satellite links used in scientific research, such as gravity detection satellite systems. The inter-satellite link makes multiple satellites form an organic whole to form a constellation system and expand the ability of a single satellite to work [4].

At present, distributed satellite systems mainly include two types: formation satellite constellation and formation satellite. A typical constellation includes the sharing of scientific data through inter-satellite links between satellites in orbits of planets or the sun. They do not rely on each other to complete autonomous onboard navigation corrections, which are transmitted through ground stations. The formation satellites need to rely on inter-satellite links to transmit navigation data to achieve fully autonomous navigation. At the same time, formation satellites also need to ensure strict positioning accuracy to meet the scientific goals of formation missions. One or several spacecraft in the formation have the function of navigation and processing, and maintain the formation or ensure a certain topology structure through the transmission of data and instructions. At the same time, the state information of the spacecraft also needs to be transmitted by inter-satellite links.

Figure 1.1 divides formation satellite systems into different classes from an inter-satellite link perspective. There are two kinds of communication links between formation satellites: satellite-ground link and inter-satellite link. If satellites transmit information through satellite-ground links, direct communication between spacecraft is generally not required, but data are collected and processed on the ground, and then integrated into scientific or navigational information to be transmitted back to the spacecraft. Usually, this method is very dependent on the ground and can be regarded as centralized control, that is, a star-shaped distributed system. Some constellations exchange and process data entirely through satellite-ground communication. Unlike constellations, formation satellites use inter-satellite communications to exchange navigation data and commands. In addition, the topological structure of the communication network of formation satellites is mainly divided into two types: star type and point-to-point type (Figure 1.1).

Figure 1.1 Distributed spacecraft communication structure division.

The use of inter-satellite links has many advantages, mainly including:

1. For communication satellites, when users who are not within the coverage area of the same satellite need to communicate, the use of inter-satellite links can eliminate satellite double hops and reduce the propagation delay. At the same time, a relay earth station dedicated to relaying signals between users in different satellite coverage areas is omitted [2, 5].
2. When the system is a constellation composed of many satellites, the use of inter-satellite links to form a complete communication network for all satellites is not only independent of the ground, but also greatly improves the system's anti-interference and anti-destroy capabilities.
3. It can be used to expand the coverage of the system. Multiple satellites are linked together through inter-satellite links, and users within the coverage area of any satellite can communicate directly with users within the coverage area of other satellites through the inter-satellite link.
4. Facilitate network management and form a global seamless network. For some low-orbit satellites, there may be no fixed earth station visibility at all in some cases (e.g. in the middle of the Pacific Ocean) due to their small coverage. At this time, it is almost the only solution to use the inter-satellite link to realize the control of the satellite on the ground and the mobile user to access the ground communication network through the inter-satellite link.
5. It is convenient to form a satellite group at the same orbital position, which is very useful for high-orbit satellites. Due to the increasing traffic volume of communications and the limited orbital positions of high-orbit satellites, it is bound to hope to make more effective use of each orbital position, and to place multiple satellites spaced about 100 km away from each other in one orbital position, so that they are interconnected by inter-satellite links to form a satellite constellation. This not only avoids the problem that a single satellite is too large to be loaded into orbit with existing launch tools, but also greatly reduces the risk of launch failure and satellite failure, and the capacity of the satellite can be adjusted according to actual needs. Incrementally, by increasing the number of satellites.
6. The space segment part of a system may be a constellation consisting of many satellites. For political and economic reasons, it...