Research on Inter-Satellite Networking Technology for Large-Scale Constellations

International Development Status

Since 2015, a large number of low Earth orbit large-scale constellation plans have been proposed both domestically and internationally. Among them, OneWeb adopts a ground networking approach, while Starlink, “Starshield,” “Blackjack,” and the large-scale resilient operational space architecture (PWSA) adopt inter-satellite networking methods.

(1) Starlink and Starshield Constellations

SpaceX launched the commercial low Earth orbit constellation Starlink project in 2015, planning to deploy a satellite constellation of 42,000 satellites. As of September 25, 2024, the total number of Starlink satellites launched reached 7,062, covering over 100 countries and regions globally, with the number of users exceeding 4 million, making it the largest, fastest-deployed, and most widely used satellite constellation to date. Initially, the Starlink project did not plan for inter-satellite links; however, starting with the V1.5 satellites, laser communication terminals were equipped to facilitate on-orbit routing and forwarding, significantly reducing global communication delays.

The Starlink constellation possesses encrypted communication capabilities and cooperates with the U.S. military to conduct various exercises involving ground weapons, drones, and unmanned vessels, demonstrating significant military application value in battlefield communication, command control, and intelligence transmission. In December 2022, SpaceX officially announced the Starshield project, planning to complete the launch and networking of 15,000 satellites within three years. The Starshield project specifically serves government, defense, and intelligence sectors, with the U.S. military planning to build a more capable defense-specific constellation based on the mature Starlink satellite platform technology, marking a key step towards military applications for the Starlink project. In the first half of 2024, Starshield completed two dedicated launches, sending 41 reconnaissance satellites into their designated orbits.

(2) OneWeb Constellation

Founded in 2012, OneWeb aims to build a high-speed broadband network covering the globe. The OneWeb constellation plans to deploy 588 satellites for global coverage and achieves resilience and backup through redundant satellites, distributed across 18 polar orbits at an altitude of 450 km. The OneWeb constellation began launching satellites in February 2019 and completed its 18th satellite launch in March 2023, reaching a total of 618 satellites and completing the deployment of the first generation constellation, beginning global commercial operations. Due to the earlier design of the OneWeb constellation, it did not adopt inter-satellite links and mainly relies on a global network of ground stations for satellite networking, with plans to introduce inter-satellite networking technology in its second generation constellation.

(3) Blackjack Project

The Blackjack project, initiated by the U.S. Defense Advanced Research Projects Agency (DARPA) in 2018, aims to utilize commercial satellite technology to build a secure, low-cost, and short-cycle military low Earth orbit satellite constellation. The Blackjack project uses laser communication links for inter-satellite networking. The first satellite was launched in January 2021, followed by the second satellite in September 2022, and four more satellites in June 2023, after which it was announced that no new satellites would be launched. Although the project has been halted, the experience gained from exploring laser inter-satellite networking technology provides important references for the development of the PWSA constellation.

(4) PWSA Constellation

In 2019, the U.S. Space Development Agency (SDA) was established and subsequently released the National Defense Space Architecture (NDSA) plan, aimed at constructing a dedicated military low Earth orbit satellite constellation to provide military satellite communication, positioning, navigation, timing, missile warning and tracking, and imaging capabilities for land/sea targets. This plan was renamed the “Large-Scale Resilient Operational Space Architecture” in January 2023, rapidly introducing new technologies through phased deployment and spiral development models to address emerging threats, marking a shift in the U.S. military’s space architecture from singularity and fragility to diversity, diffusion, and resilience.

The PWSA constellation consists of hundreds of low Earth orbit satellites divided into seven functional layers: transmission layer, tracking layer, navigation layer, hosting layer, deterrence layer, operational management layer, and support layer. Among them, the transmission layer satellites are equipped with 2 or 4 laser terminals for inter-satellite networking via laser communication links, while also carrying Ka-band ground communication links, Link16 payload, and battle management, command, control, and communication (BMC3) modules. Since 2023, the construction speed of the PWSA has significantly accelerated, completing two batches of a total of 23 Phase 0 satellites, while the development and testing of Phase 1 and Phase 2 satellites are also progressing simultaneously.

Domestic Development Status

Against the backdrop of global large-scale constellation construction and application, China is also vigorously developing various satellite constellation networks, covering high, medium, and low orbits. Among them, the Beidou constellation and “Star Network” adopt inter-satellite networking methods, while the “Qianfan” constellation (G60) and other commercial constellations mainly adopt ground networking methods, gradually transitioning to inter-satellite networking methods in the future.

(1) Beidou Constellation

The Beidou satellite navigation system is China’s independently developed and operated global satellite navigation system, which began providing regional services at the end of 2012 and achieved global service capabilities in 2020. The Beidou system employs microwave inter-satellite networking to support inter-satellite precise measurements and data transmission for autonomous operation and monitoring management of the constellation. The next generation of the Beidou system will continue to utilize a hybrid laser/microwave inter-satellite networking technology for collaborative networking of high, medium, and low orbits, providing unified and robust PNT information services that are resistant to interference and deception, and evolving towards a comprehensive PNT system.

(2) “Star Network” Constellation

The “Star Network” is China’s proposed low Earth orbit satellite internet constellation plan, which, once completed, will provide a variety of services including global broadband communication, mobile communication, and the Internet of Things. In September 2020, China submitted a frequency spectrum application for the “Star Network” constellation to the International Telecommunication Union (ITU), planning to launch a total of 12,992 satellites, including inclined orbit and near-polar orbit satellites. The “Star Network” constellation employs laser communication links for inter-satellite networking. In April 2021, China Satellite Network Group Co., Ltd. was officially established as the implementing entity for the “Star Network” project, subsequently submitting a new application to the ITU, adding 5,656 satellites to the original plan. In 2023, several experimental satellites were launched, with formal deployment expected to begin in the second half of 2024.

(3) “Qianfan” Constellation

The “Qianfan” constellation is another large-scale satellite internet construction plan in China, aiming to provide global low Earth broadband internet services. This constellation includes three generations of satellite systems and adopts a full-band, multi-layer multi-orbit constellation design. On August 6, 2024, the polar orbit 01 group satellite was successfully launched. The plan is to achieve regional network coverage with 648 satellites by the end of 2025 and to provide mobile direct connection multi-service integration with 15,000 satellites by the end of 2030.

(4) Other Commercial Constellations

China’s commercial space sector has rapidly developed in recent years, with plans for large-scale commercial constellations including the Jilin-1 constellation, “Galaxy” constellation, and “Honghu” constellation. With the miniaturization and cost reduction of laser communication terminals, commercial constellations are gradually adopting laser communication links for inter-satellite networking to reduce reliance on ground gateway stations.

Challenges and Difficulties of Inter-Satellite Networking Technology

As mentioned above, foreign countries proposed inter-satellite networking technology as early as the development of the first generation of satellite constellations. However, their early constellations were not large, and ground networking methods were sufficient to handle a small number of deployed gateway stations worldwide. In contrast, inter-satellite networking technology is complex and costly to implement, which has made foreign countries less proactive in its development. This concept has continued to influence the satellite internet era; for example, neither OneWeb nor the early Starlink constellation adopted inter-satellite networking methods. Later, as the scale of constellations expanded, inter-satellite networking gradually showed its advantages, becoming an important technical means for large-scale constellation networking.

China’s need to overcome the difficulties of global ground station deployment has driven the development and application of inter-satellite networking technology. The Beidou constellation was the first to achieve microwave inter-satellite networking applications, and most of the planned constellations now also adopt inter-satellite networking technology, reducing reliance on foreign gateway stations and meeting the networking control and diverse service transmission needs of the entire constellation. Therefore, developing inter-satellite networking technology based on laser/microwave links is an inevitable choice for China to break through the limitations of global ground station deployment and is an inherent requirement for achieving autonomous, efficient, and collaborative operation of large-scale constellation systems.

As laser inter-satellite link technology gradually matures, inter-satellite networking is evolving from traditional microwave-based networking to a hybrid microwave and laser networking technology, with significantly enhanced on-board processing capabilities to meet the growing demand for high-capacity networked transmission. Meanwhile, large-scale constellations exhibit typical characteristics such as complex constellation structures, highly dynamic topology changes, diverse service types, and uneven service flow distribution, posing numerous challenges to inter-satellite networking, specifically manifested in:

1) The large scale and complex structure of the constellation lead to complex networking control. The number of low Earth orbit satellites is rapidly increasing, from hundreds to tens of thousands, which can provide global services, and achieving collaborative networking control between satellites and ground stations with various types of inter-satellite and ground links is a significant challenge.

2) The topology changes dynamically, making rapid convergence of on-board routing difficult. The high-speed movement of large-scale constellation satellite nodes leads to dynamic changes in inter-satellite topology connections, resulting in frequent updates in on-board routing calculations, and the larger the constellation, the longer the routing calculation convergence time. Therefore, achieving rapid convergence of inter-satellite routing under large-scale constellation conditions is an important topic.

3) The diverse types of services pose challenges for quality of service (QoS) assurance. The payload systems of large-scale constellations are complex, and the service types vary, with different services having different requirements for inter-satellite transmission bandwidth, latency, and packet loss rates, necessitating resource scheduling based on service demands to achieve differentiated QoS assurance.

4) Uneven distribution of service traffic can lead to inter-satellite network congestion. The high density and wide coverage of large-scale constellation nodes, coupled with sudden traffic spikes, can result in uneven traffic distribution among satellite nodes, especially when traffic converges to ground satellites, leading to potential network congestion. Therefore, it is necessary to consider introducing traffic balancing mechanisms to improve the reliability and throughput of inter-satellite networks.

Overall Architecture

A typical large-scale constellation network system consists of three parts: the space segment, ground segment, and user segment (see Figure 1). Large-scale constellations connect various nodes in the space segment, ground segment, and user segment through multiple communication link forms, including inter-satellite links, ground control and monitoring data transmission links, cross-domain interconnection links, and information distribution links, forming an integrated information transmission network that provides broadband access, mobile communication, data relay, and IoT services to various users.

The space segment comprises high, medium, and low Earth orbit satellites and inter-satellite links, with the number of satellites ranging from dozens to tens of thousands. They are interconnected through microwave or laser inter-satellite link technologies and adopt on-board information processing and routing forwarding methods, forming a space transmission network with satellites as exchange nodes. Additionally, the space segment can interconnect with other constellation systems through laser or microwave links on certain satellite nodes, supporting cross-constellation information transmission.

Figure 1 Overall Architecture of Large-Scale Constellation Network

The ground segment includes gateway stations, operation control management centers, and ground network facilities. Gateway stations serve as gateway nodes connecting satellite networks and ground networks, capable of accessing multiple satellites within the visible range through multiple beams. Satellite data is accessed into the ground network via gateway stations, completing the transformation of satellite-ground communication protocols, thereby achieving integrated network interconnection. The operation control management center is responsible for managing satellite constellations, gateway stations, and ground network facilities, conducting comprehensive management and monitoring of the entire constellation’s network resources.

The user segment consists of various user terminal devices and application service support systems. User terminals include onboard terminals, airborne terminals, shipborne terminals, vehicle-mounted terminals, and personal mobile terminals, used to receive and process satellite link signals to obtain space-based information services.

Large-scale constellations utilize inter-satellite links to construct inter-satellite networks, primarily used to carry satellite management information and various types of payload business data, meeting the differentiated transmission needs of different business data. For example: Satellite management information transmission requires low latency, high reliability, and high security; communication services include video, text, messages, and commands, characterized by high real-time requirements, demanding low latency and high reliability; remote sensing services include raw observation data, image slice data, processing result data, etc., characterized by large data volumes, requiring high bandwidth and high reliability.

Network Protocol System

To ensure that all nodes in large-scale constellations can interconnect, a unified inter-satellite network protocol system needs to be designed and followed. Depending on the different requirements of inter-satellite networking scenarios, inter-satellite network protocols are mainly divided into two categories: network protocol systems for high-speed inter-satellite links and those for narrow-band inter-satellite links. Constellation systems running different network protocols can achieve protocol conversion and interconnection by deploying gateways.

For high-speed inter-satellite link scenarios, a standardized IPoverCCSDS protocol system can be adopted, including five layers of protocols: application layer, transport layer, network layer, link layer, and physical layer, supporting both Internet Protocol version 4 (IPv4) and IPv6 protocols. Due to advantages such as more addresses, better packet structure, stronger path selection, and enhanced security, the inter-satellite network protocol will focus on evolving towards IPv6 and its derived protocols, including new protocol technologies such as software-defined networking (SDN) and segment routing (SRv6).

For narrow-band inter-satellite link scenarios, such as communication links with bandwidth below 100 kbit/s, the overhead of IP protocol headers is too high, leading to low link resource utilization. For such special scenarios, lightweight network protocols need to be designed, optimizing IPv4/IPv6 protocol packets to significantly reduce encapsulation header overhead, freeing up more bandwidth resources for effective data transmission; and lightweight improvements to dynamic routing algorithms are needed to reduce the flooding range of routing and accelerate routing convergence calculations, further decreasing routing control overhead.

Currently, technologies represented by SRv6 have been deployed on a large scale in ground networks. SRv6 is the core technology for the evolution of IPv6 technology towards programmable networks, providing path programmability for networks; technologies represented by flow detection and network slicing have been deployed on demand in ground networks and are maturing, providing precise performance measurement and differentiated quality assurance capabilities; and application-aware networks (APN) have also been proposed. Therefore, new networks represented by “IPv6+” and their integration with 5G/6G networks will be important development directions for inter-satellite networking technology.

Network Security System

The inter-satellite network of large-scale constellations is characterized by high dynamic topology changes, an open space transmission environment, and limited on-board processing capabilities, making it susceptible to attacks. Security vulnerabilities exist in inter-satellite links, ground links, on-board computing platforms, gateway stations, and operation control management centers. For large-scale constellation networks, information security threats mainly arise from open space-ground links, communications networks in uncontrollable cross-domain areas, and data exchanges between different security trust domains, with major security risks including unauthorized access, data eavesdropping, replay attacks, data tampering, malicious program attacks, and software reconstruction anomalies.

To address the network security risks of large-scale constellations, a security protection system needs to be established at multiple levels, including physical layer, link layer, network layer, and application layer. Physical layer security technologies include frequency hopping, spread spectrum, and time hopping; link layer security technologies include signaling encryption, group routing encryption, and multi-service multi-channel encryption; network layer security technologies include boundary protection, adaptive multi-level security, and secure isolation and switching; application layer security technologies include end-to-end encryption, identity authentication, and access control.

From a practical implementation perspective, a multi-level, lightweight, and multi-modal security protection strategy should be designed. On the basis of communication data encryption, access security policies can be introduced to achieve lightweight authentication functions and controlled access capabilities for inter-satellite and ground nodes; behavior security policies can be introduced to implement inter-satellite network firewall functions, supporting filtering of illegal data packets and preventing malicious data forwarding. Finally, consideration should be given to the limited on-board processing resources, allowing for the integration of relevant network security algorithms into the on-board network routing module through lightweight design of network security strategies, thereby ensuring the secure operation of large-scale constellation networks.

Laser/Microwave Inter-Satellite Link Transmission Technology

Inter-satellite links are mainly divided into microwave inter-satellite links and laser inter-satellite links. Among them, microwave inter-satellite links primarily use Ka and Q/V frequency bands and are moving towards higher frequency bands, with multi-beam phased array technology widely applied, offering advantages such as relative technical maturity, wider beams, and easier tracking and capturing. Laser inter-satellite links have advantages of miniaturization, high transmission rates, and small beam divergence angles, providing good anti-jamming and interception performance, with good system security, but requiring higher stability from satellite platforms.

Specifically, inter-satellite links in the same orbital layer can maintain stable link relationships, primarily using laser links to ensure high-speed transmission capabilities; however, for satellites with large attitude maneuvers, microwave inter-satellite links are still needed as a supplement. Inter-satellite links between different orbital layers experience high dynamic topology changes and require one-to-many communications, primarily utilizing microwave links with multi-beam capabilities to ensure multi-user access, while laser links can serve as auxiliary supplements to meet high-speed transmission demands. Therefore, both laser inter-satellite links and microwave inter-satellite links have their advantages, and the hybrid networking of microwave and laser technologies is an important development direction for large-scale constellation networks.

Satellite-Ground Collaborative Network Control Technology

Large-scale constellation networks can adopt satellite-ground collaborative network control technology based on software-defined networking (SDN) architecture, supporting capabilities such as inter-satellite network topology planning and control, satellite-ground mobility handover topology planning and control, and high-low orbit interconnection topology planning and control. The operation control management center monitors the status of each satellite link in real time, predicting link switching changes based on satellite ephemeris data, and employing a high-low orbit and satellite-ground link mobility management strategy based on remaining survival time, dynamically adjusting routing strategies in real time to ensure continuity and reliability of data transmission.

In terms of implementation models, the network control modes of large-scale constellations mainly include ground centralized network control, on-board autonomous network control, and satellite-ground collaborative network control. Among them, satellite-ground collaborative network control technology is key to achieving efficient management of low Earth orbit constellation networks. By constructing an efficient satellite-ground collaborative architecture, real-time information exchange between ground control centers and satellites can be achieved, facilitating comprehensive management of the entire network.

Under the satellite-ground collaborative network control approach, topology dynamic management and network routing management are two important components. Network routing management needs to adapt to rapidly changing network conditions and optimize path selection to reduce latency and increase throughput, especially in cases where switching occurs between high-low orbit satellites and satellite-ground links, making inter-satellite and satellite-ground link management particularly important. Satellite-ground collaborative network control also includes information collection and processing, resource optimization allocation, health management, and security assurance, and in the future, artificial intelligence technology can be introduced to further enhance the intelligence level of network control, achieving precise prediction of network state changes and automatic strategy adjustments, thereby improving the management efficiency and service quality of large-scale constellation networks. The composition of large-scale constellation network systems is illustrated in Figure 2.

Figure 2 Composition of Large-Scale Constellation Network System

Efficient and Rapid Routing Technology for Large-Scale Constellations

Large-scale constellation networks have characteristics such as high dynamic topology, large network scale, and limited on-board processing resources, requiring routing algorithms to have small control overhead, fast convergence, and high robustness. The main routing planning methods include ground centralized routing planning, satellite-ground collaborative routing planning, and on-board distributed dynamic routing.

Ground centralized routing planning is a static routing method that mainly utilizes the predictability of satellite orbits for routing table planning. The operation control management center calculates inter-satellite and satellite-ground visibility based on satellite orbits and conducts link planning, pre-planning routing tables for each satellite according to time segments, with satellites switching to the corresponding routing tables at predetermined times. This method relies on ground processing, reducing the complexity of on-board processing; however, it cannot respond in real-time to unpredictable topology changes, and abnormal topologies usually lead to routing table failures.

Satellite-ground collaborative routing planning refers to satellites transmitting link status information back to the ground in real time, with the operation control management center conducting real-time routing planning based on satellite status information and updating routing tables for satellites to execute. This method can be combined with SDN-based network architecture, where satellite nodes are responsible for monitoring and perceiving link status, while the operation control management center collects overall network status for unified calculations, achieving a balance between on-board processing burdens and ground control accuracy. When the number of satellites is large, the burden on the ground increases, placing higher demands on satellite-ground communication links.

On-board distributed dynamic routing refers to satellites operating dynamic routing algorithms autonomously, achieving on-board autonomous dynamic routing planning capabilities. Common routing protocols include Open Shortest Path First (OSPF) and Optimized Link State Routing (OLSR).This method does not require the involvement of the operation control management center, offering advantages such as dynamic adaptability in routing and fast convergence speed, but it does require certain on-board processing capabilities.On-board, inter-satellite link status perception technology can also be introduced, utilizing link status locking monitoring and bidirectional forwarding detection fusion methods to achieve rapid perception of changes in laser/microwave link status, further accelerating routing convergence.For scenarios involving hundreds of satellites or more, improved partitioned routing algorithms can be adopted, dividing the entire constellation into multiple sub-regions to control the flooding range of routing messages, reducing routing calculation complexity and accelerating dynamic routing convergence.

Traffic Balancing Technology for Burst Services

During the transmission process of large-scale constellation networks, routing strategies that forward packets hop by hop along the shortest path can easily lead to network congestion due to the aggregation of burst traffic. Additionally, uneven distribution of network traffic can result in the underutilization of idle links on satellites, leading to sharply increased communication delays. When congestion becomes severe, high packet loss rates may occur, drastically degrading network performance.Therefore, large-scale constellation networks need to introduce traffic balancing technologies, implementing traffic balancing strategies when network traffic exceeds certain thresholds to meet the QoS assurance needs of various service types.

To address network congestion issues caused by real-time transmission of burst services, local traffic balancing algorithms based on equivalent routing can be employed, integrating resources across multiple inter-satellite links to alleviate the network load on a single path, achieving multi-path diversion and rapid relief of single-point congestion. When the main path experiences interruption, a quick switch to a backup path can ensure continuity of data transmission.

To tackle the problem that local traffic balancing cannot achieve overall network traffic optimization, a global traffic balancing algorithm based on segmented routing can be employed, combined with an SDN-based satellite-ground collaborative control architecture, utilizing overall network link status information for global diversion scheduling and traffic control, planning data packets at the source to guide traffic to idle links within the constellation network, reducing congestion levels on current links and achieving global traffic balancing optimization.

To enhance the reliability of service transmission, priority and delay-sensitive traffic scheduling algorithms can be employed, categorizing service types based on delay sensitivity and applying different levels of scheduling strategies to ensure priority transmission for delay-sensitive services. Additionally, AI-based traffic balancing strategies can be introduced, utilizing deep learning and other AI technologies to automatically extract network traffic features and conduct traffic prediction and recognition, thus achieving adaptive optimization adjustments for traffic allocation strategies, opening new avenues for addressing network congestion and traffic balancing issues.

Time and Space Reference Maintenance Technology for Large-Scale Constellations

High-precision time and space references are crucial for the autonomous operation of large-scale constellation networks. In large-scale constellation networks, satellite nodes primarily rely on a combination of satellite navigation and inter-satellite link ranging for time and frequency synchronization. Under conditions where satellite navigation is available, satellite navigation system time-frequency serves as the reference benchmark, enhancing frequency synchronization accuracy through frequency difference estimation and narrow-band phase-locked techniques, breaking through traditional co-visibility and phase-locking accuracy limitations, and utilizing geometric and dynamic methods for time difference estimation to achieve high-precision time-frequency synchronization. In conditions where satellite navigation is denied, inter-satellite ranging and time comparison data processing methods based on inter-satellite links can be employed, utilizing both co-orbital and non-co-orbital laser/microwave inter-satellite link conditions to achieve high-precision autonomous orbit determination and time synchronization for satellites, providing high-precision time and space reference support for the operation of large-scale constellation networks.

As the scale of large-scale constellations rapidly increases, inter-satellite networking, with its technical advantages of not relying on ground gateway stations and low global transmission latency, is gradually replacing the ground networking method based on transparent forwarding on satellites, becoming the primary technical means for large-scale constellation networking. Analyzing from the perspective of protocol systems, IPv6 technology has already been applied in the field of inter-satellite networking. IPv6 not only addresses the issue of insufficient network address resources but also implements various improvements and innovations over IPv4, including protocol innovations represented by segment routing, network slicing, flow detection, new multicast, application-aware networks, and technological innovations represented by network analysis, automatic tuning, and network self-healing. Therefore, new network technologies represented by “IPv6+” will be an important evolution direction for the inter-satellite network protocol system.

In recent years, the rise of software-defined networking and artificial intelligence technologies is driving inter-satellite networks towards a direction of “multi-domain hybrid heterogeneity, cloud-network-intelligence integration.” By introducing the concept of software-defined large-scale constellation networks, constructing a cloud-edge-end integrated architecture, and utilizing powerful ground cloud resources for flexible hierarchical collaborative routing control of large-scale satellite networks, it is expected to significantly enhance the resilience of networking control and the quality of network services for large-scale constellations.