Inter-satellite Links Research Articles

Design and Evaluation of Dynamic Topology for Mega Constellation Networks

Due to numerous Low Earth Orbit (LEO) satellites, urgent analysis of many temporary inter-satellite links (ISLs) is necessary for mega constellation networks. Therefore, introducing a dynamic link in topology design is crucial for increasing constellation redundancy and improving routing options. This study presents one class of static topology of satellites (STLS) and two types of dynamic topology of satellites (DTLS). Firstly, a call model based on global population density distribution is determined using world population density by provincial administrative divisions. Then, using a common simulation platform, the Dijkstra algorithm obtains random paths between 10,000 pairs of urban ground stations, adopting a time slice division strategy. Finally, 3 indexes are obtained within 66-time slices: average call distance, number of hops, and total time delay. Results show that DTLS1 reduces these indexes by 3.58%, 3.72%, and 3.57%, respectively, compared with DTLS2 under the same conditions, indicating that DTLS1 has the best network performance, transmitting traffic quickly in any direction through the reverse track, thereby verifying the related hypothesis.

Spaceborne Cooperative Detection for Distributed Sensing: Overcoming Inter-Satellite Link Limitations via Deep Information Bottleneck

We consider the cooperative detection for distributed sensing via low Earth orbit constellations. The source data gathered by the ground sensors are non-orthogonally sent to satellite access points, and then aggregated at a central satellite via inter-satellite links (ISLs). To overcome the intrinsic ISL bandwidth limitation, we consider the deep auto-encoding paradigm to jointly design the ISL transceivers among satellites, and propose a novel deep variational information bottleneck (DVIB) method which maximizes the end-to-end sensing accuracy under bandwidth constraints. Specifically, the mathematically untractable ISL bandwidth constraint is first transformed into an entropy-based format. Then a customized batch-norm layer is introduced, where the messages on ISLs are considered as latent variables and are regularized with entropy-constrained posterior for efficient compression. Compared to the benchmark, the proposed DVIB method is shown to simultaneously reduce the bandwidth overhead by 30% and enhance the sensing accuracy by 2–5 dB, validating the significance of relevant information extraction on ISLs.

Latency Optimization for Hybrid GEO–LEO Satellite-Assisted IoT Networks

Benefiting from the development of satellite onboard processing capability, the orbital computing can be realized by deploying edge computing servers on satellites to reduce the task processing latency. However, edge computing based on geostationary earth orbit (GEO) or low earth orbit (LEO) alone can hardly meet the latency requirements of satellite assisted internet of things (SIoT) services. Moreover, the uneven distribution of tasks generated by SIoT devices will also cause the load unbalancing among different satellites. In this paper, hybrid GEO-LEO SIoT networks is investigated with joint computing and communication resource allocation. To tackle the load unbalancing problem, tasks generated by SIoT devices can be processed by collaborative LEO satellites or forwarded to gateways on ground via GEO satellite. Thus, the joint task offloading, communication and computing resources allocation for the hybrid SIoT network can be formulated as a mixed integer dynamic programming problem with satellites-ground cooperation and inter-satellite cooperation via the inter-satellite links. Then, an intelligent task offloading and multi-dimensional resources allocation algorithm (TOMRA) is proposed to minimize the latency of task offloading and processing. Firstly, a method base on deep reinforcement learning is utilized to solve the subproblem of task offloading and channel allocation. And then, convex optimization is adopted to solve the sub-problem of computing resource allocation under fixed offloading and channel allocation decisions. Simulation results show that the proposed TOMRA can achieve better performance than the reference schemes.

A Link Planning and DDoS Attack Detection in SDN Based Integrated Space-Terrestrial Networks

It is expected that 5G networks would require both great dependability and stability as fundamental requirements. The introduction of a satellite component into LTE (long term evolution) networks has emerged as an attractive solution for meeting these requirements. This will enable the provision of backup interoperability to essential base stations and the routing of traffic away from crowded locations. During peak hours, the demand for their terrestrial links must be increased or even adopted in the case of anticipated breakdown or repair. Integrated spaceterrestrial networks have shown to be an advantageous design due to its broad coverage and high precision. Designing a network routing plan is not a straightforward process when the complex relative motion of low-earth-orbit (LEO) satellites and unmanned aircraft systems (UAS) is considered (UAS). To be more explicit, the major difficulty is discovering how to locate acceptable connections in timevarying network settings to offer reliable and effective UAS data transmission. This study begins with a motion analysis of satellites and UASs to discover which access satellites are most suited for usage with UASs. This is done in order to resolve the aforementioned problem. In order to provide an effective and reliable intersatellite link (ISL) deployment between access satellites, double exponential smoothing (DES) is afterwards proposed as an alternative to traditional routing. This is required to guarantee the timely delivery of UAS data. The simulation results demonstrate that the proposed DES approach is both feasible and effective. A time series-based method known as DES is used to compute the port information in an adjustable manner, and an algorithm known as SVM (Support Vector Machine) is utilized to determine if a Distributed Denial of Service (DDoS) attack has really occurred. Experiments that are typical demonstrate that the UAS may greatly reduce the controller's workload while maintaining dependable detection accuracy.

Laser Intersatellite Link Range in Free-Space Optical Satellite Networks: Impact on Latency

Laser communication terminals for establishing laser intersatellite links (LISLs) are in high demand by companies, such as SpaceX and Telesat, who want to equip their satellites with LISLs to realize free-space optical satellite networks (FSOSNs). Such terminals offer LISLs with ranges spanning from 4500 to 45,000 km. How does LISL range affect network latency in FSOSNs? To investigate this effect, we employ the satellite constellation for Phase I of SpaceX's Starlink, and examine six different LISL ranges for satellites in this constellation in three different scenarios for long-distance intercontinental data communications. We observe that the satellite connectivity and thereby the network connectivity improves with an increase in the LISL range as more and farther satellites become available within the LISL range of a satellite. A higher LISL range leads to better shortest paths offering lower average network latency. Furthermore, improvement in average network latency with increase in LISL range is seen in all scenarios.

저궤도 위성 간 레이저 통신을 위한 직접수신 광수신기 구조 연구

저궤도 위성 네트워크를 통해 전세계에 통신 서비스를 제공하는 방안에 대한 관심이 높아지고 있다. 저궤도 위성 네트워크에서 노드 간 데이터를 주고받기 위해서는 레이저 통신 기술이 필요하다. 위성 간 레이저 통신 링크는 전송 거리가 매우 길어서 링크 연결을 위해 광수신기의 수신 감도가 중요하다. 본 논문에서는 저궤도 위성 간 레이저 통신 링크 구현에 적합한 광수신기 구조를 제안한다. 저궤도 위성 간 레이저 링크 시스템의 특성을 고려하여 링크 버짓을 계산하고 요구되는 수신감도를 도출한다. 도출된 수신감도 충족을 위한 avalanche photodiode (APD)기반 광수신기와 광섬유 전치증폭기 기반 광수신기의 성능을 평가하고 최대 전송 거리 분석을 통해 저궤도 위성간 링크에 적용 가능성을 확인한다.

A Distributed Approach for Time-Dependent Observation Scheduling Problem in the Agile Earth Observation Satellite Constellation

The increasing number of agile earth observation satellites (AEOSs) in orbit have advanced maneuverable capabilities, enabling the AEOS constellation to provide richer observation services. Therefore, observation scheduling in the AEOS constellation is crucial for improving the performance of satellite remote sensing systems. This paper focuses on the problem of distributed observation scheduling in the AEOS constellation, where a period of transition time is required between two consecutive observations, and this constraint depends on the start time of observations. We define a new fitness function that not only maximizes the profit sum but also considers system load balancing. Based on the fundamental idea of a distributed performance impact (PI) algorithm, we develop a PI-based distributed scheduling method (PIDSM) that runs concurrently on all AEOSs via local inter-satellite link (ISL)-based communications. The PIDSM iterates between two phases: target inclusion and consensus and target removal. The first phase aims to select the optimal task for each AEOS, while the second phase reaches a consensus over all AEOSs and removes targets that may decrease overall fitness. Experimental results demonstrate that the PIDSM can schedule more targets, reduce communication overhead, and achieve higher fitness values than existing algorithms. Sensitivity analyses further validate the effectiveness of the PIDSM.

Trusted Autonomous Operations of Distributed Satellite Systems Using Optical Sensors.

Recent developments in Distributed Satellite Systems (DSS) have undoubtedly increased mission value due to the ability to reconfigure the spacecraft cluster/formation and incrementally add new or update older satellites in the formation. These features provide inherent benefits, such as increased mission effectiveness, multi-mission capabilities, design flexibility, and so on. Trusted Autonomous Satellite Operation (TASO) are possible owing to the predictive and reactive integrity features offered by Artificial Intelligence (AI), including both on-board satellites and in the ground control segments. To effectively monitor and manage time-critical events such as disaster relief missions, the DSS must be able to reconfigure autonomously. To achieve TASO, the DSS should have reconfiguration capability within the architecture and spacecraft should communicate with each other through an Inter-Satellite Link (ISL). Recent advances in AI, sensing, and computing technologies have resulted in the development of new promising concepts for the safe and efficient operation of the DSS. The combination of these technologies enables trusted autonomy in intelligent DSS (iDSS) operations, allowing for a more responsive and resilient approach to Space Mission Management (SMM) in terms of data collection and processing, especially when using state-of-the-art optical sensors. This research looks into the potential applications of iDSS by proposing a constellation of satellites in Low Earth Orbit (LEO) for near-real-time wildfire management. For spacecraft to continuously monitor Areas of Interest (AOI) in a dynamically changing environment, satellite missions must have extensive coverage, revisit intervals, and reconfiguration capability that iDSS can offer. Our recent work demonstrated the feasibility of AI-based data processing using state-of-the-art on-board astrionics hardware accelerators. Based on these initial results, AI-based software has been successively developed for wildfire detection on-board iDSS satellites. To demonstrate the applicability of the proposed iDSS architecture, simulation case studies are performed considering different geographic locations.

Feasibility Analysis of Autonomous Orbit Determination and Gravity-Field Recovery around Asteroids Using Inter-Satellite Range Data

Autonomous navigation and orbit determination are key problems of asteroid exploration missions. Inter-satellite range link is a type of measurement widely used in the orbit determination of Earth satellites, but not so widely used in missions around small bodies. In our study, we assume that highly accurate inter-satellite range data can be obtained around small bodies between the chief spacecraft and some low-cost deputies, and study the feasibility of simultaneous autonomous orbit determination of the spacecraft and the gravity-field recovery without the support from ground stations. After the feasibility analysis, two modified methods are proposed. Both methods demonstrate obvious improvements in both the convergence region and the accuracy. One remark is that the inter-satellite range data can be also used together with various observation data from ground stations to enhance the accuracy of the determined orbits and the gravity field.

Inter-satellite link scheduling and power allocation method for satellite networks

Inter-satellite link scheduling and power allocation method for satellite networks

Relativistic effects in BDS-3 high-accuracy intersatellite time synchronization

The Global Navigation Satellite Systems (GNSSs) apply conventional relativistic corrections to correct apparent clock offsets. However, with the continuous improvements made to time synchronization approaches and onboard atomic clocks, this model has obviously limits the accuracy improvements of BDS-3 intersatellite time synchronization, and also limited the apparent performance of BDS-3 onboard atomic clocks. This study demonstrates that the amplitude of GNSS conventional relativistic periodic errors can reach 0.08 ns for BDS-3 MEO and IGSO satellites within a day, and 0.40 ns within a month. The frequency stability of the errors’ signal can reach 1.33E-14 (approximately10000 s) and 1.78E-15 (approximately1 day), and the absolute influences on the predicted clock offset accuracy can reach 0.05 ns (1 h), 1.08 ns (1 day). Notably, the above signals caused by the model errors are found in the results of current high-accuracy intersatellite time synchronization based on BDS-3 intersatellite links (ISLs). After adopting a higher-accuracy relativistic correction model, the frequency instability of BDS-3 intersatellite clock offsets is obviously reduced, and the RMS of 24 h fitting residual can be reduced by approximately 60% from 0.12 ns to 0.05 ns. The average 2 h 95% predicted error decreases by approximately 17% from 0.38 ns to 0.32 ns. These results indicate that this high-accuracy relativistic correction model can improve both the BDS-3 satellites’ time synchronization accuracy and the BDS-3 apparent clock offset prediction capability. This study, thus contributes to time synchronization research and signal-in-space accuracy improvements for BDS-3.

Time-Varying Topology Model for Dynamic Routing in LEO Satellite Constellation Networks

With the characteristics of low-latency, seamless coverage and high bandwidth, the Low Earth Orbit (LEO) satellite network has been the promising technology for the sixth-generation mobile communication (6G) networks, especially, the inter-satellite link can improve the flexibility of inter-satellite networking and routing. However, in the existing works, since the impact of link attributes on the satellite topology has not been well investigated, it is difficult to avoid the loss of topology information for the transmission path selection, which may aggravate the unreliability of routing path. In this paper, we propose a novel time-varying topology model for the LEO satellite network, to improve the adaptability of the satellite routing. Firstly, the weighted time-space evolution graph based on the link attributes is established to construct the time-varying topology model of LEO satellite networks. Then, the utility function of the link attributes is modelled and the multi-attribute decision-making is introduced to calculate the weight of each link attribute for the quantification of the link utility. Finally, based on the topology model, the inter-satellite link utility-based dynamic routing algorithm is proposed to improve the adaptability of satellite routing. The simulation results demonstrate that the proposed routing algorithm outperforms the existing routing algorithms in terms of packet drop rate, end-to-end delay and throughput.

Demand Island Routing for LEO satellite constellations

Low Earth Orbit (LEO) constellations create a network that includes the satellites (as routing nodes) connected by Inter-Satellite Links (ISLs) and the terminals dynamically connected to one or more satellites. The combination of transient high rate changes in the network topology due to terminals handover, and the end-to-end high propagation time between the routing nodes presents a unique challenge for designing a guaranteed-bandwidth routing protocol that can support the frequent changes. In addition, terminals connected to multiple satellites can balance their traffic between multiple paths in a frame granularity based on the links conditions, and require real-time multipath guaranteed-bandwidth. Prior work focuses on end-to-end routing between multiple gateways and terminals and does not provide a multipath guaranteed-bandwidth service.This paper addresses the problem of sending traffic from a source terminal to a destination terminal connected through multiple satellites, while guaranteeing and enabling planning of the multipath service metrics (bandwidth and latency) and handling satellite handovers. A novel paradigm called Demand Islands is presented. This paradigm takes advantage of the geographical properties of the demand. The service area is split into geographical rectangles (Demand Islands), with a gateway (GW) located at the center of each island. Each Demand Island is dynamically associated with a set of satellites that cover it. We model the set of satellites as a grid. The routing algorithm uses the grid properties to provide real-time multipath guaranteed-bandwidth routing for the demand allocated to each terminal. Each satellite executes the algorithm to calculate the next hop for a received message in the time complexity of O(1). The analysis and simulation demonstrate the unique offering of the algorithm to route real-time guaranteed-bandwidth to terminals connected to multiple satellites.

Clock Ensemble Algorithm Test in the Establishment of Space-Based Time Reference

A new concept of a space-based synchronized reference network is proposed with the development of an optical frequency reference and laser inter-satellite link. To build such time reference, three clock ensemble algorithms, namely the natural Kalman timescale (NKT) algorithm, the reduced Kalman timescale (RKT) algorithm, and the two-stage Kalman timescale (TKT) algorithm are considered. This study analyzes and compares the performance of these algorithms using BDS, GPS, and Galileo satellite clock data from the GFZ GNSS clock corrections, which will be used in constructing future space-based time references. The study shows that the NKT algorithm improves frequency stability by 0.1–0.2 orders of magnitude in the short and medium term. When the satellite clock is mostly a hydrogen clock, the RKT and NKT are close, and the short and medium-term frequency stability slightly increases. In contrast, the TKT algorithm produces a timescale that improves frequency stability by 1–3 orders of magnitude. A quadratic polynomial model predicts the three timescales, with the results indicating that the short-term prediction accuracy of the satellite clock is within 1ns, and the TKT algorithm’s prediction accuracy is 1–2 orders of magnitude higher than that of the NKT and RKT algorithms. With the deployment of next-generation Low Earth Orbit (LEO) satellites equipped with higher-precision clocks, the space-based time reference system will achieve improved accuracy and greater potential for practical applications.

Fully Autonomous Orbit Determination and Synchronization for Satellite Navigation and Communication Systems in Halo Orbits

This paper presents a solution for autonomous orbit determination and time synchronization of spacecraft in Halo orbits around Lagrange points using inter-satellite links. Lagrange points are stable positions in the gravitational field of two large bodies that allow for a sustained presence of a spacecraft in a specific region. However, a challenge in operating at these points is the lack of fixed landmarks for orbit determination. The proposed solution involves using inter-satellite links to perform range and range-rate measurements, allowing for accurate computation of the spacecraft’s orbit parameters without the need for any facilities on Earth. Simulations using a fleet of three satellites in Near Rectilinear Halo Orbits around the Earth–Moon Lagrange point, proposed for the Lunar Gateway stations, were conducted to demonstrate the feasibility of the approach. The results show that inter-satellite links can provide reliable and accurate solutions for orbit determination with a DRMS error lower than one meter (90th percentile) and synchronization errors of around one nanosecond. This solution paves the way for a fully autonomous fleet of spacecraft that can be used for observation, telecommunication, and navigation missions.

A compass time-space model-based virtual IP routing scheme for NTSN satellite constellations

Recently, Non-Terrestrial Satellite Networks (NTSTs) have gained more and more attentions due to global coverage, low latency, and high-speed communications. The routing scheme is one of the primary challenges for NTSNs, due to the mega scale of an NTSN constellation and the dynamic topology feature. To solve many pressing problems, a Compass time-space Model-based Virtual IP (CMVIP) routing scheme is proposed in this paper. In order to compensate for discontinuities in existing topology models, a compass-shaped time-space model is proposed. It can be adapted with Inter-Satellite-Link (ISL) and Ground-Satellite-Link (GSL) transmissions. A distributed algorithm with multiple optimization objectives and multiple constraints is applied for routing path discovery. To more realistically verify the specific scheme, a traffic model that supports two different services is proposed. Access and bearer services are the two main applications of an NTSN. Experimental results demonstrate that the proposed scheme achieves superior Quality-of-Service (QoS) performance. In addition, comparison results demonstrate that the CMVIP routing scheme is superior to the Virtual-Topology-based Shortest Path (VT-SP) routing algorithm.

An Intersatellite Link Assignment Design for Megaconstellation Based on NSGA-II

The exploitation of intersatellite links (ISLs) makes the LEO satellite constellation based Internet service widely favored due to its advantages of low-latency and high-speed communications. However, considering the time-varying network topology, limited on-board resources, and high node failure probability, the design of link assignment has become one of the key challenges for the future megaconstellation. To solve this problem, we adopt the idea of modified finite state automaton (FSA) to model the LEO satellite network and use nondominated sorting genetic algorithm (NSGA-II) to optimize the minimum residual capacity and the maximum transmission delay. The simulation results show that our method is more invulnerable and flexible, which is fit for megaconstellations. This work can provide comprehensive guidance on intersatellite link assignment for future megaconstellations.

A Method for Autonomous Generation of High-Precision Time Scales for Navigation Constellations.

The time maintenance accuracy of the navigation constellation determines the user positioning and timing performance. Especially in autonomous operation scenarios, the performance of navigation constellation maintenance time directly affects the duration of constellation autonomous navigation. Among them, the frequency stability of the atomic clock onboard the navigation satellite is a key factor. In order to further improve the stability of the navigation constellation time-frequency system, combined with the development of high-precision inter-satellite link measurement technology, the idea of constructing constellation-level synthetic atomic time has gradually become the development trend of major GNSS systems. This paper gives a navigation constellation time scale generation framework, and designs an improved Kalman plus weights (KPW) time scale algorithm and time-frequency steer algorithm that integrates genetic algorithms. Finally, a 30-day autonomous timekeeping simulation was carried out using the GPS precision clock data provided by CODE, when the sampling interval is 300 s, the Allan deviation of the output time scale is 5.73 × 10-14, a 71% improvement compared with the traditional KPW time scale algorithm; when the sampling interval is 1 day, the Allan deviation is 9.17 × 10-15; when the sampling interval is 1 × 106 s, the Allan deviation is 8.87 × 10-16, a 94% improvement compared with the traditional KPW time scale algorithm. The constellation-level high-precision time scale generation technology proposed in this paper can significantly improve the stability performance of navigation constellation autonomous timekeeping.

Design and performance of a Martian autonomous navigation system based on a smallsat constellation

Deciphering the genesis and evolution of the Martian polar caps can provide crucial understanding of Mars' climate system. The growing scientific interest for the exploration of Mars at high latitudes, and the need of minimizing the resources onboard landers and rovers, motivates the need for adequate navigation support from orbit. We propose a novel concept based on a constellation that can support autonomous navigation of different kind of users devoted to scientific investigations of those regions. We study two constellations, that differ mainly for the semi-major axis, composed of 5 small satellites (based on the SmallSats design being developed in Argotec), offering dedicated coverage of the Mars polar regions. We focus on the architecture of the inter-satellite links (ISL), the key elements providing both ephemerides and time synchronization for the broadcasting of the navigation message. Our concept is based on suitably configured coherent links, able to suppress the adverse effects of on-board clock instabilities and to provide excellent range-rate accuracies between the constellation's nodes. The data quality allows attaining good positioning performance for both constellations with a largely autonomous system. Indeed, we show that ground support can be heavily reduced by employing an ISL communication architecture.

Orbit design of satellite quantum key distribution constellations in different ground stations networks

In the field of Cryptography, Quantum Key Distribution (QKD) is an application of Quantum Information theory that obtained a great deal of attention in recent years. It allows to establish secret keys between two or more parties, in a much safer way than that implemented by classic cryptography (based on discrete logarithms and factorization of prime numbers). The most promising way of realizing a QKD network (especially over great distances) in the near future is by a constellation of satellites. This paper considers the problem of optimizing the orbits of the satellites in order to maximize the minimum key length shared in a network of ground stations over a fixed amount of time. Different networks of stations are considered and the influence of their geographical disposition on the design and the performance index is highlighted. The networks considered are: a global constellation, a regional European constellation, and two in which there are groups of stations in two different narrow bands of latitude. The effect of Inter-satellite links is then taken into account and how, in some cases, they can improve the performances. Finally the daily performance of the considered constellations are analyzed.