Contact Plan Research Articles

Large-volume LEO satellite imaging data networked transmission scheduling problem: Model and algorithm

With the current miniaturization and cost reduction trend, many small imaging satellites are deployed in low Earth orbit (LEO). Efficient scheduling of transmission links is crucial for handling the large volume of imaging data in the time-varying inter-satellite broadband communication networks. This ensures optimal utilization of inter-satellite link resources and gives rise to a novel problem known as the Large-volume LEO satellite Imaging Data Networked Transmission Scheduling Problem (LLSIDNTSP). This problem requires the integrity of the imaging data received on the ground station. A Specific Time-Evolving Graph (STEG) model is introduced to formulate this problem, while a highly effective Sequential Two-Phased Heuristic Algorithm (STPHA) is proposed to solve it. STPHA determines an optimized Contact Plan (CP) with a Contact-Fast Construction Algorithm (CFCA) in the first phase. With this CP as input, it produces an optimal transmission schedule with Linear Programming in the second phase. The CFCA incorporates a novel heuristic called Mission Residual Volume Factor(MRVF), which guides the selection of a proper inter-satellite and satellite-to-ground link at each construction step. Extensive simulation results demonstrate the effectiveness of the proposed STPHA. In particular, it can achieve the optimal solution for small-sized scenarios with one-hundredth of the computing time used by the exact solver CPLEX. Also, it is consistently better than other heuristic algorithms adapted from the literature for large-sized scenarios. Additional experimental analysis is conducted to showcase the effectiveness of the innovative components of STPHA.

A Conceptual Model of Remote Sensing Data Routing in the Grouping of Communication Satellites of a Multi-Satellite Space System

The use of low-orbit constellation of small or super-small satellites for solving problems of remote sensing of the Earth is a promising direction for the development of space activities. In order to ensure the communication of these satellites with ground-based points in advanced space systems, it is planned to use groupings of low-orbit communication satellites. At that, a certain formation of this grouping is considered. It is assumed that the satellites are evenly distributed in several orbital planes, and each of them has a connection with two neighboring satellites in its plane and two satellites in neighboring planes. The object of research in the article is the problem of routing, namely, the search for routes for relaying data streams from remote sensing satellites to ground-based information reception points. The proposed approach to solving the routing problem uses the following network specifics. The nodes of the network are remote sensing satellites, communication satellites and objects of ground infrastructure. In this case, you can highlight two fragments of the network. Communication satellites form the first fragment of the network, and the second one — the communication channels of these satellites with ground infrastructure objects and remote sensing satellites. The topology of the first network fragment is static, and the topology of the second fragment is dynamically changing. However, the dynamics of changes in the topology of this network fragment is predictable. It can be calculated based on satellite flight simulations and described as a contact plan that defines the time parameters of satellite communication sessions with ground stations. The solution of the problem is based on an agent-based approach. Satellite agents form the overlay layer of the network and, based on information interaction, provide route search, traffic balancing, and data transmission without delays at network nodes. The article offers information interaction schemes that provide both centralized and distributed route search options.

On the scalability of battery‐aware contact plan design for LEO satellite constellations

SummarySize and weight limitations of low‐Earth orbit (LEO) small satellites make their operation rest on a fine balance between solar power infeed and power demands of supporting communication technologies, buffered by on‐board battery storage. As a result, the problem of planning battery‐powered intersatellite communication is a very difficult one. Nevertheless, there is a growing trend toward constellations and mega‐constellations that are to be managed using sophisticated software support. In earlier work, we have discussed how the effective construction of contact plans in delay‐tolerant satellite networking can profit from a refined model of the on‐board battery behavior. This paper presents a profound study of the scalability of the approach and discusses how to tailor it to the needs arising in the management of mega‐constellations, together with a variety of improvements to the base approach. Results show that efficient mega‐constellation operations are compromised, encouraging further research on the area.

Modifi cation of the CGR-Algorithm on Data Routing in a Communication Network of Satellite Constellation

Communication networks in space systems involving the use of satellite constellations are DTN networks (Delay and Disruption Tolerant Networks). The establishment of communication channels in space communication networks has certain specifics: communication channels can be planned. In this regard, the CGR approach (Contact Graph Routing) is considered as the most promising solution to the problem of data routing. At the basis of this approach, taking into account this specificity, the calculation of the contact plan is considered. On the basis of this plan in the network nodes contact graphs are calculated, which are used to search the shortest data transmission routes. The paper proposes two interrelated solutions as a modification of this approach: the route search based on the contact plan, i.e. without calculation and use of the contact graph, and an adaptive method of finding the set of shortest routes required for routing. The essence of the first solution is as follows. In the standard CGR approach, the graph vertices correspond to the planned contacts between the network nodes, and the edges correspond to the data storage processes in the network nodes. In contrast, in the proposed approach, the vertices of the graph correspond to the nodes of the network, and the edges of the graph and their weight are determined dynamically, in the process of finding the shortest routes. The second solution is based on the concept of the planning front, which means a list of the closest contacts in time. The required routes are divided into a certain number of pools. Each pool combines the routes that use the specified contact from the planning front. The planning front is updated in two cases. If the network topology changes, the completed or not established contacts are replaced by subsequent ones with the same network nodes that are closest in time. If message traffic grows, a certain extension of the planning front and the use of additional route pools are performed. The article concludes with a description and justification of the expected advantages of the proposed approach.

Assessment of Satellite Contacts Using Predictive Algorithms for Autonomous Satellite Networks

Upcoming Low Earth Orbit Satellite Networks will provide low-latency and high downlink capacity necessary for future broadband communications and Earth Observation missions. This architecture was proposed at the beginning of the 90's, although it has just recently re-gained popularity thanks to the so-called Mega-Constellations. This network is composed of satellites that have Inter-Satellite Links (ISL) to communicate between them. Due to the satellite motion, an ISL is a temporal contact between two satellites characterized by a lifetime in which the communication remains feasible. The determination of a route between distant satellites is a challenging problem in this context. However, the satellite follows a well-known deterministic orbit trajectory, being feasible the prediction of its position by propagating a trajectory model over time. The Contact Graph Routing protocol uses this feature to determine the evolution of the routes by pre-computing on-ground a planning of the satellite contacts. This centralized ground-dependent solution cannot be directly applied in the Internet of Satellites paradigm, which proposes the autonomous deployment of heterogeneous satellite networks without pre-assuming any specific satellite system architecture. Following this concept, the present work proposes a distributed algorithm by which a satellite predicts neighbor contacts, and generates a global contact plan without trajectory propagation. To achieve this solution, an ISL has been modeled as a “close approach” between two satellites, which is characterized by their relative motion. The present work details the predictive algorithm, and evaluates its performance in two scenarios with a hybrid satellite constellation and a mega-constellation.

Contact Plan Design With Directional Space-Time Graph in Two-Layer Space Communication Networks

In the two-layer space communication network (TLSCN), communications can be performed to obtain higher throughput and lower latency by introducing various types of spatial nodes. However, the mobility of spatial nodes and the connectivity of spatial links result in the time-varying network topology, and intermittent link connection. This further leads to the lack of continuous contact, unreliable transmission, and high transmission cost. In this article, contact plan design (CPD) is employed to address the above problems by increasing the contact reliability while decreasing the contact cost. First, a directional space-time graph (DSTG) is constructed by considering the motion trajectory of spatial nodes and the time evolution nature of TLSCN. Afterwards, based on DSTG, we propose three CPD methods with greedy algorithm by considering the limited computing power of nodes. The three methods can optimize the objective functions of the total contact reliability, total contact cost, and invalid probability cost, respectively. The simulation results show that the proposed CPD methods can effectively improve the contact reliability, reduce the contact cost of TLSCN, and perform well with the increase of network density.

The views of social workers and special guardians on planning contact for special guardianship children

Purpose Special guardianship order (SGO) assessments require social workers to make plans and recommendations for ongoing post-SGO contact between the child and the parents. However, there is very little policy to inform and guide practitioners on how these duties should be undertaken, and no studies that describe current practice. The purpose of this paper is to investigate how the recommending of contact in special guardianship cases is currently working, by holding focus groups with social workers and special guardians. This paper reports on the results of a study to examine what contact plans social workers are recommending, the thinking behind their decisions and the views of the special guardians who have to make those plans work. Design/methodology/approach The research involved a mixed methods approach comprising of an online questionnaire, two focus groups for social workers and two focus groups for special guardians. This paper describes the second part of the study and reports on the qualitative results from the four focus groups. The methodology was based on a pragmatist theoretical position, and used an interpretivist approach and elements of the analytical procedure of grounded theory in order to generate inductive research. The focus group method was chosen as the best way to gather rich information on the opinions and ideas of practitioners who are recommending contact and the carers who are managing it. Findings Participants provided a wide range of views on the issues, with practitioners describing the challenges of planning contact, and special guardians explaining the problems they faced with the parents. Involving special guardians in the study gave a chance to include the different perspectives of the people who have to make the contact recommendations work, and contrast their views on contact planning with those of the professionals. The study makes recommendations for practice, which recognise the difficulty of preparing an initial contact plan that will remain relevant for years ahead. Research limitations/implications The number of focus groups the author held was limited by the author’s own personal resources and the time the author had available, and one group only had three social workers on the day. The author’s involvement affected the responses, and the author’s questions dictated the issues that were commented on, but the answers were the opinions that the participants wanted to express. The nature of the approach means that no two sets of focus group results would ever be the same. And as the direction of the discussions was largely dictated by the participants, the coverage of all aspects of contact was probably inconsistent. Practical implications This research sheds light on a crucial area of social work permanency planning, that has suffered from a lack of previous research, in order to better inform future practice. The paper reports on what contact plans social workers are recommending, the thinking behind their decisions and the views of the special guardians who have to make those plans work. It concludes with recommendations for improving future special guardianship policy and practice. Social implications The research clearly raises a number of specific difficulties faced by special guardians and problems with current policy and practice. These include the special guardians’ lack of understanding about contact, the difficulty for social workers of long-term planning, the challenge posed by uncooperative parents who behave badly, the view of carers for the need for a greater emphasis on the quality and reliability of contact, and the challenge to careful contact planning posed by the adversarial court process. Originality/value Special guardianship has had a major impact on permanency planning since its introduction 12 years ago. However, apart from one DfE study in 2014, very little research has been produced to inform policy and practice. There have been no studies specifically on contact in special guardianship cases, despite contact being one of the two major factors in determining the success of SGO placements. This study has provided the first in-depth evaluation of social worker contact planning in special guardianship, and the first investigation of special guardians’ views on contact.

Application of Probabilistic Method to Evaluate the Rock Mass Foundation Influence over the Probability of Failure by Sliding of Hydraulics Structures: Case Study of Baixo Iguaçu HPP Spillway

In the last decades, accidents involving dams and other hydraulic structures have become more frequent, increasing social pressure on the existing dams safety conditions. This fact demands commitment of professionals and companies with new plant project safety approaches. The failure of a hydroelectric concrete structure can be caused by several factors, however the main causes of failure are related to exceptional floods and foundation-related problems, which are the focus of this paper. In order to understand the influence of the rock mass characteristics, especially the contact plan with the concrete structure, this work considers the variability of geotechnical parameters, drainage efficiency, specific gravities and seismic acceleration to determine the probability of failure of these structures. To achieve this objective, the work makes use of a case study, the spillway of Baixo Iguacu HPP, on Parana/Brazil during its design and construction phase, determining the probability of failure by sliding. The study reproduces the foundation condition throughout a FEM hydrogeological model and determines the acting uplift through seepage analysis. The probability of failure was determined by Monte Carlo method using the probability distribution of each variable, performing 5,000,000 evaluations for each simulation. Simulations were performed for flow range varying from the ecological flow (200 m3/s) to exceptional floods with 10,000 years of recurrence time (56,362 m3/s) and its respective water levels. The most critical condition found was the extended operation at the minimum (ecological) flow, while for a normal operation, at the flow of 2598 m3/s, the order of the probability of failure is one in a million. The paper show how using hydrogeological model can lead to more realistic values of uplift pressures than using simplified methods recommended in the international guidelines, and consequently lead to a more realistic probability of failure.

Distributed Contact Plan Design for GNSSs

Next generation global navigation satellite systems (GNSSs) will embrace intersatellite links capability as an enabler of navigation and data transfer. However, platform restrictions will require of suitable contact plan design (CPD) schemes, which have been traditionally assumed centralized . After discussing the requirements of distributed CPD, we propose a first scheme in this class. Simulation results on the BeiDou GNSS prove that satisfactory metrics can be obtained enabling valuable and real autonomy for future GNSSs.

Heuristic computing methods for contact plan design in the spatial-node-based Internet of Everything

To satisfy the increasing demands of high-speed transmission, high-efficiency computing, and real-time communications in the high-dynamic and heterogeneous networks, the Contact Plan Design (CPD) has attracted continuous attention in recent years, especially for the spatial-node-based Internet of Everything (IoE). In this paper, we study the NP-hardness of contact scheduling and the attenuation of atmospheric precipitation in the spatial-node-based IoE. Two heuristic computing methods for contact plan design are proposed by comprehensively considering the time-varying topology, the intermittent connectivity, and the adaptive transmission in different weather conditions, which are named Contact Plan Design-Particle Swarm Optimization (CPD-PSO) and Contact Plan Design-Greedy algorithm with the Minimum Delivery Time (CPD-GMDT) separately. For the population-based algorithm, CPD-PSO not only solves the CPD problem with a limited-resource condition, but also dynamically adjusts the search scope to ensure the continuous searching capability of the algorithm. For the CPD-GMDT that makes CP decisions based on the current state, the algorithm uses the idea of greedy algorithm to schedule Satellite-Platform Links (SPLs) and Inter Satellite Links (ISLs) respectively using the strategies of optimal matching and load balancing. The simulation results show that the proposed CPD-PSO outperforms Contact Plan Design-Genetic Algorithm (CPD-GA) in terms of fitness and delivery time, and CPD-GMDT presents better overall delay than Fair Contact Plan (FCP).

An Intelligent Computing Method for Contact Plan Design in the Multi-Layer Spatial Node-Based Internet of Things

Computational Intelligence (CI) has been addressed as a great challenge in recent years, particularly the aspects of routing, task scheduling, and other high-complexity issues. Especially for the Contact Plan Design (CPD) that schedules contacts in dynamic and resource-constrained networks, a suitable CI algorithm can be exchanged for a high-quality Contact Plan (CP) with the appropriate computational overhead. Previous works on CPD mainly focused on the optimization of satellite network connectivity, but most of them ignored network topology characteristics. In this paper, we study the CPD issue in the spatial node based Internet of Things (IoT), which enables the spatial nodes to deliver data cooperatively via intelligent networking. Specifically, we first introduce a Multi-Layer Space Communication Network (MLSCN) model consisting of satellites, High Altitude Platforms (HAPs), Unmanned Aerial Vehicles (UAVs), and ground stations, on which a Time-Evolving Graph (TEG) is used to illustrate the CPD process. Then, according to the characteristics of each layer in the MLSCN, we design the corresponding CPs for the inter-layer contacts and intra-layer contacts. After that, a CI algorithm named as Multidirectional Particle Swarm Optimization (MPSO) is proposed for inter-layer CPD, which utilizes a grid-based initialization strategy to expand the diversity of individuals, in which a quaternary search method and quaternary optimization are introduced to improve efficiency of particle swarms in iterations and to ensure the continuous search ability, respectively. Furthermore, an optimized scheme is implemented for the intra-layer CPD to reduce congestion and improve transmission efficiency. Simulation results show that the proposed CPD scheme can realize massive data transmission with high efficiency in the multi-layer spatial node-based IoT.

Channel-Aware Mission Scheduling in Broadband Data Relay Satellite Networks

Mission scheduling algorithms are envisioned as critical to satisfy the increasing mission requirements in broadband data relay satellite networks, which is severely influenced by time-varying inter-satellite contacts (i.e., potential available communication links) and differentiated satellite downlink contacts. Nevertheless, the intertwined effect of such two types of contacts on mission schedules poses daunting challenges for the efficient mission scheduling design. In this paper, to achieve fair performance among user satellites, we maximize the minimum number of successfully scheduled missions over all user satellites by jointly optimizing contact plan design, power allocation (PA) in relay satellites, and mission schedules based on the time-expanded graph. The formulated problem is a mixed-integer nonlinear program optimization problem that is challenging to solve. For tractability purpose, we equivalently decompose the problem into a PA problem and an optimal PA-based mission scheduling (OPA_MS) problem, which is still a mixed-integer linear program. We further devise a new two-stage scheme to efficiently solve the OPA_MS problem. Simulation results validate the significant gains of the proposed algorithm in mission completion number and necessitate the consideration of the time-varying and differentiated inter-satellite and downlink contacts.

Caring Together 2 Breathe Easy: an integrated care trial to reduce readmission and improve the health care experience for people with COPD

Introduction: As evidenced in “Perils of Place – Identifying hotspots of health inequality” (Duckett, 2016), Caboolture (Queensland) was highlighted as having a 10 year history of higher than average readmission rates for patients with Chronic Obstructive Pulmonary Disease (COPD). A new Integrated Pathway of Care is being trialled for COPD patients (n=30). Key collaborators include: the Caboolture Hospital Emergency Department and Respiratory Service; GP practices, Queensland Ambulance Service and key community services. The model of care, known as Caring Together 2 Breathe Easy was developed in collaboration with Canterbury Health in New Zealand. This model employs a patient focus to improve QoL by implementing COPD Action Plans and ensuring that the patient, their GP, Ambulance and the Emergency Department utilise this plan, including alternatives for acute care. A “Breathe Easy” team: GPLO, Clinical Nurse; Physiotherapist and Psychologist, work with the participants to implement action and intervention plans. Participants are provided with a pack which includes their COPD action plan, contact plan and educational materials. Aims and Measures: Reduced ED presentations and re-admission rate within 28 days Improved QoL measured by COPD Assessment Tool (CAT) and meeting individual goals Identification of secondary anxiety and depression (K-10) and linkage to supports Improved GOLD Score Increased access and connection to health and community services Developing integrated pathways with health and community services. Targeted Population and Stakeholders: Patients with a diagnosis of COPD and a history of re-presentations and/or re-admissions to the Caboolture Hospital within 28 day within the past two years (N=30) Participants reside in the Caboolture community which has recognised unmet health needs and high levels of socioeconomic disadvantage (Source: The Social Health of Australia: Data by population area, Population Health Information Data Unit 2016) Timeline 2017: Feb: Cohort selected March- April: Recruitment, Assessments May-Jun: Action Plans July-October: Interventions Nov-Dec: Evaluation Highlights: COPD Action Plans developed for all patients; High levels of consumer engagement with Breathe Easy Team and Project; Interventions commenced; Improved engagement and co-operation across secondary hospital, primary care, ambulance and community agencies. Sustainability: The intention is to upscale and embed the “Breathe Easy” pathway trial at an organisational and local community level. Transferability: If successful, the “Breathe Easy” trial outcomes will contribute to models of integrated care that can be applied to a range of patient groups requiring integrated care. Conclusions: Caboolture Hospital “Breathe Easy” project is demonstrating high levels of engagement and support from consumers, health care providers and the community. The multi-level evaluation data will provide evidence and inform viability for up-scaling and consolidation as an alternative health care service model in Caboolture and more broadly in Metro North HHS.

Cascade optimization design of inter-satellite link enhanced with adaptability in future GNSS satellite networks

Inter-satellite communication and inter-satellite ranging are the foundation of autonomous navigation for satellite navigation systems. Due to cost limitations, it has been proposed in recent years to equip each satellite with one spot beam antenna, which points to different satellites according to a polling mechanism, resulting in an intermittently connected satellite network. This poses the problem of how to design the inter-satellite link (ISL) contact plan, which determines the evolution of network topology and has important effects on system performance. We propose a new framework for the ISL contact plan design in satellite navigation systems. Considering contact plan design as a multi-parameter and multi-objective optimization problem, the cascade optimization design (COD) is proposed as a method simple to implement and optimize the parameters of the contact plan. COD considers network load and geometry of satellites and has a good adaptability. In the proposed framework, both communication and measurement requirements are taken into account while the contact sequence and the slot length are optimized in two steps. Simulation results show that COD guarantees zero packet drops and achieves the least average delay with a selected network load. When the packet arrival rate is 0.45 packets/s, the packet drop ratio of COD is zero, while that of the traditional simulated annealing design (SAD) is 2.58%. The delay of COD is 22.97 s, which is only two-fifth of the SAD value of 58.77 s. At the same time, using COD the average autonomous navigation weighted dilution of precision decreases from 1.4408 to 0.9671. COD also has strong robustness and performance regardless of the onboard buffer size.

Distributed contact plan design for multi-layer satellite-terrestrial network

In multi-layer satellite-terrestrial network, Contact Graph Routing (CGR) uses the contact information among satellites to compute routes. However, due to the resource constraints in satellites, it is extravagant to configure lots of the potential contacts into contact plans. What's more, a huge contact plan makes the computing more complex, which further increases computing time. As a result, how to design an efficient contact plan becomes crucial for multi-layer satellite network, which usually has a large scaled topology. In this paper, we propose a distributed contact plan design scheme for multi-layer satellite network by dividing a large contact plan into several partial parts. Meanwhile, a duration based inter-layer contact selection algorithm is proposed to handle contacts disruption problem. The performance of the proposed design was evaluated on our Identifier/Locator split based satellite-terrestrial network testbed with 79 simulation nodes. Experiments showed that the proposed design is able to reduce the data delivery delay.

Assessment of Astronauts’ Workload with Task-Irrelevant Auditory Probes In Manually Controlled Spacecraft Rendezvous and Docking

Introduction. The rendezvous and docking (RVD) operation is a challenging task of high risk for astronauts. It is an essential part of missions involving more than one spacecraft such as transportation missions for the International Space Station supply. While intensive automation is an indispensable ingredient of the modern RVD operations, manual docking remains one of the critical skills for an astronaut. Manual RVD is a cognitively demanding task that requires a constant sustained attention. The astronaut has to perform a precise docking by minimizing the linear and rotational speeds of the spacecraft using minimum control actions, thus, minimizing the fuel consumption. The performance in RVD operation can be modulated by the astronauts’ workload. Efficient assessment techniques can be used on the ground both for human factor evaluation of the system design and training purposes, as well as in space for adaptive astronaut-spacecraft interaction. In this study, we evaluate whether task-irrelevant auditory probes can be an effective assessment method of astronauts’ workload. Materials and Methods. The docking operation was simulated using the simulator of the Moscow Aviation Institute, which uses two stereographic projectors and 3D-glasses reproduces the immersive experience of the SOYOUZ module docking to the International Space Station (ISS). An astronaut can operate two levers: left lever controls the translational speed (Vx, Vy, Vz), while the right one controls the rotational speed (Wx, Wy, Wz) of the spacecraft. The numerical values of both speeds are displayed on the screen. The levers can be operated with or without feedback. With the feedback, when a lever is released, the corresponding speed is automatically reduced to 0. Visual guiding cues can also be displayed on the screen: a dotted circle indicating the direction of the speed vector, a cross corresponding to the projection of the docking in the current configuration, and a corridor represented with four trajectory lines perpendicular to the final contact plan of the docking. Three scenarios were used to modulate the participants’ workload. In the first scenario, both levers feedback and visual cues were present. In the second scenario, only feedback was present. In the third one, only visual cues were present. Three participants with docking simulator experience performed three times each docking scenario. The workload was measured using task-irrelevant auditory probes. The participants were presented with 200 ms high (2 kHz) and low (1 kHz) tones in 20/80% proportion with a variable inter-stimuli interval from 2 to 3 seconds. They were asked to verbally report the rare high tones and their response times were measured. The participants evaluated each scenario using NASA-TLX questionnaire dimensions. Results. The participants performed fewer input actions for the scenario n°2 with visual cues only. While most efficient for fuel consumption, the second scenario induced the least accurate final docking positions, and was considered by participants as the most demanding both mentally and physically. The response times were consistent with the subjective and objective difficulty of the scenarios: 475.3 ms on average for the scenario n°2 vs 412.4 ms for the scenario n°1 and 387.6 ms for the scenario n°3. Conclusion. The presented work showed that a simple technique of task-irrelevant auditory probes can be used to assess the astronauts’ workload during manual controlled spacecraft rendezvous and docking. The participants of this experiment in SOYOUZ module simulator showed less accurate docking in a scenario without lever feedback. They also evaluated this scenario as particularly cognitively and physically demanding. The response times to irrelevant high-pitch tones were in line with the objective (performance) and subjective (questionnaire) measurement of the scenarios’ difficulty. Therefore, this simple technique can be used in such situations as manual RVD in order to evaluate the astronauts' workload.

Mission Aware Contact Plan Design in Resource-Limited Small Satellite Networks

Small satellite networks (SSNs) are playing an increasing role in nowadays earth observation due to their less development cost and energy consumption. In SSNs, it is pivotal to transmit a huge amount of data for differentiated missions to ground stations. Nevertheless, due to limited transponders and energy budget, not all contacts, i.e., potential available communication links, are feasible in data delivery. Besides, satellite downlink channel conditions are indeed time-varying due to atmospheric precipitation. Therefore, one daunting challenge is searching for feasible contacts termed as contact plan design with consideration of the differentiation for missions. In this paper, we exploit an extended time-evolving graph to characterize network resources. Based on the graph, we formulate the design of mission-aware contact plan, aiming at maximizing network profit in terms of sum weighted data volume as a mixed-integer linear programming. Due to its NP-hardness, we propose a primal decomposition method to efficiently solve the formulated problem by exploiting its special structure. To further reduce the complexity, we propose a link metric considering the issues of residual energy of satellites, time-varying satellite downlink contact capacity, and the differentiation for missions in the conflict graph. Based on the conflict graph, we devise a heuristic algorithm to design contact plan. Simulation results demonstrate the efficiency of the proposed algorithms and necessitate the consideration of the time-varying downlinks and the differentiation of missions for contact plan design.

Dynamic Contact Plan Design in Broadband Satellite Networks With Varying Contact Capacity

Contact plan design (CPD) aims to schedule conflicting forthcoming communication opportunities (i.e., contacts). In CPD, the capacity of established contacts is indeed time-varying. Neglecting such variations in contact capacity inevitably leads to system performance deterioration. In this letter, we develop a new dynamic CPD algorithm while considering the time-varying property of contacts in broadband data relay satellite networks. The novelty lies in that we treat the flow optimization problem in a time-expanded graph as a queue-stability-related stochastic optimization problem, such that the problem can be solved independently slot by slot. Furthermore, we prove that the proposed online algorithm can approach arbitrarily close to the optimal throughput. Simulation results validate the efficiency of our proposed algorithm.

An evolutionary approach towards contact plan design for disruption-tolerant satellite networks

Delay and disruption tolerant networks (DTNs) are becoming an appealing solution for satellite networks where nodes can temporarily store and carry in-transit data until a link with a suitable next-hop becomes available. Since satellite trajectories and orientation can be predicted, on-board routing schemes can base these forwarding decisions on a contact plan comprising all forthcoming communication opportunities. In general, contact plans are previously calculated on ground where their design can be optimized to consider not only available spacecraft resources but also the expected traffic which is largely foreseeable in space applications. Despite optimal contact plan design procedures exist, their computation complexity might result prohibitive even for medium-sized satellite networks. In this work, we propose an evolutionary algorithm to provide sub-optimal yet efficient and implementable contact plans in bounded time. In particular, we depict specific strategies such as encoding and repairing techniques to later evaluate the algorithm performance in a typical scenario demonstrating its usefulness for planning future DTN-based satellite networks.

Traffic-aware contact plan design for disruption-tolerant space sensor networks

Delay and disruption tolerant networks (DTNs) are becoming an appealing solution for extending Internet boundaries across challenged network environments. In particular, if node mobility can be predicted as in space sensor networks (SSNs), routing schemes can take advantage of the a-priori knowledge of a contact plan comprising forthcoming communication opportunities. However, the design of such a plan needs to consider available spacecraft resources whose utilization can be optimized by exploiting the expected data which is largely foreseeable in typical Earth observation missions. In this work, we propose Traffic-Aware Contact Plan (TACP): a novel contact plan design procedure based on a Mixed Integer Linear Programming (MILP) formulation which exploits SSNs predictable properties in favor of delivering efficient and implementable contact plans for spaceborne DTNs. Finally, we analyze a low orbit SSN case study where TACP outperforms existing mechanisms and proves to be of significant impact on enhancing the delivery of sensed data from future space networks.