Distributed Satellite Cluster Network:a Survey

DONG Fei-hong(董飞鸿)，WANG Jing-chao(王敬超)，YANG Jie(杨 杰)，CAIChun-xiao(蔡春晓)

1 College of Communication Engineering，PLA University of Science and Technology，Nanjing 210007，China

2 Institute of China Electronic System Engineering Corporation，Beijing 100141，China

Distributed Satellite Cluster Network:a Survey

DONG Fei-hong(董飞鸿)1，2*，WANG Jing-chao(王敬超)2，YANG Jie(杨 杰)2，CAIChun-xiao(蔡春晓)2

1 College of Communication Engineering，PLA University of Science and Technology，Nanjing 210007，China

2 Institute of China Electronic System Engineering Corporation，Beijing 100141，China

The ability of the monolithic satellite，satellite orbit (especially GEO)，and radio resource are very lim ited，so the development of distributed satellite cluster network(DSCN)receives more and more worldw ide attention.In this paper，DSCN is surveyed and the study status of DSCN architecture design is summarized.The formation flying of spacecrafts，reconfiguration，networking，and app lied research on distributed satellite spacecraft are described in detail.The DSCN w ill provide a great technology innovation for space information network，satellite communications，satellite navigation，deep space exploration，and space remote sensing.In addition，this paper points out future trends of the DSCN development.

distributed satellite cluster network(DSCN);architecture; formation flying;networking;space information network

Introduction

W ith the rapid grow th of satellites in the space，the information exchange technology is more and more complex.The satellites function in“bent pipe”fashion w ill notmeet the requirements，which exchange information ground-based.So the space-based information transport systems are studied in many countries，such as US，UK，and China.The USM inistry of National Defense proposed the global information grid (GIG)［1］.To achieve its aim，the transformational satellite communications system(TSAT)was established on 3rd Sep.2002 by authority of the undersecretary of the Air Force.To develop and coordinate implementation of transformational communications architecture(TCA) to maintain America＇s asymmetric information and decision superiority and address the need of replacing numerous aging legacy systems［2］.This architecture w ill provide a constellation of interoperable satellites (hereafter called distributed satellite cluster(DSC))，and objectively improve connectivity and data transfer capability while removing bandw idth constraints for worldw ide.

Distributed satellite cluster(DSC)consists of numbers of satellites which distribute on the same or adjacent orbit positions.The satellite vehicles cooperate to achieve a particular mission.They are interconnection and intercommunication through inter-satellite links.The DSC can fast reassembly through redundancy when vehicles or inter-satellite links fail.The DSC is also named as payload distributed communication satellite system(DCSS)，distributed space system(DSS)，and fractionated spacecraft cluster(FSC)in general，despite that they may have somewhat different.DSC is an alternative to the monolithic satellite: divides or“fractionates” a system，distributing the individual sensors，on board processers，and mass storages，etc.There aremany systems such as space based group(SBG)，F6(future，fast，flexible，fractionated，freeflying spacecraft)，and SkyLAN［35］.

Distributed satellite clusters network (DSCN) is a backbone network in the space which consists of numbers of DSCs in geostationary earth orbit(GEO).The DSCs intercommunicate through inter-DSCs laser links.The links are capable of transm ission rates of 1-10 Gbps.The distance between two DSCs is about 7×104km to achieve worldw ide coverage.The users of the DSCN include deep space spacecrafts，space-based vehicles，air-based aircrafts，groundbased users，and sea-based ships，etc.The service information includes voice，image，video，and remote sensing data，etc.In this paper，we consider that each of the researched system in section 4 is a DSCN［6］.

1 Design of DSCN Architecture

Different from conventional satellite systems，the DSCN is a novel space vehicle in terms of architecture design.The architecture is a top-level design for DSCN.So much work has been done into DSCN architecture design.There are mainly three aspects:(1)DSCN satellite nodes spatial layout design (namely topology structure design)，(2)DSCN satellite nodes data transmission(namely inter-satellite communication)，and (3)DSCN satellite nodes payload division.

1.1 Topology structure design

The DSCN typical topology structures include line topology，ring topology，star topology，and mesh topology，as shown in Fig.1.These topologies have high regularity.So they always have zero entropy or low entropy［7］.For example，the TSAT space segment consists of a constellation of five geostationary satellites and Continental United States(CONUS) ground gateway elements(CGGEs).The five geostationary satellites can be connected in various ways，including a ringor a line(string).Regardless of the physical topology，they can form a fullmesh logical topology.The use of a fullmesh reduces the space forwarding load，and perm its better traffic engineeringmanagement［8］.

The line topology is a simple network topology，which is easy to operate and maintain.Because the satellite in this topology is easy to adjust attitude，and the topology has no central node.The A-train(or the p.m.train)is line topology［9］in Fig.2(a).However，the disadvantage of line topology is that each node only has links w ith its neighbors，which provides an average path length of O(n/3).W ith the development of large scale network，the information w ill be relayed in the whole nodes.This w ill result in low information transm ission efficiency and poor robustness.

In the ring topology，the data transm its double directions through the network.Every node has two neighbors，one is sender and the other is receiver.One exam ple is that three GEO satellites are connected as ring topology to achieve worldw ide coverage.The ring has better robustness than that of line，because each node is at an equal position in the network.If one node breaks down，the ring can just avoid connecting it and forms new ring network immediately.However，the ring has no central node.It is hard to operate and maintain.Furthermore，w ith the development of network scale，the data will be relayed in a multi-hop way.This w ill result in low information transm ission efficiency and large time delay.The average path length between each vehicle is O(n/4).

The star topology is a master-slave structure，in which the center satellite plays a role of hub and others surround the hub.Each slave node has a link with the master node，and sends data to the master node.Communication among the slave nodes should be through the center hub.This structure isw idely used in satellite communication because of its high communication efficiency and easy implementation.The defense advanced research project adency(DARPA) system F6 is an example in Fig.2(b).The advantage of star topology is that the average path length between each vehicle is O(2).So the average path length does not increase w ith the enlarging of network scale.The star has high information transm ission efficiency and low time delay.If slave node breaks down，it w ill affect little on network performance.However，when the master node breaks down，the whole network w ill be disintegrated.This is the disadvantage of star topology.Furthermore，the hub satellite traffic load is high，which may lead to a network jam.

The mesh topology has the best robustness among typical topologies，because each node is at an equal position in the network，and network has no center node.There are somany links among the nodes.If one node breaks down，it w ill affect network performance little.Every node is directly connected，so the information transfers in time.The average path length between each vehicle is O(1).However，the disadvantages ofmesh are obvious.Every node has link w ith each other，when the scale of the network expands the number of links w ill increase rapidly w ith O(n2).This is a big grow th of the demand for inter-satellite links.

1.2 Inter-subsatellite communication

The information stream transmission technology willmainly solve the cooperative transmission for every subsatellite in DSCN.The cluster w ill be connected through inter-satellite communication.The main functions of inter-satellite communication are that:(1)the reliable and stabilized intersatellite links are established among hub satellites and client satellites in physical layer;(2)subsatellites are efficient and flexible access in link layer;(3)self-organizing wireless satellite cluster networks and multi-hop information transmission are achieved in network layer;(4)the service-oriented transmission validity is guaranteed in transport layer and application layer［10］.The complex space environment，low power and process ability are negative factors for inter-satellite communication.

1.3 Payload division

Thepayload division of DSCN satellite can be achieved in three wayswhich are defined as homogeneous，heterogeneous，and hybrid.The first one is the decomposition into identical or functionally similar modules considering the redundancy of a subsystem or decoupling multiple payloads.An example of homogeneous is Iridium satellites communication system because the payloads are decoupled.The second one is the decomposition of a given spacecraft into many dissim ilar subsystems or functional elements.This is called heterogeneous.The functionality of the system is distributed into many distinct and independent infrastructure and payload modules［11］.Typically，a cluster is composed of identical satellites.This happens for Technology Satellite 21 Century (Techsat-21)，Canadian Advanced Nanospace eXperiment (CanX)-4 -5，Three Corner Satellite(3CSat)，and Tiny，Independent，Coordinating Spacecraft(TICS)［12］，but it has been also studied on the possibility to have different payloads on the flight platforms，as foreseen for F6 or SBG［1314］.The third class called“hybrid”can be also considered as combining homogeneous and heterogeneous.All of these three types are shown in Fig.3.This would also provide a measure of homogeneity w ithin DSC.

The payload division of DSCN satellite can also be defined as nodes distributed， antennas distributed，processing distributed， storage distributed， etc.For exam ple，the processing distributed DSC is that the clusters share the processors in each vehicle，called distributed pool of processors.The characteristics of these processors are low power，inexpensive，and commercial quality.There are about 10 to 100s of processors in DSC.The DSC has fault tolerance.If several processors fail，we sim ply don't care.Fault tolerance is the gradual loss of system functionality due to one or more failures.This attribute depicts the difference in the response of distributed and monolithic spacecraft to the conditionswhere the system turns out to be not robust.For exam ple，the change in functionality for various stimuli and conditions can be dramatic for monolithic spacecrafts while a well-designed distributed architecture should degrade capability incrementally［15］.

2 Formation Flying of Spacecrafts

One of themain challenges of DSCN is the precise control technology.The relative position of satellites in cluster formation flying is very important.The separated distance affects cluster collision and sensing performance.

2.1 Collision avoidance

One important area of research that must converge to a solution before these satellite clusters are fully operational is that of collision avoidance，both in terms of orbital debris and among the satellites in the cluster itself.Much work has gone into predicting risk from orbital debris on satellites in all orbits.It is clear that some common orbital patterns are becoming less attractive due to the increase in debris from older spacecraft.Recentwork has focused on evaluating the effects that future space constellation concepts w ill have on the orbital debris problem and vice versa.Studies have shown that a single collision w ithin a cluster would not only drastically increase the risk to the other satellitesw ithin the cluster，but also the debris would propagate and potentially render the orbit useless for many years to come.Future space cluster concepts thereforew ill require a highly reliable system of control loops，fault detection，and collision avoidance for full deployment［16］.

The cluster is strung out through tens of kilometers of space;the station keeping tolerances required to avoid collision has been shown to be wellw ithin the state of the art.Onemath can be described as that:each satellite have a capability to derive its own location，using GPS，active ground ranging，or other techniques［14］.

2.2 Autonomous operation

Because of the vehicle perturbations，the cluster formation will changewith time.Thus，the autonomousoperation technology is needed to adjust and maintain the flying formation at the right moment.Or when themission requirements change，the network can be reconfigured to adapt the new environment.These can also occur when the subsatellite goeswrong or is destroyed.The DSCN autonomous operation technology can adapt at the complex space environment and choose the optimal space formation scheme［17］.This is one of the key features of the DSCN.For example，the autonomous operation system may propose amaneuver scheme by optimizing the multi-constraint including influences of the sun，space reflection control，system hardware ability，inter-satellite communication ability，power consume，etc.

3 Reconfiguration， Self-healing， and Networking

Self-reconfiguration is the typical feature of DSCN.The self-reconfiguration is established by many inter-satellite communication links which can form a network dynam ic selfhealing mechanism.When parts of the nodes go w rong，the information processing and transmission ability w ill not totally lose.In fact，it justslightly decreasesbecause of on board quick self-healmechanism.W ith the new sub nodes joining in the network，the information processing and transm itting ability w ill be rapidly recovered and enhanced.

The reconfiguration and self-healing mechanismare shown in Fig.4.The link control and neighbor node discover protocol can adjust the transmitted power and antenna direction.Thisw ill bring about a hop-by-hop connection recovery in the perspective of link reestablishing.Another perspective is the sub-satellite controlmechanism，which w illmove the subsatellite vehicle to a right orbit to recover the network when the inter-subsatellite link does not work.The third perspective is regionalization and reconfiguration.The recovery region topology optimizing strategy will be studied.This is always a multi-objective optimization problem(MOP).The combination of these three views above can finally achieve the aim of self-reconfiguration self-healing.

The vehicles inter DSCN can be connected through Ethernet and every device/subsystem talks TCP/IP protocol.The paper studies also show that vehicles inter DSCs can use 2.4GHz WiFiand 802.11 s protocol［18］.

However，in our opinion vehicles in DSC are connected through inter-satellite laser links，whose transm ission rates are 1 to 10Gbps［19］.The distance between each vehicle varies from 1 to 100 km.In the case of formation flying，the satellites need to be controlled precisely to keep a specific configuration.However cluster flying does not require a strict relative position control as long as collision avoidance is ensured.

4 Application Research

There aremany distributed satellite cluster networks，such as SBG，and SkyLAN in GEO;The am.Train，The pm.train，F6， Techsat-21［20］， and Space Technology-3 in LEO; Magnetosheric Multiscale［21］，Simbol-X in HEO;and Solar Terrestrial Relations Observatory［22］，Laser Interferometric Space Antenna［23］in heliocentric orbit.The research fields include satellite communication，satellite navigation，astroobservation，meteorologic，hydrographic，etc.In this section，we w ill just introduce the three of them because of the length limitation.

4.1 SBG

SBG is a new architecture for space systems in which a single system is implemented by a group of spacecraft.SBG also implies a radically new approach to theway we acquire and think aboutspace assets［3］.SBG is proposed by Applied Physics Laboratory of Johns Hopkins University and The U.S.National Security Space Office in 2007.The idea of the project is to divide the system function into many subsatellites.The fundamental functions are mainly concentrated on hub satellite (or service satellite).So there are three kinds of satellites:hub satellite，mission spacecraft，and group support satellites in the view of satellite function.The SBG architecture is shown in Fig.5.The designed architecture mainly has the characteristics as follows.Firstly，each subsatellite just achieves onem ission.Secondly，multiple router satellites w ill be used to achieve worldw ide coverage.Thirdly，the orbitalmaneuver and cluster reconfiguration are the key technologies.The use of DSC w ill lead to single subsatellite low complexity and R＆D costs.The new technique can be applied in the SBG quickly.This is also a responsive space system.

4.2 F6

The F6 program has three parts:the F6 developer＇s kit (FDK)，the F6 tech package(F6TP)，and the on-orbit demonstration testbed.All the modules are united by information exchange.The FDK consists of standard interfaces.The F6TP consists of modular hardware applicable to a w ide range of spacecrafts.The on-orbit demo testbed pertains to affordable satellite buses.The F6 architecture is shown in Fig.6.The F6 system program architecture is applied to reducing the new space threats，and establishing future-oriented flexible effective spacecraft system structures.The monolithic spacecraft is fractionated into many composable modules.The w ireless energy transmission，w ireless data transmission，and on board reconfiguration w ill be used.The fractionated modules can be rapidly made and independently launched.Themodules w ill make up a full function virtual spacecraft system.This system has ability of function redefinition on board scalability and maintainability.

4.3 SkyLAN

SkyLAN is a cluster of geostationary satellites for broadband communications.The project proposes a space local area network system which distributes several small satellites in GEO.The small satellites are connected through inter-satellite links.This project aims to provide mobile satellite service (MSS)，fixed satellite service(FSS)，and broadcasting satellite service(BSS).

5 Conlusions

In this paper，an overview of DSCN is given in the view of mainly current research directions.These include architecture design，satellite cluster formation flying，reconfiguration，selfhealing mechanism，networking，and application research.The SBG，F6，and SkyLAN are introduced as the main content of section 4.In fact，the DSCN has been applied in many fields，such as satellite communication，satellite navigation，astroobservation，meteorologic，and hydrographic.The orbits include GEO，HEO，LEO，and heliocentric orbit.The DSCN w ill play a big role in the space information network.

Acknow ledgment

We thank Dr.Zhang Gengxin，professor of Institute of Communication Engineering，PLA University of Science and Technology for his beneficial suggestions and prom ising instruction on satellite network performance analysis.We are also grateful to Dr.Li M in for generously sharing information about distributed satellite cluster.Finally，we would like to thank the anonymous reviewers for their constructive comments.

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TN927 Document code:A

1672-5220(2015)02-0332-05

date:2014-10-10

s:National Natural Science Foundations of China(Nos.61032004，91338201，and 61231011);National High Technology Research and Development Program of China(863 Program)(No.2012AA121605)

* Correspondence should be addressed to DONG Fei-hong，Email:dfh_sinlab@163.com