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- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
LASER INTER-SATELLITE LINKS TECHNOLOGY State of the art resource covering key technologies and related theories of inter-satellite links Laser Inter-Satellite Links Technology explores satellite networking as a growing topic in the field of communication technology, introducing the definition, types, and working frequency bands of inter-satellite links, discussing the number of orbital elements of the spacecraft motion state under two-body motion and their conversion relationship, and establishing the basic demand model for inter-satellite link network, chain topology model, and transmission protocol model. The book focuses on the analysis and introduction of the principles and error sources of microwave and laser inter-satellite ranging, including the basic composition, workflow, and constraints of the laser inter-satellite link, and related design principles of the inter-satellite laser transmitter and receivers. Later chapters also discuss theories and methods of acquisition, alignment, and tracking, the impact of alignment errors on performance, and inter-satellite link modulation and its implementation. Specific sample topics covered in Laser Inter-Satellite Links Technology include: * Pulse position modulation (PPM), differential pulse position modulation (DPPM), digital pulse interval modulation (DPIM), and double-head pulse interval modulation (DH-PIM) * Basic demand model of inter-satellite link network application, including basic configuration of constellations and inter-satellite transmission networks * Inter-satellite ranging accuracy, principles of microwave inter-satellite ranging, and analysis of microwave ranging error sources * Effect of tracking error on the beam distribution at the receiving end and influence of tracking and pointing error on communication error rate Laser Inter-Satellite Links Technology serves a completely comprehensive resource on the subject and is a must-have reference for experts and scholars in aerospace, along with graduates and senior undergraduates in related programs of study.
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Veröffentlichungsjahr: 2022
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Table of Contents
Cover
Series Page
Title Page
Copyright Page
Author Biography
Preface
1 Introduction
1.1 Connotation of Inter‐Satellite Link
1.2 Types of Inter‐Satellite Links
1.3 Band Selection of Inter‐Satellite Link
1.4 Microwave Inter‐Satellite Link
1.5 Laser Inter‐Satellite Link
References
2 Development History of Laser Inter‐Satellite Link
2.1 Development Stage of Laser Inter‐Satellite Link
2.2 Development Status of Laser Inter‐Satellite Link Technology in Various Countries
2.3 Experience and Inspiration
References
3 Spacecraft Orbits and Application
3.1 Overview
3.2 Kepler's Laws
3.3 Two‐Body Motion and Orbital Parameters
3.4 Near‐Earth Space Orbits and Applications
References
4 Basic Model of Constellation Inter‐Satellite Link Networking
4.1 Application Requirements of Satellite Navigation Inter‐Satellite Links
4.2 Basic Requirement Model of Inter‐Satellite Link Network Application
4.3 Inter‐Satellite Link Network Chain Topology Model
4.4 Inter‐Satellite Link Network Protocol Model
References
5 Principles of Laser Inter‐Satellite Ranging
5.1 Principle of Inter‐Satellite Ranging
5.2 Inter‐Satellite Ranging Accuracy
5.3 Principle of Microwave Inter‐Satellite Ranging
5.4 Principle of Laser Inter‐Satellite Ranging
References
6 Composition of Laser Inter‐Satellite Link
6.1 Basic Structure of Laser Inter‐Satellite Link
6.2 Workflow of Laser Inter‐Satellite Link
6.3 Constraints
6.4 Transmitter Design
6.5 Receiver Design
References
7 Inter‐Satellite Laser Capture, Aiming, and Tracking System
7.1 Introduction
7.2 Acquisition
7.3 Pointing
7.4 Tracking
7.5 APT System Terminal Structure
References
8 Inter‐Satellite Laser Link Tracking Error
8.1 Definition of Alignment Error
8.2 Alignment Error Model and Factor Analysis
8.3 Analysis of Tracking and Pointing Error Sources of Inter‐Satellite Laser Communication System
8.4 Satellite Platform Vibration Suppression Scheme
References
9 Inter‐Satellite Link Laser Modulation Mode
9.1 Block Diagram of Inter‐Satellite Link Optical Communication System
9.2 Typical Incoherent Optical Modulation (IM/DD)
9.3 Coherent Optical Communication Modulator and Modulation Principle
9.4 Comparison of Communication Performance of Laser Modulation Schemes
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Composition of microwave frequency bands.
Table 1.2 Frequency allocation of inter‐satellite links.
Table 1.3 Inter‐satellite link frequency bands allocated by ITU.
Table 1.4 Frequency bands of inter‐satellite links between domestic and for...
Table 1.5 Comparison of multiple access modes.
Chapter 2
Table 2.1 Status of research on laser inter‐satellite links in various coun...
Chapter 3
Table 3.1 The significance of the number of orbital elements.
Chapter 4
Table 4.1 Comparison of basic multiple access modes.
Table 4.2 Inter‐satellite link application data types.
Table 4.3 Inter‐satellite link budget table.
Table 4.4 Communication loss budget of some mesh topology stars.
Chapter 5
Table 5.1 Comparison of laser ranging pulse width and ranging accuracy.
Chapter 6
Table 6.1 Basic link data.
Table 6.2 Main performance parameters of GEO platform related to laser comm...
Table 6.3 Main related performance parameters of LEO platform related to la...
Table 6.4 The error distribution of the OICETS satellite at open loop state...
Table 6.5 The vibration spectrum density of some typical platforms.
Chapter 7
Table 7.1 Comparison of transmissive and reflective telescopes.
Table 7.2 Correction factor corresponding to the different meteorology, whe...
Table 7.3 Coarse sight subsystem control indicators.
Chapter 8
Table 8.1 Vibration types and their effects on the inter‐satellite laser co...
Table 8.2 Satellite vibration angles at different sampling frequencies.
Chapter 9
Table 9.1 Symbol information of different header sequences.
List of Illustrations
Chapter 1
Figure 1.1 Distributed spacecraft communication structure division.
Figure 1.2 Schematic diagram of GEO/GEO inter‐satellite link.
Figure 1.3 Schematic diagram of GEO–LEO interplanetary link.
Figure 1.4 Typical advanced space laser communication device. (a) US moon–Ea...
Figure 1.5 Schematic diagram of high‐speed laser inter‐satellite link.
Chapter 2
Figure 2.1 Major foreign research institutes for laser inter‐satellite links...
Figure 2.2 US laser inter‐satellite link development program.
Figure 2.3 LLCD system composition.
Figure 2.4 LADEE and LLST optical terminal module. (a) LADEE and Laser commu...
Figure 2.5 LCRD mission structure diagram.
Figure 2.6 LCRD spaceborne optical terminal.
Figure 2.7 ILLUMA‐T optical terminal and International Space Station deploym...
Figure 2.8 DSOC task diagram.
Figure 2.9 DSOC deep space exploration data transmission rate curve.
Figure 2.10 ESA's laser inter‐satellite link network construction diagram.
Figure 2.11 EDRS equipped with LCT‐135 laser terminal developed by German Te...
Figure 2.12 OPTEL‐μ micro laser communication system. (a) System components;...
Figure 2.13 Schematic diagram of the HICALI plan.
Chapter 3
Figure 3.1 Schematic diagram of elliptical orbit.
Figure 3.2 Orbit eccentricity.
Figure 3.3 Schematic diagram of orbit inclination.
Figure 3.4 Ascension of the ascending node, argument of perigee and true per...
Figure 3.5 Schematic diagram of satellite coverage of the ground.
Figure 3.6 Schematic diagram of ground coverage under the constraint of mini...
Chapter 4
Figure 4.1 Simulation diagram of hybrid constellation structure.
Figure 4.2 Time slot division change diagram of a satellite.
Figure 4.3 Schematic diagram of time slot access in STDMA mode.
Figure 4.4 Schematic diagram of narrow beam antenna for inter-satellite link...
Figure 4.5 Schematic diagram of the composition of the constellation communi...
Figure 4.6 Schematic diagram of inter‐satellite topology attributes.
Figure 4.7 Inter‐satellite visibility analysis. (a) Inter‐satellites are not...
Figure 4.8 Common network topology diagram. (a) Ring topology; (b) star topo...
Figure 4.9 Schematic diagram of partial mesh topology in STDMA mode.
Figure 4.10 The relationship between the communication rate
R
b
(kbps) and the...
Figure 4.11 Inter‐satellite link network layer model.
Chapter 5
Figure 5.1 The principle and timing relationship of two‐way incoherent infor...
Figure 5.2 Schematic diagram of pseudo‐range measurement.
Figure 5.3 Schematic diagram of two‐way measurement between satellites.
Figure 5.4 The relationship between satellite centroid and phase center in m...
Chapter 6
Figure 6.1 Block diagram of the laser inter‐satellite link system.
Figure 6.2 Workflow of laser inter‐satellite link.
Figure 6.3 The error of precise orbit positioning.
Figure 6.4 The schematic drawing of the relative position in the midst of th...
Figure 6.5 The schematic drawing of the impact of the sun spectra on LEO.
Chapter 7
Figure 7.1 APT system basic schematic.
Figure 7.2 The relationship structure diagram of scanning area, alignment ar...
Figure 7.3 The schematic diagram of scanning‐scanning.
Figure 7.4 The schematic figure of acquisition path. (a) The rough scanning ...
Figure 7.5 Laser communication link diagram.
Figure 7.6 Gauss beam propagation characteristics.
Figure 7.7 A diagram of the spot in the field of view. (a) Coarse alignment ...
Figure 7.8 Schematic diagram of the composite axis control structure of the ...
Figure 7.9 Terminal structure diagram of inter‐satellite laser communication...
Figure 7.10 Coarse sight subsystem composition.
Figure 7.11 Coarse sight subsystem working flow chart.
Figure 7.12 Incremental and absolute encoder markings. (a) Incremental; (b) ...
Figure 7.13 Coarse aiming control system block diagram.
Figure 7.14 Precision aiming control subsystem.
Figure 7.15 Double‐loop control system block diagram of precision aiming.
Figure 7.16 Precision aiming control system block diagram.
Chapter 8
Figure 8.1 Schematic diagram of alignment error.
Figure 8.2 The main components of factors that cause alignment errors.
Figure 8.3 Schematic diagram of tracking and aiming error.
Figure 8.4 Relationship between relative light intensity and tracking error....
Figure 8.5 The effect of tracking error on the signal optical power at the r...
Figure 8.6 The relationship between tracking error and bit error rate.
Figure 8.7 Tracking error source of inter‐satellite laser communication syst...
Figure 8.8 The measured power spectrum curve of the LANDSAT‐4 satellite.
Figure 8.9 Working principle diagram of CCD spot detector.
Figure 8.10 Schematic diagram of the relationship between the spot center an...
Figure 8.11 Schematic diagram of two passive vibration isolation structures....
Figure 8.12 Active control structure.
Figure 8.13 Structural diagram of precision aiming control with vibration in...
Figure 8.14 Control results of precision sighting system with vibration inte...
Figure 8.15 Structure block diagram of feedforward vibration suppression.
Figure 8.16 Simulation results of feedforward vibration suppression algorith...
Chapter 9
Figure 9.1 IM/DD system composition block diagram.
Figure 9.2 Block diagram of the receiving end of the coherent system.
Figure 9.3 PPM modulation timing diagram.
Figure 9.4 DPPM modulation timing diagram.
Figure 9.5 The corresponding timing diagram of several modulation methods un...
Figure 9.6 LiNbO
3
crystal integrated optical phase modulator structure.
Figure 9.7 MZM modulator structure.
Figure 9.8 Parallel and cascaded QPSK modulation structures.
Figure 9.9 Cascaded 8PSK modulation structure.
Figure 9.10 8PSK modulation of QPMZM structure.
Figure 9.11 8QAM modulation structure. (a) All‐optical 8QAM modulation struc...
Guide
Cover Page
Series Page
Title Page
Copyright Page
Author Biography
Preface
Table of Contents
Begin Reading
Index
WILEY END USER LICENSE AGREEMENT
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IEEE Press445 Hoes LanePiscataway, NJ 08854
IEEE Press Editorial BoardSarah Spurgeon, Editor in Chief
Jón Atli Benediktsson
Andreas Molisch
Diomidis Spinellis
Anjan Bose
Saeid Nahavandi
Ahmet Murat Tekalp
Adam Drobot
Jeffrey Reed
Peter (Yong) Lian
Thomas Robertazzi
Laser Inter‐Satellite Links Technology
Jianjun Zhang
China Academy of Space Technology, Beijing, China
Jing Li
Beijing Institute of Technology, Beijing, China
Copyright © 2023 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.
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Author Biography
Jianjun Zhang, PhD, Professor. He received his PhD degree from the Institute of Optoelectronics, Chinese Academy of Sciences, in 2010. He is now Professor at Beijing Institute of Spacecraft System Engineering, China Academy of Space Technology. He is also the member of the Youth Science Club of China Electronics Society, member of the Edge Computing Expert of China Electronics Society, Chairman of the “Space (Aerospace) Information Technology” Professional Committee of China Electronics Society, and member of the Satellite Application Expert Group of China Aerospace Society. He is mainly engaged in satellite navigation system design and advanced spatial information system technology based on cognitive mechanism. He has presided over several major projects such as the National Natural Science Foundation's major research project, the final assembly fund, the 863 project, and the development project of the Science and Technology Commission of the China Academy of Space Technology. He has published more than 50 SCI/EI search papers in international journals and conferences, authorized more than 20 invention patents at home and abroad, and published 3 monographs. He won the third prize of National Defense Science and Technology Progress Award with the first completion person.
Jing Li, PhD, Associate Professor, Supervisor. She received her PhD degree from the Beijing Institute of Technology in 2011. She is now Associate Professor at the School of Automation, Beijing Institute of Technology. She is an expert member of the “Space Information Technology” Youth Committee of China Electronics Society. Her main research direction is robot environmental awareness, image detection and target tracking, and multi‐sensor information fusion. She has presided over more than 10 projects including the National Natural Science Youth Fund, the Postdoctoral Special Fund, the Key Laboratory of the Ministry of Education, and the Science and Technology Cooperation. She has published 25 academic papers (including 10 SCI papers and 15 EI papers) and the book “Image Detection and Target Tracking Technology”, granted 7 national invention patents as the first author and the National Science and Technology Progress Award 2 (ranked 8th). The guided postgraduates have awarded the second prize of the 14th China Graduate Electronic Design Competition, the second prize of the first China‐Russia (Industrial) Innovation Competition, and the second prize of the 14th National College Student Smart Car Competition.
Preface
In recent years, with the rapid growth of the demand for satellite applications in people’s daily life, various needs are more and more inseparable from the navigation system, and the role of the satellite navigation system in people’s daily life is becoming more and more important. The development of satellite navigation systems plays an important role in national economic construction and military defense work. Therefore, the world’s major powers regard the construction of modern global navigation satellite system GNSS as a major strategic basic resource allocation. The four existing GNSS systems include GPS in the United States, Galileo in Europe, GLONASS in Russia, and Beidou in China, as well as other satellite navigation systems under construction or to be built in the future, and their related augmentation systems.
The normal operation of the ground station is an important support for the stable operation of the existing GNSS system. If the ground station is destroyed during combat or in other situations, the entire GNSS system will be paralyzed, and the consequences will be unimaginable, which will bring benefits to the entire country. The damage is irreparable. The inter-satellite link is also an important part of the GNSS system, which links the navigation satellites in the same orbit or between different orbits through transmitters, antennas, and receivers. Reducing the cost of system construction is one of the benefits brought by ISL technology. The inter-satellite link ranging and communication functions can effectively reduce the number of ground stations required by the system. Even if there is no ground station support, the system can still be used in cycles. Normal work is especially important for countries with limited resources and no global deployment capability. The Block-IIR series satellites in the second-generation GPS system of the United States are the only inter-satellite link systems with ranging and communication functions among all the GNSS navigation satellites currently in orbit. In addition, the construction of inter-satellite links, whether it is the third-generation GPS system or GNSS in other countries, is still in the stage of research and demonstration.
The International Telecommunication Union (ITU) has not yet allocated a laser band inter-satellite link, and the radio frequency link covers several radio frequency bands from ultra-high frequency (UHF) to extremely high frequency (EHF) allocated by the ITU. Compared with traditional inter-satellite radio frequency technology, satellite laser communication technology is known as a new generation of space communication technology. Laser carrier has some unique advantages: large communication capacity, good concealment performance, high bandwidth and transmission rate, strong anti-interference ability, no radio frequency license, etc., are required. At the same time, according to the modulation method and the detection method of the receiving end, the inter-satellite laser communication can be divided into incoherent – Intensity Modulation/Direct Detection (IM/DD) and coherent systems. The former uses light intensity modulation and direct detection schemes, which use precise modulation techniques (such as BPSK, QPSK, M-QAM, etc.) combined with coherent reception techniques (homodyne and heterodyne) schemes. Studies have shown that the IM/DD communication system cannot approach the theoretical limit of detection sensitivity in the actual inter-satellite link environment, and the sensitivity and wavelength selectivity of the coherent method are greatly improved compared to the IM/DD system.
China is promoting the construction of the Beidou second-generation satellite navigation system in an orderly manner, but for various reasons, China currently does not have global geographic strategic resources, and can only set up ground stations in China or parts of Asia. Therefore, the research on the inter-satellite link of the navigation system and the construction task is urgent and affects the overall work. Precise ranging and data transmission between navigation satellites is the premise for the system to achieve precise positioning and time synchronization. It can be seen that the application of laser technology to the inter-satellite link of the navigation constellation can enhance the anti-interference ability and confidentiality of the system at the same time, and what is more expected is to significantly improve the inter-satellite ranging accuracy and communication rate, thereby enhancing the survivability of the entire GNSS system. It is foreseeable that the application prospect of the inter-satellite link laser ranging and communication integration technology is immeasurable, so the research on the laser inter-satellite link technology is indispensable.
1Introduction
1.1 Connotation of Inter‐Satellite Link
With the rapid development of aerospace technology, national interests are gradually expanding beyond the traditional territory, territorial waters, and airspace, expanding and extending to the ocean, space, and electromagnetic space. With the rapid development of space technology, space has increasingly become a new source of international strategic competition commanding heights. Space technology embodies the political strength, economic strength, and scientific and technological strength of a country, and has become a strategic means for countries to demonstrate the progress of their national strength and defend their international status. In order to safeguard our maritime rights and interests, defend our space rights and interests, and ensure the expansion of national interests, China must focus on a global perspective and develop satellite navigation, communication, and other systems on a global scale. To this end, it is necessary to use space‐based networking methods to break through the limitations of land and solve problems such as satellite full‐orbit operation management, constellation autonomous operation, and rapid response to complex tasks, in order to ensure the development of space systems [1].
Satellite technology is the primary breakthrough for occupying space resources. After going through two stages of single‐satellite application and constellation application, the satellite field has gradually moved toward networking. The networking between satellites must first require that the information and data between satellites can be interconnected. Considering factors such as security and national strategy, the number of satellite ground stations that can be established is very limited, and most of them are limited to the country. When the ground station is strictly limited, the establishment of an inter‐satellite link has become one of the most important necessary conditions for inter‐satellite networking [2, 3].
Once a communication network is established between satellites through inter‐satellite links, the satellites are no longer isolated individuals, but a whole with a considerable scale, and satellites can only complete a small number of tasks to complete a general a lot of work. Under a good inter‐satellite link network, the satellite not only has more powerful functions, but also its robustness and anti‐interference have also been greatly enhanced.
The constellation network is a very complex space network. This is because in the inter‐satellite link network, there are not only a large number of space nodes, but also many ground nodes are also part of the network, the composition is complex and changeable, the access is flexible and irregular, with typical flat and centerless features. These features require a high degree of flexibility and adaptability to build inter‐satellite networks. In addition, the inter‐satellite link is not a single, pure communication link. While carrying a certain rate of communication tasks, it may also need to complete high‐precision inter‐satellite measurement functions at the same time. In the navigation system, the inter‐satellite link needs to meet the core requirements of precise orbit determination and time synchronization of the system, autonomous navigation applications, etc., and support the use of satellite–ground joint orbit determination and the transmission capability of measurement data for autonomous navigation. The construction put forward higher requirements.
Inter‐satellite link (or crosslink) refers to the link between satellites and can also be extended to the link between spacecraft. The inter‐satellite link can perform functions such as inter‐satellite communication, data transmission, inter‐satellite ranging, and inter‐satellite measurement and control. Different space systems have different functions of inter‐satellite links. The inter‐satellite link of the communication satellite constellation can reduce the number of satellite–ground hops and communication delay; the inter‐satellite link of the reconnaissance formation system can increase the aperture of the virtual camera and improve the resolution; the inter‐satellite link of the navigation satellite constellation can support autonomous operation and improve the resolution. Positioning accuracy, the inter‐satellite link of the relay satellite system can increase the measurement and control arc of the user satellite. In addition, there are some inter‐satellite links used in scientific research, such as gravity detection satellite systems. The inter‐satellite link makes multiple satellites form an organic whole to form a constellation system and expand the ability of a single satellite to work [4].
At present, distributed satellite systems mainly include two types: formation satellite constellation and formation satellite. A typical constellation includes the sharing of scientific data through inter‐satellite links between satellites in orbits of planets or the sun. They do not rely on each other to complete autonomous onboard navigation corrections, which are transmitted through ground stations. The formation satellites need to rely on inter‐satellite links to transmit navigation data to achieve fully autonomous navigation. At the same time, formation satellites also need to ensure strict positioning accuracy to meet the scientific goals of formation missions. One or several spacecraft in the formation have the function of navigation and processing, and maintain the formation or ensure a certain topology structure through the transmission of data and instructions. At the same time, the state information of the spacecraft also needs to be transmitted by inter‐satellite links.
Figure 1.1 divides formation satellite systems into different classes from an inter‐satellite link perspective. There are two kinds of communication links between formation satellites: satellite–ground link and inter‐satellite link. If satellites transmit information through satellite–ground links, direct communication between spacecraft is generally not required, but data are collected and processed on the ground, and then integrated into scientific or navigational information to be transmitted back to the spacecraft. Usually, this method is very dependent on the ground and can be regarded as centralized control, that is, a star‐shaped distributed system. Some constellations exchange and process data entirely through satellite–ground communication. Unlike constellations, formation satellites use inter‐satellite communications to exchange navigation data and commands. In addition, the topological structure of the communication network of formation satellites is mainly divided into two types: star type and point‐to‐point type (Figure 1.1).
Figure 1.1 Distributed spacecraft communication structure division.
The use of inter‐satellite links has many advantages, mainly including:
For communication satellites, when users who are not within the coverage area of the same satellite need to communicate, the use of inter‐satellite links can eliminate satellite double hops and reduce the propagation delay. At the same time, a relay earth station dedicated to relaying signals between users in different satellite coverage areas is omitted [
2
,
5
].
When the system is a constellation composed of many satellites, the use of inter‐satellite links to form a complete communication network for all satellites is not only independent of the ground, but also greatly improves the system’s anti‐interference and anti‐destroy capabilities.
It can be used to expand the coverage of the system. Multiple satellites are linked together through inter‐satellite links, and users within the coverage area of any satellite can communicate directly with users within the coverage area of other satellites through the inter‐satellite link.
Facilitate network management and form a global seamless network. For some low‐orbit satellites, there may be no fixed earth station visibility at all in some cases (e.g. in the middle of the Pacific Ocean) due to their small coverage. At this time, it is almost the only solution to use the inter‐satellite link to realize the control of the satellite on the ground and the mobile user to access the ground communication network through the inter‐satellite link.
It is convenient to form a satellite group at the same orbital position, which is very useful for high‐orbit satellites. Due to the increasing traffic volume of communications and the limited orbital positions of high‐orbit satellites, it is bound to hope to make more effective use of each orbital position, and to place multiple satellites spaced about 100 km away from each other in one orbital position, so that they are interconnected by inter‐satellite links to form a satellite constellation. This not only avoids the problem that a single satellite is too large to be loaded into orbit with existing launch tools, but also greatly reduces the risk of launch failure and satellite failure, and the capacity of the satellite can be adjusted according to actual needs. Incrementally, by increasing the number of satellites.
The space segment part of a system may be a constellation consisting of many satellites. For political and economic reasons, it is impossible to build a large terrestrial network that would allow any one of the satellites in the system to be able to see one of the ground control earth stations at any time. At this time, an inter‐satellite link can be used to interconnect a satellite in the system that cannot be seen by the ground control earth station to another satellite (such as a high‐orbit satellite) that can simultaneously see the ground control earth station and the satellite [
6
,
9
].
On the other hand, the use of inter‐satellite links also increases some design difficulties, including the need to increase the transceiver equipment necessary to maintain one or more inter‐satellite links, including transceiver antennas, transceiver, radio frequency equipment, modulation and demodulation equipment, and the necessary baseband processing equipment. The satellite is required to have onboard processing function to distinguish whether the signal is sent to the inter‐satellite link or the downlink. At the same time, it needs to have the necessary switching equipment to realize the onboard routing and exchange of the signal. Furthermore, the communication over the inter‐satellite link should be transparent and the signal quality should not be degraded. All of these will inevitably increase the complexity of the satellite, and may increase the power burden of the satellite [7].
1.2 Types of Inter‐Satellite Links
The type of inter‐satellite link depends on the user with different usage requirements. However, its basic division includes two kinds: one is division by space domain and the other is division by frequency domain. Inter‐satellite links are divided by frequency domain: ITU has allocated 14 frequency bands for inter‐satellite links, ranging from UHF to EHF (190 GHz), including unallocated laser bands. That is, it can be summarized as microwave, millimeter wave, and laser links in three frequency bands. The inter‐satellite link can be divided into the following two situations according to the airspace, that is, according to the satellite orbit [8–10]:
Inter‐satellite links between satellites of the same orbit type, such as: GEO/GEO, LEO/LEO, etc.
The inter‐satellite links between satellites of the same orbit type are further divided into interstellar links in the same orbital plane and interstellar links in different orbital planes. Since the relative positions of the interstellar link satellites in the same orbital plane are fixed, we generally only analyze the different interplanetary link within the orbital plane. The interstellar links of the same orbit type mentioned in the subsequent chapters refer to the interstellar links in different orbital planes.
Under the same coverage as shown in Figure 1.2, the GEO/GEO or MEO/MEO inter‐satellite link can greatly improve the communication capacity of the system; when covering different areas, it can greatly increase the area of communication coverage; at the same time, it can improve the ground station. The minimum elevation angle can improve the quality of communication; it can also reduce the restrictions on satellite orbital positions and establish a global satellite communication network.
Figure 1.2 Schematic diagram of GEO/GEO inter‐satellite link.
The LEO satellite inter‐satellite link can make up for the two shortcomings of LEO satellite communication: one is that the coverage of a single satellite is very small, and the other is that the continuous communication time of a single satellite is very short, such as for Motoroh’s iridium system, the maximum sustainable communication time of a single satellite is only 16 minutes. If the ground gateway station is used as a bridge for information communication between satellites, dozens or hundreds of ground stations may be required. Therefore, for the LEO system, it is unrealistic to rely on the ground station alone to communicate information between the satellites of the system. The inter‐satellite link can realize the aerial networking of the satellite mobile communication system. In some cases, the satellite communication signal can reach the ground station only once or not at all before reaching the final communication user, thereby greatly saving the satellite mobile communication system. Invest in the ground segment and enable rapid transfer of information.
Interplanetary links between satellites of different orbit types, such as GEO/LEO, MEO/LEO inter‐satellite links, also known as inter‐orbital links (IOL).
