Ground Station Design and Analysis for LEO Satellites E-Book

Shkelzen Cakaj

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 394

Veröffentlichungsjahr: 2022

Beschreibung

Tutorial for analytical and scientific approaches related to LEO satellites ground station performance, including math, experiments, and simulations. Ground Station Design and Analysis for LEO satellites provides complete instructions and steps for ground station performance evaluation, including stations dedicated for scientific or communication purposes, and offers the reader an enhanced learning experience by proposing 40 ideas related to ground station performance assessment. Each idea goes over the math analysis, experiment or simulation, the methodology applied, the results, and a conclusion. This approach provides the reader with the opportunity to compare theoretical results with on-site results, guiding the reader towards intelligent and practical performance evaluation and enhancement. The text also considers the future emerging developments of LEO satellites and their challenges and applications, including multimedia and other scientific applications. Stemming from the highly qualified author's research work of roughly 20 years, Ground Station Design and Analysis for LEO Satellites includes information on: * Interference aspects, covering intermodulation interference modeling for LEO satellite ground stations and downlink adjacent interference for LEO search and rescue satellites * Sun synchronization, covering nodal regression and Sun synchronization of low Earth orbits through inclination angle. * Horizon plane and communication duration, covering communications duration with LEO satellites under ideal, practical and designed horizon plane for LEO satellite ground stations * Figure of merit and system noise temperature, covering composite and antenna noise temperature, Sun noise experiment and desensibilization measurements at LEO satellite ground stations Ground Station Design and Analysis for LEO Satellites is an innovative and advanced tutorial-based resource that will appeal to satellite engineers (operators/vendors) working on the operation, maintenance, and performance evaluation of ground stations, as well as postgraduate students/early-stage researchers wishing to obtain knowledge on this state-of-the-art technology.

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Ground Station Design and Analysis for LEO Satellites

Analytical, Experimental and Simulation Approach

Shkelzen Cakaj

Copyright © 2023 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.

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To my wife Naime and our childrenGresa, Vesa and Genti

Shkelzen

Preface

I belong among the experts who have combined their professional career in the telecommunications industry with academic scientific research. I have been involved for 40 years in the development and business processes in Kosovo's telecommunication system, while for the last 20 years I was also engaged in academic scientific activities at universities in Republic of Kosovo and Republic of Albania. My entire scientific research interest was dedicated to the performance of ground stations for LEO (low Earth orbiting) satellites, with an outcome of 40 scientific published papers worldwide, exclusively related to LEO ground stations.

Throughout my career, I have been involved in several technological telecommunication infrastructure generations, each one providing significant improvements, enhancing the services toward a better social future. Thus, to comprehend the technological effect into our lives, one could simply compare the lifestyle of our grandparents with that of our nieces and nephews, which covers the period of approximately 100 years. The difference is unimaginable. From my perspective, this tremendous difference, including the current focus on worldwide communications, stems from the two crucial technological achievements from the past century – the satellite systems and Internet. The interoperation of both, enabled impressive long distance multimedia communication services, virtual meetings and the long‐distance social interoperability.

Recently, significant international efforts are oriented toward the framework definition for a global satellite communication system which would be fully integrated within the existing terrestrial system. The most convenient structures for such a development are the LEO satellites. The latter operate closer to Earth compared to the other orbits, providing significantly lower latency, which is crucial for reliable and safe communications. The LEO satellites combined with the ground stations as a part of the satellite‐terrestrial integrated network, through their interoperability, are the future of communication infrastructure, intending to offer the global Earth coverage with broadband multimedia services, from South Africa to Tibet, from Alaska to New Zealand. But such an approach involving many satellites will result in new scientific challenges, not only in the integrated satellite‐terrestrial networks, but also pertaining to the sky transformation itself.

Except for communication purposes, LEO satellites are seriously and effectively applied for scientific missions as well. Potential applications are vast, including but not limited to the remote sensing of oceans, analyses of Earth's climate changes, mapping, Earth's imagery with high resolution, navigation, management of Earth's resources, astronomy, military, agriculture, and even humanitarian efforts for search and rescue services.

Therefore, it is to be expected that such missions will be further developed in the future, especially in the fields where similar experiments cannot be done by means on the Earth. Thus, ground stations must be established to communicate with satellites to further these missions. Their performance is crucial for reliable communication; thus, they must be carefully and multidimensionally analyzed, which is the subject of this book. The same approach should be applied to the access points on ground dedicated for internet access.

My passion for LEO satellites began when I joined the LEO satellite MOST (Microvariability and Oscillations of Stars) ground station at the Vienna Technical University in Austria. In September 2003, in the capacity of a scientific guest, I joined a working group at the Institute for Communication and Radiofrequency Engineering of Technical University in Vienna. The group was tasked to work on the implementation of a satellite ground station in Vienna, dedicated for communication with a Canadian space observation LEO microsatellite – MOST.

For more than three years starting in 2005, I worked with the Department for Radio‐Communication and Microwave Engineering at the Zagreb University in Croatia, focusing on my PhD thesis and analyzing the satellite ground station performances in the urban areas. These analyses, titled “Rigorous Analysis on Performance of LEO Satellite Ground Station in Urban Environment,” are published in the International Journal of Satellite Communications and Networking, and form the core scientific contribution of my PhD thesis.

In 2009, I was supported by the Fulbright program for my postdoctoral research. The research area treated the simulation and implementation protocols for local user terminals dedicated for search and rescue services supported by satellites. I spent three months working at the United States National Oceanic and Atmospheric Administration (NOAA), analyzing the performance of the local user terminals and applying simulations for hypothetical distress events resulting into conclusions about the performance of local user terminals under different circumstances.

My overall scientific engagement over two decades related to the satellite ground stations performance dedicated for LEO satellites, is fourfold: (i) atmospheric impairments, (ii) coverage area from LEO satellites, (iii) ground station's ideal and designed horizon plane, and (iv) communication duration optimization between LEO satellites and appropriate ground stations. All of these aspects interlinked together are also treated, for the overall performance evaluation of the ground station, usually expressed through ground stations' Figure of Merit.

In March 2021, I contracted COVID‐19 and became severely ill. I fought to survive for several weeks. In moments of subtle hope, I promised myself that if I got out of bed, I would condense my scientific work in the form of a single book. COVID touched and woke up my motivational instinct for publishing this book!

This book reflects the consolidated research of 20 years, including mathematical analyses, experiments, and simulations that could serve as a guide for the ground station performance evaluation at any of the worldwide LEO stations. My approach focuses on four key components: idea, methodology, results, and conclusions stemmed from my research, as an innovative tutorial and an advanced level guide to ground stations analysis and design. In my view, this makes the difference!

Usually, in the literature, the satellite ground stations are treated as a chapter within a book for satellite communications or more generally for satellite systems. My intention through this book is to bring to the readers a deeper insight, not only about the satellite ground station organization but also about the performance of each separate block and the entire ground station performance evaluation toward the safe and reliable functionality, considering technical characteristics of devices and the environmental circumstances.

This book is organized into 10 chapters and ends with a few short, final remarks. Being aware that there are readers who, for different reasons, might not be able to read the entire book, I have been careful that each chapter is organized and compacted in such a manner that it can also be read as single one, designed to provide the necessary and expected information for the respective readers.

Chapter 1 covers the most general concepts of the satellite ground station organization, providing an overview of the single and double antenna system configuration. Ground station subsystems and respective components are described, followed by the Figure of Merit interpretation, including both ground station equipment parameters and external environmental factors impact. For purposes of illustration, the chapter provides a brief description of the Canadian satellite MOST (Micro‐variability and Oscillation of STars) and the respective ground station implemented in Vienna, which is further applied for the interpretation purposes throughout the entire book.

Chapter 2 focuses on rain attenuation since it is considered as the most impactful atmospheric factor on the radio waves propagation. Behind the general aspect of the rain attenuation, based on the rain attenuation path geometry, the modeling of rain attenuation is provided. The modeling approach is applied for the rain attenuation calculation for different European cities randomly chosen. The appropriate data provide hints about the atmospheric impairments attenuation to be applied due to the link budget related calculations anywhere.

The downlink performance is expressed through the downlink budget. The Figure of Merit is the main downlink performance indicator, depending on the implemented equipment characteristics and environmental factors, represented by system noise temperature. System noise temperature consists of composite noise temperature and antenna noise temperature. In Chapter 3, two scenarios of the downlink budget are compared – for the single antenna and double antenna configurations. This chapter ends with the experimental measurement of the Figure of Merit, based on the Sun Flux Density, experimentally confirming the mathematical calculations previously applied.

Chapters 4 and 5, respectively, consider the ground station horizon plane and the coverage area of the LEO satellites, issues that are closely interrelated. The coverage area is the fraction of Earth's surface covered by the LEO satellite and the horizon plane determines the communication zone between the satellite and the appropriate ground station. Three types of ground station horizon planes are analyzed in details, the ideal, practical, and designed one. The mathematical and geometrical correlation between the ideal and the designed horizon plane are established as well. The time efficiency factor is implemented in order to quantify the difference in communication duration under ideal and designed horizon plane. Further, the individual and global coverage from the LEO satellites is clarified. This is accompanied with specifically calculated coverage for the LEO satellites, proving their low coverage area. Chapter 5 also includes a geometrical confirmation of the handover process due to the global coverage by the LEO constellation.

In Chapter 6, the Sun's synchronized orbits are analyzed, as they are very useful for the photo imagery missions. The inclination window for LEO altitudes to be Sun synchronized is calculated. Further, under that inclination window, the perigee deviation for the Sun synchronized orbits is interpreted. Chapter 7 presents the launching satellite procedure and discusses coplanar Hohmann transfer analysis under the different LEO altitudes. Chapter 7 ends with a detailed procedure of geostationary altitude attainment form the Kourou launching site.

The LEO satellites applied for the search and rescue missions are considered in Chapter 8. Simulation of local user terminals dedicated for search and rescue services is provided, assuming the hypothetical local user terminal and four randomly supposed distress locations. The existing LEOSAR and the future to be finalized MEOSAR constellation are compared. Toward the end of the book, the interference issue is considered throughout Chapter 9. The intermodulation products are analyzed and the modeling for in advance interference identification is given. The adjacent interference is analyzed through the typical example of the interference avoidance for the case of the NOAA constellation. Modulation index application for the interference identification accompanied with practical records is provided in this chapter as well. Finally, Chapter 10 outlines the need for further scientific research. The first one relates to free space loss compensation through dynamic bandwidth tunability, while the second relates to the wideness horizon plane parameter to be applied in the future for the analysis of the LEO ground station performance.

The book ends with “Closing Remarks,” in which I emphasize the three most important events (in my opinion) in satellite systems in the last 10 years. The first one related to developments of April 2014, when the first stage of the Falcon 9 rocket had made a controlled power landing on the surface of the Atlantic Ocean. The second relates to the Orbit Fab, a San Francisco‐based space‐industry startup, that has developed end‐to‐end refueling service using its Rapidly Attachable Fluid Transfer Interface (RAFTI). Finally, the third one is related to the launching of satellites from the International Space Station (ISS). These achievements, beside economic advantages, will also support the longer satellite life in space, for the better and longer coexistence with terrestrial systems.

I retired in the middle of March 2022, but I am still engaged in the education system in Republic of Kosovo and Republic of Albania, teaching new generations about the principles and the future of the satellite communications. In the future, I will still keep my friendship with LEO satellites.

Acknowledgments

I would like to deeply thank Professor Arpad L. Scholtz and Dr. Werner Keim for their support and excellent cooperation during my time spent at the Institute for Communications and Radiofrequency Engineering of Technical University in Vienna, Austria, to study satellite ground station performance in Vienna for communication with the MOST satellite. My gratitude also goes to Professor Kresimir Malaric from the faculty of Electrical and Computing Engineering of Zagreb University, Croatia, for his excellent guidance as I worked on my doctoral thesis. For my postdoc research, as Fulbright alumnus, at NOAA‐SARSAT‐NSOF in Suitland, Maryland, US, related to the application of LEO satellites in search and rescue services, I acknowledge Micky Fitzmaurice, for support and open cooperation. I recognize my colleagues, Professor Bexhet Kamo and Professor Elson Agastra, from the faculty of Information Technology of Polytechnic University of Tirana, Albania, for their careful reading of the manuscript and valuable suggestions. Finally, I would like to acknowledge senior editor Sandra Grayson for her support from my first idea for this book and also Kimberly Monroe‐Hill and Becky Cowan for carefully managing the editing and technical support, respectively.

1LEO Satellite Ground Station Design Concepts

1.1 An Overview of LEO Satellites

Satellites are an important part of telecommunication infrastructure worldwide, carrying large amounts of multimedia traffic. Since their inception around 60 years ago, communication satellites have been a major element in worldwide communication infrastructure and networking. More than 40 countries own satellites for communication, commercial, science, and even humanitarian purposes. But only few of them have building and launching capabilities.

The basic resources available for satellite communications are orbits and radio frequency (RF) spectrum. The orbit is the path in space followed by the satellite; the frequency allocations are subject to international agreements managed and controlled by international bodies.

Different types of orbits are possible, each suitable for a specific application or mission. Generally, satellites follow an elliptical orbit with a determined eccentricity laid on the orbital plane defined by space orbital parameters (Maral and Bousquet 2005; Maini and Agrawal 2011). Thus, the space orbital parameters, known as Kepler's elements (usually given as two‐lines elements), determine the position of the satellite in space (space slot). Orbits with zero eccentricity are known as circular orbits. The circularity of the orbit simplifies the analysis, compared to the elliptical one. The movement of the satellite within its circular orbit is represented by altitude, radius velocity, and orbital time.

Satellites' circular orbits are categorized as geosynchronous Earth orbits (GEO), medium Earth orbits (MEO), and low Earth orbits (LEO). The main difference among them is in the altitude above the Earth's surface, which further impacts the velocity and the orbital period of the satellite in the appropriate orbit (Maral and Bousquet 2005; Maini and Agrawal 2011). Only the circular orbits are of the further concern through this book, more exactly, the LEO satellites and the appropriate ground stations.

Communication between the satellite and a ground station is established when the satellite is consolidated in its own orbit and it is visible from the ground station. The link that transmits radio waves from the ground station to the satellite is called uplink, and from satellite toward the ground station is downlink.

The orbits of altitudes ranging from 300 km up to around 1400 km above the Earth's surface are defined as LEO, and the satellites consolidated to these orbits are known as the LEO satellites. The lower altitude range is limited by the Earth's atmosphere – more accurately, by the level above the Earth's atmosphere where there is almost no air, so the satellite's speed reduction and drag down is avoided. The inner Van Allen belt limits the higher altitude range (Van Allen radiation belt 2020). The Van Allen belt is known as a space radiation zone and has undesired effects on satellites' payload and platform (electronic components and solar cells can be damaged by this radiation); thus, the belt should not be used for the accommodation of LEO satellites.

LEO satellites move at around 7.2–7.5 km/s velocity relative to a fixed point on the Earth (ground station). Satellites' orbital period is in the range of 90–110 minutes. The communication duration between the satellite and the ground station takes 5–15 minutes over 6–8 times during the day (Cakaj and Malaric 2007a), all these dependent on orbital altitude. The characteristics of LEOs are the shortest distance from the Earth compared with other orbits and consequently less time delay. These characteristics make them very attractive for communications but also for other applications (Cakaj 2021).

Thus, in addition to communications, LEO satellites are also applied for scientific and research purposes, more specifically under circumstances where no on‐ground means are appropriate. Dynamics on climate changes, remote sensing applications for oceans, different astronomic observations, ions density records in the ionosphere, and very specific humanitarian applications related to search and rescue services are some of activities carried out by LEO satellites, activities that are too difficult or impossible to be implemented on Earth. For these activities within satellite structures, the instruments or devices (telescope, cameras, probes, sensors, etc.) for the appropriate application or mission are installed (Zee and Stibrany 2002; Cakaj et al. 2010a). Usually, LEO satellites dedicated for scientific purposes or remote sensing applications are accommodated in specifically designed orbits, known as the Sun synchronized orbit. The Sun synchronization feature enables a treated area on the ground from the satellite to be observed under similar illumination conditions due to different satellite passes (Cakaj et al. 2009).

These satellites provide opportunities for investigations for which alternative techniques are either difficult or impossible to apply. Thus, it may be expected that such missions will be further developed soon, especially in fields where similar experiments by purely Earth‐based means are impracticable. Ground stations (access points) must be established to communicate with such satellites, and the quality of communication depends on the performance of the satellite ground station, in addition to that of the satellite.

Communications‐integrated satellite‐terrestrial networks used for global broadband services have gained a high degree of interest from scientists and industries worldwide. The most convenient structures for such use are LEO satellites, since they fly closer to the Earth compared to the other orbits, and consequently provide significantly lower latency, which is essential for reliable and safe communications. Among these efforts is the Starlink satellites constellation, developed and partly deployed by the US company SpaceX. The constellation is planned to be organized in three spatial shells, each made up of several hundreds of small‐dimensioned and lightweight LEO satellites specially designed to provide broadband services, intending to offer global Earth coverage through their interoperability, combined with the ground stations as a part of the satellite‐terrestrial integrated network. On October 24, 2020, 893 satellites were situated in orbit of altitudes of 550 km under different inclinations, determining the first Starlink orbital shell (Cakaj 2021).

This would suggest that in the near future, worldwide broadband services provided by integrated satellite‐terrestrial communication networks will be a part of daily communication activities, demands for which will rapidly increase, so operators should carefully manage operation and distribution of real‐time services toward maximizing the downlink data throughput related to the broadband requirements without significantly affecting the mission cost (Botta and Pescape 2013; Garner et al. 2009). Therefore, future satellite payloads and platforms must become more flexible, lightweight, and smaller, easier to be launched, and reconfigurable related to the EIRP and coverage, to provide large capacity at the lowest cost, toward the main goal of the worldwide coverage with broadband services and other scientific missions, as well.

According to the worldwide coverage missions, their network architecture in space could be categorized into single‐layer (one‐shell) networks and multilayer networks. A single‐layer network provides intercommunication between only satellites of the same altitude, whereas multilayer networks enable communications between satellites in different shells. Multilayer networking is more complex but is advocated for its flexibility in providing more sustainable global coverage, seamless handovers, and reliable communications.

LEO satellites used at the end of the past century were known as microsatellites because of their light weight and small dimensions. Later, nanosatellites were developed as more convenient structure for launching process, since less energy is required to launch such satellites into the LEO space slot. But recently, it has been possible to launch nanosatellites from the International Space Station (ISS) (List of spacecrafts deployed from the International Space Station 2020). Related to the launching process, LEOs play an additional role as the first space shell for the satellites toward geosynchronous (geostationary) orbits, due to the three‐step transfer process (known as Hohmann transfer) (Cakaj et al. 2015a).

LEO satellites and appropriate ground platforms (access points) now represent a very useful system, not only for the main mission as communication is but also for research scientific missions. Through LEO satellites and appropriate platforms, anywhere on the globe can be provided data about the water dirtiness of the river Amazon, about new exoplanets, natural disasters, air or marine disasters, how the wheat is growing in South Africa, how many refugees are crossing the borders, ice melting, and increasing seawater level, for example.

Related to the last item, the satellite Sentinel‐6 Michael Freilich, launched on November 21, 2020, from Vandenberg launching site in California and consolidated into the LEO orbit of altitude of 1336 km under 660 inclination, will measure the sea level around the globe for the next five years. The mission is collaboration between NASA and the European Space Agency (see Figure 1.1) (Sentinel‐6 Michael Freilich 2021).

Finally, as the nineteenth century was deeply marked with the steam machines, this century will be marked by LEO satellites, hopefully for the better life on Earth! These tools provide opportunities not only for communications but also for scientific purposes, including Earth and space observation. LEO satellites serving as “eyes” in the sky might also prove useful for world peace!

Figure 1.1Sentinel‐6 Michael Freilich spacecraft.

Communication with such missions is enabled through the ground stations; thus, the performance of the ground station is crucial for such missions, and will be elaborated on throughout this book.

1.2 Satellite System Architecture

The scheme of a typical satellite communication system architecture is shown in Figure 1.2 (Maral and Bousquet 2005). It includes a ground segment, space segment, and control segment.

The operational satellite receives the radio waves transmitted by the ground station. This is called uplink. The received signals by satellite are processed, translated into another radio frequency, and amplified on‐board. In turn, these signals are further transmitted to the receiving ground station. This is called downlink. Uplinks and downlinks are based on radio frequency modulated carriers' principles. Carriers are modulated by baseband signals, including analog or digital, conveying information for communication or for other purposes.

The space segment contains one or several active and spare satellites organized in a constellation. The satellite is an artificial body orbiting around the Earth as “flying” trans‐receiver, either for communication or scientific purposes. Each satellite consists of a payload and platform (bus). The payload consists of the receiving and transmitting antennas and all electronics that support the reception and the transmission of radio carriers. The satellite's payload has two main functions:

To amplify the received carriers for retransmission to the downlink

. Large distance between the ground station and the satellite causes the carrier's power at the input of the satellite's receiver to be too low. Thus, power must be amplified to feed the satellite's transmit antenna toward users on ground within its coverage area.

Frequency conversion

. Frequency conversion is required to increase isolation between the receiving input and transmitting output (avoiding the re‐injection into the receiver). In

Figure 1.3

, the transparent satellite payload is given, making clear the uplink/downlink isolation.

Transparent payload belongs to a single antenna beam satellite where each transmit and receive antenna generates only one beam. Figure 1.3 shows that carriers are power amplified, and frequency down converted. The amplifying chain associated with each sub‐band is called satellite channel or transponder. The bandwidth splitting is achieved using a set of filters. Regenerative payload (multibeam) antennas would have many inputs/outputs as up beams/down beams. Routing of carriers from one up beam to a given down beam implies on‐board switching at radio frequency. LEO satellites for scientific purposes usually use single‐beam antennas.

Figure 1.2 Typical satellite communication system architecture.

Figure 1.3 Transparent payload.

Figure 1.4 Satellite platform block scheme.

The satellite platform consists of subsystems that permit the payload to operate. These subsystems are: structure, power supply, temperature control, altitude control, and communication subsystem (see Figure 1.4).

The structure provides the necessary mechanical support. The electrical power supply subsystem provides the necessary DC power. The altitude control subsystem stabilizes the satellite and controls its orbit. The thermal control system maintains the temperature of various subsystems within tolerable limits. All these functions are controlled by on‐board computerized subsystem.

Some missions, in principle, can be realized just by a single satellite, but for real‐time continuity of services and large or full Earth's coverage, the space segment must be organized as single‐layer or multilayer constellation. A single‐layer network provides intercommunication between only satellites of the same altitude, whereas multilayer networks enable communications between satellites in different orbital shells. Multilayer networking is more complex, but it is more preferable.

Figure 1.5 One Web's segment of single shell network.

Active satellite projects related to an integrated satellite‐terrestrial communications network include the Iridium constellation with 66 satellites (Cochetti 2015), the OneWeb constellation with 648 satellites (De Selding 2015; Pultarova and Henry 2017), Amazon, which has filed to launch 3,236 spacecrafts in its Kuiper constellation (Sheetz 2019), and Telesat, with the initiative of having a 117‐spacecraft constellation (Foust 2018). Figure 1.5 illustrates a segment of OneWeb's existing network showing its longitudinal orbit planes (Constellation One Web 2022).

In my view, the most serious activities have been taken by SpaceX, whose Starlink constellation is planned to consist of thousands of small LEO satellites, deployed in three shells (layers), dedicated to maximizing broadband internet services toward global Earth coverage, and combined with ground stations (trans‐receivers), to be organized as a satellite‐terrestrial integrated network for real time worldwide broadband services. The Starlink single layer constellation at altitude of 550 km is given in Figure 1.6 (The real benefit of SpaceX‐Starlink, highspeed internet, 2022).

The ground station