TELECOMMUNICATION - PART 4 - SATELLITES

Satellite Technologies: A Comprehensive Overview

Introduction

Satellites have become indispensable in today's world, enabling a wide range of applications in various sectors. This article provides a comprehensive overview of satellite technologies, including their orbits, categories, communication systems, atmospheric effects, and ground station infrastructure.

Satellite Orbits

Satellites are placed in different orbits depending on their applications. The main types of satellite orbits are:

Low Earth Orbit (LEO): LEO satellites orbit at altitudes between 500 km and 2,000 km. They are typically used for radar applications, environmental monitoring, and communications. The new space industry has made LEO SATCOM affordable, paving the way for satellite mega-constellations like Starlink and Kuiper.

Medium Earth Orbit (MEO): MEO covers a wide altitude range between GEO and Low Earth Orbit (LEO). Satellites in MEO typically support global navigation satellite systems (GNSS), communications, and environmental monitoring applications.

Geostationary Earth Orbit (GEO): Satellites in GEO orbit at an altitude of 35,800 km above the equator, co-rotating with the Earth. They provide continuous coverage of a specific region and are often used for broadcast signal transmission.

Highly Elliptical Orbit (HEO/HELO): Satellites in Highly Elliptical Orbits follow an elongated path around the Earth, with a low perigee and a high apogee. This orbit is particularly useful for providing coverage over polar regions and high-latitude areas where GEO satellites have limited visibility. HEO satellites are commonly used for communications, early warning systems, and scientific missions requiring extended dwell time over specific regions.

Sun-Synchronous Orbit (SSO): SSO is a near-polar orbit in which a satellite passes over the same region of the Earth at the same local solar time each day. This consistency makes it ideal for Earth observation, weather monitoring, and reconnaissance missions. SSO ensures uniform lighting conditions, which is particularly useful for imaging applications.

Azimuth and Elevation: Azimuth and elevation are essential parameters for determining the position of a satellite relative to an observer on Earth.

• Azimuth is the horizontal angle measured clockwise from the north, ranging from 0° to 360°. For example, an azimuth of 45° means the satellite is in the northeast direction.
• Elevation is the vertical angle between the observer’s horizon and the satellite, ranging from 0° (on the horizon) to 90° (directly overhead, known as the zenith).

Azimuth and Elevation in Different Orbits:

• In GEO, azimuth and elevation angles do not change over time, providing consistent coverage for fixed ground stations.
• In MEO and LEO, azimuth and elevation angles vary dynamically during a satellite overflight, requiring tracking mechanisms to maintain communication.
• In HEO/HELO, azimuth and elevation angles change significantly, demanding advanced tracking and predictive adjustments.
• In SSO, azimuth and elevation angles shift in a predictable manner, maintaining a stable observation schedule.

Satellite Ground Path and Field of View:

• Field of View (FOV): Defines the area on the ground where electromagnetic waves from the satellite can be detected and decoded. It is typically measured in kilometers in terms of an antenna swath width. The larger the FOV, the more extensive the coverage area, but this often comes at the cost of reduced resolution or signal quality.
• Overflight Duration: The time during which electromagnetic waves from the satellite can be reliably detected and decoded, particularly relevant for LEO and MEO satellites. The duration depends on the satellite’s speed, altitude, and the observer’s location on Earth.
• Nadir: The direction from the satellite toward the Earth's center, defined by the shortest path between the satellite and the Earth's surface. The nadir point continuously changes as the satellite moves in its orbit, defining the satellite’s ground track.
• Satellite Ground Path: The trajectory on the Earth's surface traced by the satellite's nadir direction. It represents the path along which the satellite moves relative to the ground. This path is crucial for understanding satellite coverage and planning observational or communication activities.
• Slant Path: The maximum distance between the edge of the field of view and the satellite. This measurement is crucial for worst-case link budget calculations, as it affects signal strength and communication reliability. Atmospheric interference and signal attenuation increase with longer slant path distances, making link adjustments necessary for stable communication.

In the following image, the red line represents the satellite's ground track, while the shaded area indicates its field of view. As the satellite orbits, its ground track traces a path on the Earth's surface, and its field of view covers a specific swath of terrain beneath it.

Satellite Orbital Inclination: Defining the Celestial Tilt

In the vast expanse of space, satellites trace intricate paths around our planet, each meticulously designed to fulfill a specific mission. While aspects like altitude and orbital period often take center stage, another critical parameter governs a satellite's journey: orbital inclination. This chapter delves into the concept of orbital inclination, exploring its definition, significance, classifications, relationship with latitude, effects on ground tracks, and real-world applications.

Defining Orbital Inclination

Orbital inclination is defined as the angle between a satellite's orbital plane and the Earth's equatorial plane. Imagine slicing the Earth in half along the equator; this defines the equatorial plane. Now picture the plane in which a satellite orbits the Earth. The angle formed between these two planes is the orbital inclination.

Measured in degrees, inclination can range from 0 to 180:

• 0 degrees: The satellite orbits directly above the equator in the same direction as Earth's rotation (Eastward). This is known as an equatorial orbit.
• 90 degrees: The satellite passes over both poles, orbiting perpendicular to the equator. This is called a polar orbit.
• 180 degrees: The satellite orbits the equator but in the opposite direction of Earth's rotation (Westward).

Significance of Orbital Inclination

Orbital inclination plays a crucial role in determining a satellite's coverage area and its suitability for specific applications.

• Coverage Area: Inclination dictates the latitudes over which a satellite will pass. A satellite with a higher inclination will reach higher latitudes, while a lower inclination confines the satellite's coverage to areas closer to the equator.
• Mission Requirements: Different missions demand different inclinations. For instance, Earth observation satellites often utilize polar orbits to monitor polar regions, while communication satellites serving equatorial regions favor low inclination orbits.

Classifying Orbits by Inclination

Based on inclination, satellite orbits are broadly classified into:

• Low Inclination Orbits (0-30 degrees): Primarily used for communication and observation satellites focused on equatorial and tropical regions.
• Medium Inclination Orbits (30-60 degrees): Offer a balance between coverage of lower and higher latitudes, often employed for navigation and surveillance.
• High Inclination Orbits (60-90 degrees): Ideal for observing polar regions, conducting scientific research at high latitudes, and monitoring global weather patterns.
• Polar Orbits (exactly 90 degrees): Essential for Earth observation, remote sensing, and scientific studies requiring coverage of the entire globe.
• Sun-Synchronous Orbits: A special type of polar orbit where the satellite passes over the same spot on Earth at the same local solar time each day. This is crucial for consistent lighting conditions in Earth observation.

Inclination and Latitude

The highest latitude a satellite reaches in its orbit is equal to its inclination. For example, a satellite with an inclination of 70 degrees will pass over latitudes as high as 70 degrees North and 70 degrees South. This direct relationship makes inclination a critical factor in determining a satellite's coverage zone.

Effects on Ground Tracks

A satellite's ground track, the path it traces on the Earth's surface, is significantly influenced by its inclination.

• Equatorial Orbit: The ground track follows the equator.
• Polar Orbit: The ground track traces lines of longitude, passing over both poles.
• Inclined Orbit: The ground track oscillates between the latitude limits dictated by the inclination, creating a sinusoidal pattern on the Earth's surface.

The ground track's shape and repeat cycle are vital considerations for mission planning, ensuring consistent coverage and revisit times for specific areas of interest.

Applications of Orbital Inclination

Orbital inclination is tailored to meet the unique requirements of various satellite applications:

• Communication Satellites: Geostationary orbits (0 inclination, fixed position over the equator) provide continuous communication coverage to specific regions.
• Earth Observation Satellites: Polar and Sun-synchronous orbits enable comprehensive monitoring of Earth's land, oceans, and atmosphere.
• Navigation Satellites (GPS, Galileo, GLONASS): Utilize medium inclination orbits to ensure global coverage for positioning and timing services.
• Scientific Research Satellites: Employ a range of inclinations depending on the specific scientific objectives, from studying Earth's magnetosphere to observing celestial objects.

Altitude and Line of Sight: Establishing the Link

In the realm of satellite communications and remote sensing, understanding the relationship between a satellite's altitude and its line of sight (LOS) is fundamental. These two parameters dictate a satellite's coverage area, its ability to communicate with ground stations, and the quality of the data it collects. This chapter delves into the intricacies of altitude and line of sight, exploring their definitions, significance, interplay, effects on communication and observation, and practical implications.

Defining Altitude

Altitude, in the context of satellites, refers to the distance between the satellite and the Earth's surface directly below it (the nadir). It's typically measured in kilometers (km) or miles (mi).

It's important to distinguish altitude from other measures of a satellite's position:

• Apoapsis and Periapsis: These refer to the highest and lowest points in an elliptical orbit, respectively, relative to the center of the Earth.
• Semi-major Axis: This is half the longest diameter of the elliptical orbit.

Altitude specifically focuses on the distance from the Earth's surface, making it a crucial parameter for understanding a satellite's operational environment.

Defining Line of Sight (LOS)

Line of sight (LOS) refers to the direct, unobstructed path between a satellite and a point on the Earth, typically a ground station or another satellite. For a successful communication or observation link, the LOS must be free from obstacles such as terrain, buildings, or atmospheric interference.

The concept of LOS is rooted in basic geometry: a straight line connecting two points. However, in the context of satellites, the curvature of the Earth and potential atmospheric effects add complexity to determining a true LOS.

The Interplay of Altitude and LOS

Altitude and LOS are intrinsically linked. A satellite's altitude directly influences its potential LOS:

• Higher Altitude: A higher altitude generally increases the potential LOS, as the satellite can "see" a larger portion of the Earth's surface. This translates to a wider coverage area and the possibility of communicating with ground stations over greater distances.
• Lower Altitude: A lower altitude restricts the potential LOS, limiting the coverage area. However, lower altitudes can offer benefits such as higher resolution for Earth observation due to the satellite's closer proximity to the surface.

Effects on Communication

In satellite communication, altitude and LOS are critical for establishing and maintaining a reliable link:

• Signal Strength: The strength of the signal received by a ground station is affected by the distance to the satellite, which is related to altitude. Free space path loss, the reduction in signal power as it travels through space, increases with distance.
• Latency: The time it takes for a signal to travel to the satellite and back is influenced by the satellite's altitude. Higher altitudes result in longer latency.
• Coverage Area: As discussed, altitude directly affects the satellite's coverage area, determining which ground stations can communicate with it.

Effects on Observation

For Earth observation satellites, altitude and LOS impact the quality and type of data collected:

• Resolution: Lower altitudes generally allow for higher resolution images as the satellite is closer to the observed objects.
• Coverage Area: Altitude affects the swath width, the area of Earth that the satellite can observe in a single pass. Higher altitudes provide wider swaths.
• Atmospheric Effects: The amount of atmosphere the signal passes through affects the data quality. Lower altitudes mean less atmosphere, potentially reducing atmospheric distortion.

Practical Implications

Understanding altitude and LOS is crucial for:

• Satellite Mission Design: Choosing the appropriate altitude is a critical step in mission planning, balancing coverage needs with communication and observation requirements.
• Ground Station Placement: The location of ground stations must be carefully considered in relation to the satellite's orbit and altitude to ensure a clear LOS.
• Interference Mitigation: Obstacles in the LOS can cause signal blockage or interference, requiring careful planning and potentially the use of techniques like diversity to overcome these challenges.

Satellite Categories

Satellites are classified based on their function, target orbit, size, and weight. The main categories include:

1. Conventional Satellites: These encompass heavy and large satellites weighing over 1,000 kg. They are commonly employed for internet connectivity, TV broadcasting, weather forecasting, and defense purposes.
2. Medium Satellites: Weighing between 500 kg and 1,000 kg, medium satellites are utilized for navigation, internet connectivity, Earth observation, and scientific research.
3. Small Satellites: This category comprises mini satellites (300-500 kg), micro satellites (10-100 kg), and nano satellites (1-10 kg). These smaller and more versatile satellites support a variety of applications, including internet connectivity, remote sensing, and scientific research.

CubeSat Architecture: A Standardised Approach to Nano-satellite Design

In the rapidly evolving landscape of space technology, CubeSats have emerged as a disruptive force, democratising access to space and enabling a wide range of innovative applications.

These standardised nano-satellites, with their unique modular design, have become a popular platform for research, education, and commercial ventures.

This chapter delves into the architecture of CubeSats, exploring their standard form factor, internal components, structural design, and the advantages this standardisation brings.

The CubeSat Standard: Units (U)

At the heart of the CubeSat concept lies its standardised form factor. The size of a CubeSat is specified in units (U), where 1 U represents a 10 cm x 10 cm x 10 cm cube. This modular approach allows for flexibility and scalability:

• 1U CubeSat: The basic building block, measuring 10 cm on each side.
• Multiple-U CubeSats: Larger CubeSats can be constructed by combining multiple units. Common configurations include 2U (10 cm x 10 cm x 20 cm), 3U (10 cm x 10 cm x 30 cm), and even larger variations like 6U and 12U.

This standardised sizing is crucial for several reasons:

• Simplified Integration: It allows for standardised launch deployment systems, making it easier and more cost-effective to launch CubeSats as secondary payloads.
• Modularity and Interoperability: Components designed for CubeSats are often interchangeable, fostering a growing ecosystem of readily available parts.
• Educational Value: The simplicity of the form factor makes CubeSats an ideal platform for students and researchers to gain hands-on experience in satellite development.

Internal Architecture: Components and Subsystems

Despite their small size, CubeSats are fully functional satellites, incorporating all the essential subsystems found in larger spacecraft:

• Payload: This is the mission-specific component, which can include cameras, sensors, communication equipment, or experimental devices. The payload defines the purpose of the CubeSat.
• Command and Data Handling (Camp;DH): The brain of the CubeSat, responsible for controlling all other subsystems, processing data, and communicating with the ground station.
• Attitude Determination and Control System (ADCS): Maintains the CubeSats orientation in space. This is crucial for ensuring that solar panels are pointed towards the sun for power and that instruments are oriented correctly for observation or communication.
• Communication System: Enables the CubeSat to communicate with ground stations on Earth. This typically includes a transceiver and an antenna.
• Power System: Generates, stores, and distributes power to all subsystems. Solar panels are commonly used for energy generation, and batteries provide storage.
• Structure: Provides the physical framework for the CubeSat, holding all components together and protecting them during launch and operation.

5.3 Structural Design: Meeting the Challenges of Space

The structural design of a CubeSat must meet the rigorous demands of the space environment:

• Durability: The structure must withstand the forces of launch and the harsh conditions of space, including extreme temperatures and vacuum.
• Light-weighting: Minimising weight is crucial for cost-effective launch. CubeSat structures are often made from lightweight materials like aluminum alloys.
• Standardisation: The structure must adhere to the CubeSat standard dimensions to ensure compatibility with launch systems and deployers.

Advantages of CubeSat Architecture

The standardised architecture of CubeSats offers numerous advantages:

Low Cost: Compared to traditional satellites, CubeSats are significantly cheaper to develop and launch, opening up space access to a wider range of users.

Rapid Development: The availability of standardised components and simpler design allows for faster development cycles.

Educational Opportunities: CubeSats provide valuable hands-on experience for students and researchers, fostering the next generation of space scientists and engineers.

Innovation and Experimentation: The low cost and rapid development cycle make CubeSats an ideal platform for testing new technologies and conducting scientific experiments in space.

The CubeSat specifications were developed in 1999 by:

Jordi Puig-Suari, professor at California Polytechnic State University (Cal Poly)

Bob Twiggs, professor at Stanford University Space Systems Development Laboratory

CubeSats are very cost-efficient and versatile, often coming with commercial off-the-shelf (COTS) components for their electronics and structure. Originally used for technology demonstrations, the CubeSat industry has grown quickly in recent years. The miniature satellites are used in various missions, e.g. remote sensing, wireless communications, research and education, emission monitoring and earth observation payloads for LEO orbits.

ESAs Cubesat Fleet: Small Satellites, Big Impact

The European Space Agency (ESA) has embraced the innovative potential of Cubesats, miniature satellites built from standardised cubic units, typically 10cm per side. These small yet powerful platforms have revolutionised access to space, offering cost-effective solutions for a wide range of scientific and technological missions. ESAs Technology Cubesat fleet, as illustrated, showcases the remarkable versatility and adaptability of these tiny explorers.

This diverse collection of Cubesats represents a significant investment in space technology development, demonstrating ESAs commitment to fostering innovation and expanding our understanding of Earth and the cosmos. From studying our planets atmosphere and monitoring climate change to venturing out to explore asteroids and the lunar surface, each Cubesat plays a crucial role in advancing scientific knowledge and pushing the boundaries of whats possible in space.

The modular design of Cubesats allows for a high degree of flexibility and customization. As seen in previous picture, the fleet encompasses a variety of sizes, denoted by the U designation (e.g., 3U, 6U, 12U), indicating the number of standard units combined. This scalability enables missions to be tailored to specific requirements, optimizing both functionality and cost-effectiveness.

Several key missions highlight the breadth of ESAs Cubesat program:

• Earth Observation: Cubesats like SIMBA are dedicated to monitoring crucial climate variables, providing valuable data for understanding and addressing the challenges of climate change. PICASSO focuses on studying the atmosphere, contributing to our knowledge of weather patterns and atmospheric composition.
• Space Technology Demonstration: Missions such as GOMX-3 and GOMX-5 serve as testbeds for new platform technologies and next-generation constellation technologies, paving the way for future advancements in space infrastructure. GOMXAB further explores the complexities of constellation technologies, essential for providing continuous global coverage for various applications.
• Exploration and Science: Venturing beyond Earth orbit, Cubesats like M-ARGO are designed to demonstrate asteroid rendezvous and in-situ resource utilization, crucial steps for future human exploration and potential resource extraction. The HERA Cubesats, part of the larger HERA mission, will observe the asteroid deflection assessment, providing invaluable insights into planetary defense strategies. Furthermore, LUMIO and VMMO, in collaboration, will measure lunar surface impact hazards and investigate in-situ resources on the Moon.
• Specialized Scientific Investigations: CUBESPEC utilizes stellar spectroscopy to analyze light from distant stars, contributing to our understanding of stellar evolution and the composition of exoplanetary systems. Qarman is designed to study atmosphere re-entry, providing critical data for improving spacecraft design and ensuring safe return to Earth. PRETTY demonstrates GNSS reflectometry, a technique used to measure various Earth surface characteristics, while Sunstorm focuses on measuring X-ray fluxes, contributing to our understanding of solar activity and its impact on space weather. RadCube measures space radiation and magnetic fields, providing essential data for protecting satellites and astronauts from the harsh space environment.

The ESAs Technology Cubesat fleet represents a significant leap forward in space exploration and utilisation. By leveraging the advantages of miniaturisation, modularity, and cost-effectiveness, these small satellites are making a big impact on our understanding of Earth, our solar system, and the universe beyond.

Satellite Frequency Bands: Orchestrating Communication in Space

Satellites rely on radio waves to communicate, and the specific frequencies they use are carefully allocated to prevent interference and ensure efficient operation. These designated ranges of frequencies are known as frequency bands, analogous to lanes on a highway, each dedicated to specific satellite services and applications. Understanding these bands is crucial to comprehending how different satellite systems function and interact.

While the fundamental concept of frequency bands is universal, the nomenclature can vary. Organizations like the International Telecommunication Union (ITU), the Institute of Electrical and Electronics Engineers (IEEE), and the North Atlantic Treaty Organization (NATO) have each developed their own naming conventions. This can sometimes lead to confusion, but the underlying frequency ranges remain consistent. Think of it like different musical instruments playing the same note – the sound is the same, but the instrument producing it is different.

The following table provides an overview of commonly used frequency bands for various satellite services and applications. It's important to note that this is not an exhaustive list, and specific allocations within each band can vary by region and country. This table offers a general understanding; for precise and detailed information on global frequency band allocations and their corresponding applications, the "Worldwide Spectrum Allocations" poster is an invaluable resource. This poster provides a comprehensive guide to navigating the complex landscape of radio frequency assignments in the satellite domain. Consulting this document is highly recommended for anyone needing precise regulatory information.

Satellite Services and Use Cases: A World Connected

Satellites have become indispensable in our modern world, underpinning a vast array of services that permeate nearly every aspect of our lives. From the moment we check our weather app to navigating across unfamiliar terrain, satellites play a crucial role in connecting us, informing us, and enabling a multitude of essential functions. It's hard to imagine a world without the seamless communication and precise positioning they provide. This chapter explores the diverse landscape of satellite services and highlights some key use cases across private, public, and industrial sectors.

The tables below provide an overview of common satellite services, categorized into two aspects: one table highlights satellite services in relation to their applications, while the other focuses on satellite services based on their respective orbits. It is important to note that this is not an exhaustive list, as satellite technology continues to evolve with new and innovative applications.

Note: This article is part of a multi-article series. In the next installment, I will analyze Satellite Communication, exploring its technologies, applications, and impact on modern connectivity.