Satellite communications (SatComs) have recently entered a period of renewed interest motivated by technological advances and nurtured through private investment and ventures. The present survey aims at capturing the state of the art in SatComs, while highlighting the most promising open research topics. Firstly, the main innovation drivers are motivated, such as new constellation types, on-board processing capabilities, non-terrestrial networks and space-based data collection/processing. Secondly, the most promising applications are described, i.e., 5G integration, space communications, Earth observation, aeronautical and maritime tracking and communication. Subsequently, an in-depth literature review is provided across five axes: i) system aspects, ii) air interface, iii) medium access, iv) networking, v) testbeds & prototyping. Finally, a number of future challenges and the respective open research topics are described.

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Satellite Communications in the New Space Era: A Survey and Future Challenges

Small satellite systems enable a whole new class of missions for navigation, communications, remote sensing, and scientific research for both civilian and military purposes. As individual spacecraft are limited by the size, mass, and power constraints, mass-produced small satellites in large constellations or clusters could be useful in many science missions such as gravity mapping, tracking of forest fires, finding water resources, etc. The proliferation of small satellites will enable a better understanding of the near-Earth environment and provide an efficient and economical means to access the space through the use of multi-satellite solution. Constellation of satellites provide improved spatial and temporal resolution of the target. Small satellite constellations contribute innovative applications by replacing a single asset with several very capable spacecraft, which opens the door to new applications. Future space missions are envisioned to become more complex and operate farther from Earth, and will need to support autonomous operations with minimal human intervention. With increasing levels of autonomy, there will be a need for remote communication networks to enable communication between spacecraft. These space-based networks will need to configure and maintain dynamic routes, manage intermediate nodes, and reconfigure themselves to achieve mission objectives. Hence, inter-satellite communication is a key aspect when satellites fly in formation. In this survey, we present the various research being conducted in the small satellite community for implementing inter-satellite communications based on the open system interconnection (OSI) model. This survey also reviews the various design parameters applicable to the first three layers of the OSI model, i.e., physical, data link, and network layer. Based on the survey, we also present a comprehensive list of design parameters useful for achieving inter-satellite communications for multiple small satellite missions. Specific topics include proposed solutions for some of the challenges faced by small satellite systems, enabling operations using a network of small satellites, and some examples of small satellite missions involving formation flying aspects.

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Survey of Inter-Satellite Communication for Small Satellite Systems: Physical Layer to Network Layer View

Small satellite systems enable whole new class of missions for navigation, communications, remote sensing and scientific research for both civilian and military purposes. As individual spacecraft are limited by the size, mass and power constraints, mass-produced small satellites in large constellations or clusters could be useful in many science missions such as gravity mapping, tracking of forest fires, finding water resources, etc. Constellation of satellites provide improved spatial and temporal resolution of the target. Small satellite constellations contribute innovative applications by replacing a single asset with several very capable spacecraft which opens the door to new applications. With increasing levels of autonomy, there will be a need for remote communication networks to enable communication between spacecraft. These space based networks will need to configure and maintain dynamic routes, manage intermediate nodes, and reconfigure themselves to achieve mission objectives. Hence, inter-satellite communication is a key aspect when satellites fly in formation. In this paper, we present the various researches being conducted in the small satellite community for implementing inter-satellite communications based on the Open System Interconnection (OSI) model. This paper also reviews the various design parameters applicable to the first three layers of the OSI model, i.e., physical, data link and network layer. Based on the survey, we also present a comprehensive list of design parameters useful for achieving inter-satellite communications for multiple small satellite missions. Specific topics include proposed solutions for some of the challenges faced by small satellite systems, enabling operations using a network of small satellites, and some examples of small satellite missions involving formation flying aspects.

Survey of Inter-satellite Communication for Small Satellite Systems: Physical Layer to Network Layer View

Advances in modern technology have aided the development of a class of miniaturized satellites called SmallSats that typically weigh less than 500 kg. Key members of this family are CubeSats. CubeSats can weigh as little as 1.33 kg, with a typical volume of 10 ? 10 ? 10 cm3. Their potential has motivated the scientific community to revisit existing spacecraft technologies to make them suitable for CubeSats.

For Satellites, Think Small, Dream Big: A review of recent antenna developments for CubeSats.

Given the increasing number of space-related applications, research in the emerging space industry is becoming more and more attractive. One compelling area of current space research is the design of miniaturized satellites, known as CubeSats, which are enticing because of their numerous applications and low design-and-deployment cost. The new paradigm of connected space through CubeSats makes possible a wide range of applications, such as Earth remote sensing, space exploration, and rural connectivity. CubeSats further provide a complementary connectivity solution to the pervasive Internet of Things (IoT) networks, leading to a globally connected cyber-physical system. This paper presents a holistic overview of various aspects of CubeSat missions and provides a thorough review of the topic from both academic and industrial perspectives. We further present recent advances in the area of CubeSat communications, with an emphasis on constellation-and-coverage issues, channel modeling, modulation and coding, and networking. Finally, we identify several future research directions for CubeSat communications, including Internet of space things, low-power long-range networks, and machine learning for CubeSat resource allocation.

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CubeSat Communications: Recent Advances and Future Challenges

Realizing inter-satellite links is a must for ensuring the success of cubesat swarm missions. Nevertheless, it has hardly been considered until now. The communication systems for cubesats have to deal with a few peculiar demands regarding consumed power, geometry and throughput. Depending on the type of application, required data rates can go up to tens of megabits per second, while power consumption and physical size are limited by the platform. The proposed communication scheme will combine power-efficient modulation and channel coding with multiple access and spread spectrum techniques, enabling the deployment of multiple satellites. Apart from this, the antenna system has to be designed such that links can be established and maintained independent of the satellites' orientation. An electrically steerable radiation pattern is achieved by placing antennas on each face of the cube. Conformal beamforming provides the system with 5 dBi gain for any desired direction of transmission, eliminating the need for attitude control. Furthermore, using planar antennas reduces the complexity of the mechanical part as they require no deployment.

Inter-satellite links for cubesats

New science drivers have recently emerged in radio astronomy for observation of low-frequency radio waves, below 30 MHz. Exploring this frequency requires a space-based radio telescope with a very large aperture that is impossible to realize in a monolithic fashion. A distributed system consisting of a swarm of 50 or more nano-satellites is used to realize such an instrument. Equipped with low-frequency antennas, the very small spacecraft provide the needed aperture to capture and sample ultra-long electromagnetic waves. The distributed low-frequency telescope has to fulfill multiple tasks in which drawbacks such as the size and the limited power available are overcome by the large number of satellites. Sending the processed data to a base station is one of these aforementioned tasks that is critical for the functionality of the system. In our paper we analyze the challenges of downlinking data from a swarm of nano-satellites to Earth and propose a diversity scheme that helps the system to achieve its mission.

Swarm to earth communication in OLFAR

Swarm-to-Earth communication in OLFAR

Low-frequency space observation is one of the developing directions in radio astronomy, as scientists try to reveal more of the universe. The Orbiting Low Frequency Array for Radio Astronomy project is aimed at developing a distributed radio telescope sensitive to ultra-long waves, by placing an array of antennas, far away from any terrestrial interference. It will consist of over 50 small satellites grouped in a swarm capable of collecting and processing astronomical data. The system will use its resources to the limit and will need an efficient communication topology in order to guarantee functionality. In this paper we present a clustering scheme that reduces data distribution efforts at the cost of decreasing imaging redundancy.

OLFAR: Adaptive topology for satellite swarms

The Orbiting Low Frequency Antennas for Radio Astronomy (OLFAR) project aims to develop a space-based low frequency radio telescope that will explore the universe's so-called dark ages, map the interstellar medium, and discover planetary and solar bursts in other solar systems. The telescope, composed of a swarm of at least fifty satellites working as a single instrument, will be sent to a location far from Earth in order to avoid the high Radio Frequency Interference (RFI) found at frequencies below 30 MHz, originating from Earth. The OLFAR telescope is a novel and complex system, requiring not-yet proven technologies and systems, therefore, a number of key technologies are still to be developed and proven. Most of these can be tested on Earth, but four aspects in particular require in-space verification. Those are (1) the satellite's propulsion and attitude control systems, and (2) their interactions with the large science antennas, as well as the (3) payload system itself and finally (4) the in-space interferometry and 3D-imaging. Furthermore, the RFI environment in the intended target orbits is mostly unknown. Indeed, only three satellites missions have previously been launched into orbit shedding light on the RFI environment, but sufficiently detailed measurements allowing for the creation of a usable RFI model have never been performed. To carry out both the hardware qualification and RFI measurements, a few pathfinder missions are deemed in order. This paper describes these pathfinders in detail; outlining the scientific objective, the technologies being demonstrated as well as the missions' roadmap which revolves around a novel systems engineering approach. This approach resembles those used in certain fast-paced industries where development is heavily parallelised and products are launched as soon as opportunities arise. This will be combined with in-space upgrading of mission firmware to allow for high flexibility within the limited time and budget constraints of these pathfinders.

A. Budianu

Papers

Inter-satellite links for cubesats

Swarm to earth communication in OLFAR

Swarm-to-Earth communication in OLFAR

OLFAR: Adaptive topology for satellite swarms

The road to OLFAR - a roadmap to interferometric long-wavelength radio astronomy using miniaturized distributed space systems