A technical comparison of three low earth orbit satellite constellation systems to provide global broadband

Highlights

• Technical overview of the architecture of OneWeb, SpaceX and Telesat LEO constellations.

• Comparison in terms of satellites in LoS, frequency allocation, beam characteristics.

• Analysis of ground segment requirements and impact on system capacity.

• Statistical estimates of the total system throughput (sellable capacity) for each system.

Abstract

The idea of providing Internet access from space has made a strong comeback in recent years. After a relatively quiet period following the setbacks suffered by the projects proposed in the ’90s, a new wave of proposals for large constellations of low Earth orbit (LEO) satellites to provide global broadband access emerged between 2014 and 2016. Compared to their predecessors, the main differences of these systems are: increased performance that results from the use of digital communication payloads, advanced modulation schemes, multi-beam antennas, and more sophisticated frequency reuse schemes, as well as the cost reductions from advanced manufacturing processes (such as assembly line, highly automated, and continuous testing) and reduced launch costs. This paper compares three such large LEO satellite constellations, namely SpaceX's 4425 satellites Ku-Ka-band system, OneWeb's 720 satellites Ku-Ka-band system, and Telesat's 117 satellites Ka-band system. First, we present the system architecture of each of the constellations (as described in their respective FCC filings as of September 2018), highlighting the similarities and differences amongst the three systems. Following that, we develop a statistical method to estimate the total system throughput (sellable capacity), considering both the orbital dynamics of the space-segment and the variability in performance induced by atmospheric conditions both for the user and feeder links. Given that the location and number of ground stations play a major role in determining the total system throughput, and since the characteristics of the ground segment are not described in the FCC applications, we then run an optimization procedure to minimize the total number of stations required to support the system throughput. Finally, we conclude by identifying some of the major technical challenges that the three systems will have to overcome before becoming operational.

Introduction

The idea of providing Internet from space using large constellations of LEO satellites has re-gained popularity in the last years. Despite the setbacks suffered by the projects proposed in the decade of the ’90s [1], a new wave of proposals for large low Earth orbit (LEO) constellations of satellites to provide global broadband emerged between 2014 and 2016. A total of 11 companies have applied to the Federal Communications Commission (FCC) to deploy large-constellations in non-geostationary satellite orbits (NGSO) as a means to provide broadband services. These new designs range from 2 satellites, as proposed by Space Norway, to 4,425 satellites, as proposed by SpaceX. Due to the large number of satellites in these constellations, the name “mega-constellations” was coined to refer to these new proposals.

The main differences of these new mega-constellations compared to their predecessors from the 90's (e.g., Iridium, Globalstar, Orbcomm), are the increased performance that results from the use of digital communication payloads, advanced modulation schemes, multi-beam antennas, and more sophisticated frequency reuse schemes, as well as cost reductions from advanced manufacturing processes and reduced launch costs. In addition to reduced costs and increased technical capabilities, the increasing demand for broadband data, as well as the projections of growth of the mobility (aerial, maritime) markets, provided major incentives for the development of these systems.

Of the 11 proposals registered within the FCC, there are three that are in an advanced stage of development, with launches planned in the next 3 years: OneWeb's, SpaceX's, and Telesat's.

This paper reviews the system architecture of each of these mega-constellations, as described in their respective FCC filings (as of September 2018), and highlights the similarities and differences amongst the three systems. We then proceed to estimate the total system throughput using a novel statistical framework that considers both the orbital dynamics of the space-segment, the variability in performance induced by atmospheric conditions for the user and feeder links, and reasonable limits on the sellable capacity.

Using large constellations of LEO satellites to provide global connectivity was first proposed in the 90's, fueled by the increasing demand for cellular and personal communications services, as well as general Internet usage. Among the LEO systems proposed, some were cancelled even before launch (e.g., Teledesic, Celestri, Skybridge), whereas others filed for bankruptcy protection shortly after the beginning of operations (e.g., Iridium, Globalstar, Orbcomm) [2].

Multiple technical reports were published (mostly by the constellation designers themselves) outlining the architecture of each of the proposed systems: Sturza [3] described the technical aspects of the original Teledesic satellite system, a 924 satellite constellation; Patterson [4] analyzed the 288 satellites system that resulted from downsizing the original proposal; the Iridium system was comprehensively described by Leopold in several papers [5,6]; and Globalstar's constellation was analyzed by Wiedeman [7].

From the comparative approach, Comparetto [8] reviewed the Globalstar, Iridium, and Odyssey systems, focusing on the system architecture, handset design and cost structures of each of the proposals. Dumont [9] studied the changes these three systems went through from 1991 to 1994. Evans [10] analyzed different satellite systems for personal communications in different orbits (GEO, MEO, and LEO), and later compared the different proposals for Ka-band [11] and Ku-band [12] systems in LEO. The approaches followed in these references were mostly descriptive in nature, providing overviews on the architectures of the various LEO systems. On the other hand, Shaw [13] compared quantitatively the capabilities of the Cyberstar, Spaceway, and Celestri proposals assessing variables such as capacity, signal integrity, availability, and cost per billable T1/minute.

The research related to the new LEO proposals is scarce and has focused on analyzing debris and impact probabilities [14,15], as well as comparing the qualities of LEO and GEO systems in serving maritime and aeronautical users [16]. In particular, Le May [14] studied the probability of collision for SpaceX and OneWeb satellites operating in the current LEO debris environment, while Foreman [15] provided several policy recommendations to address orbital debris concerns after analyzing the number of encounters between satellites and space debris. Finally, McLain [16] compared the two aforementioned systems against multiple geostationary, very-high-throughput satellites, and concluded that the latter offer a simpler, less risky, and more economical path to providing large for the aeronautical and maritime industries.

This paper adopts a similar approach as Evans [10] to compare the proposals of OneWeb, Telesat, and SpaceX. We first describe each of the systems, and then, we conduct a comparative analysis for some additional aspects of the constellations. The second half of this paper is devoted to estimating the performance (in terms of total system throughput and requirements for the ground segment) of the three systems.

The objectives of this paper are twofold. First, to present the system architecture on a consistent and comparable basis of OneWeb's, Telesat's, and SpaceX's constellations, while conducting a technical comparison between them; second, to estimate the total system throughput and requirements for the ground segment for each of the proposals using a statistical method that considers both the orbital dynamics of the space-segment and the variability in performance induced by atmospheric conditions both for the user and feeder links.

This paper is structured as follows: Section 2 discusses the different system architectures for the three systems conceived by Telesat, OneWeb and SpaceX; Section 3 introduces the methodology to estimate the total system capacity and derive the requirements for the ground segment.; Section 4 presents the results in terms of total system throughput and number of gateway and ground station locations required by each of the mega-constellations; Section 5 identifies the major technical challenges that we believe these systems still have to overcome before becoming operational; and Section 6 presents our overall conclusions.