Large Constellations Research Articles

Development of an international compendium of guidelines for the long-term sustainability of outer space activities

The Earth's orbital space environment constitutes a finite resource that is being used by an increasing number of States, international intergovernmental organizations and non-governmental entities. The proliferation of space debris, the increasing complexity of space operations, the emergence of large constellations and the increased risks of collision and interference with the operation of space objects are raising concerns about the long-term sustainability of outer space activities. Addressing these developments and risks requires international cooperation. In 2010, the United Nations Committee on the Peaceful Uses of Outer Space (UN COPUOS) established a working group to address these matters and to propose a set of voluntary, non-binding guidelines for States that represent a compendium of internationally recognized measures for, and commitments to, ensuring the long-term sustainability of outer space activities and, in particular, enhancing the safety of space operations. This paper describes the efforts of this Working Group in the context of other multilateral space security initiatives. The first twelve guidelines, which were agreed in 2016, are briefly discussed, as well as the linkages of this process with the recommendations contained in the report of the Group of Governmental Experts on Transparency and Confidence-Building Measures in Outer Space Activities. The paper ends with some reflections on factors shaping the negotiations of the remaining draft guidelines as the Working Group enters its final round of negotiations in 2018.

Dimensional and scale analysis applied to the preliminary assessment of the environment criticality of large constellations in LEO

Several analytical expressions, based on reasonable simplifying assumptions, were developed for the assessment of the environment criticality of large constellations and huge numbers of small satellites in low Earth orbit. They can provide preliminary quantitative answers to difficult questions, with no need of complex models and computations. Moreover, a specific figure of merit was introduced for gauging the environment criticality of new large constellations: the collision rate percentage increase. Because several systems might be operated at the same time, an alert threshold of no more than 10% per single constellation seemed a wise suggestion. Finally, several quantitative examples and the associated results were presented and discussed.

Effect of large constellations on lifetime of satellites in low earth orbits

A 2010 study found, assuming business-as-usual satellite operations and replenishment activities, that for large satellites operating in the low Earth orbit (LEO) regime, a mean lifetime reduction over the next 50 years of ∼13% might be expected (i.e., a satellite with a mean lifetime of 10 years in a no-debris environment would see a reduction of 1.3 years in an environment that included space debris), with much of this reduction coming from degradation of solar panels by small, un-trackable debris. Since 2010, proposals have been put forward that would add thousands of new satellites to the LEO environment within the next few years, many more than the number projected in the 2010 study.The current study reexamines the earlier prediction by assuming that over 5000 new small satellites are operating in high LEO orbits, consistent with proposals announced by commercial companies. As with the 2010 study, this projection includes the effects of collisions of both tracked objects, objects greater than 10 cm in LEO, and small debris down to 1 mm—a size that the 2010 study showed could “sandblast” and reduce the power output of solar panels—and adds changes in the debris environment due to the previously un-modeled new satellites colliding in their constellations and with other objects as they undergo constellation replenishment activities.Primary results are that satellites being disposed from or added to the new constellations and debris associated with collisions involving these objects could potentially double the reduction in the mean operational lifetime of satellites operating in LEO. As in the 2010 study, the degradation of solar panels due to small debris impacts is a significant factor in this degradation, with the mean lifetime reduction increasing from the 13% predicted in the earlier study to as much as 60% in some cases (i.e., a large satellite with a mean lifetime of 20 years in a no-debris environment could have its mean lifetime reduced by as much as 12 years). Results suggest that minimizing collisions during replenishment activities and making solar panels and space hardware more resilient to small debris impacts will be increasingly important in the future.

Construction and Labeling of Triangular QAM

We propose two algorithms: one is for building and the other for labeling triangular QAM (TQAM) constellations. A root distribution algorithm (also referred as RD) is used to construct TQAM constellations of low average energy. The proposed labeling algorithm, termed the layer labeling algorithm, results in bit-to-symbol mappings that minimize the average Hamming distance between constellation points. The strength of the algorithms lies in their applicability to large constellations, while currently known solutions rely on a manual design of good TQAM constellations. This letter describes the very details of the algorithms and compares the performance, in terms of $BER(E_{b}/N_{0})$ , of the proposed solution with the one that can be found in the literature.

Probabilistic Equalization With a Smoothing Expectation Propagation Approach

In this paper, we face the soft equalization of channels with inter-symbol interference for large constellation sizes, $ \mathtt {M}$ . In this scenario, the optimal BCJR solution and most of their approximations are intractable, as the number of states they track grows fast with $ \mathtt {M}$ . We present a probabilistic equalizer to approximate the posterior distributions of the transmitted symbols using the expectation propagation (EP) algorithm. The solution is presented as a recursive sliding window approach to ensure that the computational complexity is linear with the length of the frame. The estimations can be further improved with a forward–backward approach. This novel soft equalizer, denoted as smoothing EP (SEP), is also tested as a turbo equalizer, with a low-density parity-check (LDPC) channel decoder. The extensive results reported reveal remarkably good behavior of the SEP. In low dimensional cases, the bit error rate (BER) curves after decoding are closer than 1 dB from those of the BJCR, robust to the channel response. For large $ \mathtt {M}$ , the SEP exhibits gains in the range of 3–5 dB compared to the linear minimum mean square error algorithm.

On Southern Things

On Southern Things Bernard L. Herman (bio) Click for larger view View full resolution Thinking of southern things, we are often drawn to the canonical object—an oyster po'boy or Charleston single house or Edgefield pottery or kudzu. Edgefield District, South Carolina Face Jug, ca. 1862, alkaline glazed stoneware with kaolin inserts, 6⅝ × 5⅛ in. (16.83 × 13.02 cm.). Photograph by Jim Wildeman, courtesy of the Chipstone Foundation 2012.4. [End Page 7] Material culture is best understood as the history and philosophy of objects. It proceeds from the idea that objects, tangible and imagined, locate the entirety of human experience and understanding. We are simply creatures that know and make sense of the world and our places within it through things. Southern things superintend the vast and diffuse array of objects that ground the many, often conflicted, sometimes nostalgic ideologies of a regional identity that is at once singular and plural. The question that emerges at the heart of this collection of essays, images, and poems is not what are southern things, but rather how are things southern. Each of the contributions that follow provides a fragment of an answer. Southern things are not necessarily objects with regional pedigrees discovered through metrics of makers, locales, collections, and consumers. Southern things are those objects that are shaped, molded, and presented to the world as an amalgam of many deeply conflicted identities forged in a crucible of race and class and connected to a region and its diaspora. How else can you reconcile the nascar circuit in California or the blues in Chicago or red velvet cupcakes served with claret at Selfridges in London or the unending reinvention of poverty foods (for example, shrimp and grits) as haute cuisine? To grasp how southern things do this work invites some thoughts on how we might go about the critical practices of engaging things through interpretive acts. At the outset, it is useful to draw a distinction between object-centered and object-driven approaches. An object-centered approach seeks to represent the artifact in all of its materiality. In the narrowest sense, an object-centered approach has as its first imperative something of a connoisseurial or curatorial gesture. We want to understand why an object looks the way it does, where it fits into a larger array of objects, and then where and how that array intersects with larger constellations. Among the core questions of an object-centered approach are those of maker(s), media, craft, manufacture, category, antecedents, function, and more. In essence, we want to know why the object looks the way it does. Thus, Emily Hilliard's interview and photographic essay with West Virginia broom maker Jim Shaffer epitomizes an object as a made thing via the close documentation of process through photographs and recorded interview. Handmade brooms conform neatly within long-held genres of southern handicraft. The same cannot be said for a set of bejeweled false teeth from McComb, Mississippi. Recuperating the origin story for the Rhinestone Cowboy's dentures, Jennifer Joy Jameson interviewed the dentist who fabricated the dazzling chompers. What links Jameson's and Hilliard's contributions is their common goal of documenting skill, process, and product. An object-driven approach originates from a different but parallel set of concerns about what and how things convey meaning. The core assumption is that objects operate as signs, communicating meaning and structuring social relationships [End Page 8] in ways that are infinitely variable and always contingent on contexts of circumstance and shared ideologies of everyday life. A key distinction in object-driven approaches is the recognition that things offer wellsprings of questions. Too often, material culture studies relegate objects to the role of illustration or as subjects on which imported meanings are inscribed. But, objects ask questions—and the kinds of questions they pose offer unexpected avenues of inquiry and insight. Dale Rosengarten's recitation of the fate of two silver bowls, Shana Klein's reprise of Harriet Beecher Stowe's orange grove, and Rebecca Sharpless's recounting of the place of "Dutch" ovens held in southern kitchens all begin with objects that speak to larger historical currents. Their entries in this collection remind us that...

The 59th Colloquium on the Law of Outer Space at the 67th International Astronautical Congress

The 59th International Institute of Space Law (IISL) Colloquium on the Law of Outer Space was held from 27–30 September 2016 in Guadalajara, Mexico, in conjunction with the 67th International Astronautical Congress (IAC). Comprised of seven technical sessions and one session for interactive presentations, the Colloquium focused on a range of thematic issues in space law, and featured papers from almost seventy presenters. Topics discussed included the legal aspects related to the remote sensing principles, regional cooperation and Latin American space activities, large satellite constellations, space resource utilization, planetary defence, and spectrum management. The 8th Nandasiri Jasentuliyana Keynote Lecture was delivered by incoming IISL president Kai-Uwe Schrogl, entitled ‘Space Law and Diplomacy’. The Manfred Lachs Space Law Moot Court semi-finals and world finals were also held in Guadalajara, as well the General Assembly of the IISL.

Cutoff Rate of Sparse Code Multiple Access in Downlink Broadcast Channels

For the sake of supporting massive connectivity in the future 5G networks, non-orthogonal multiple access (NOMA) techniques are advocated. As a promising NOMA technique, in recent years sparse code multiple access (SCMA) has attracted substantial attention. However, there is a paucity of studies on the theoretical analysis of its error-freely achievable data rate, especially, in the downlink context. Hence, we derive the cutoff rate of SCMA in downlink broadcast channels, which indicates the lower-bound of a system’s error-freely achievable rate. However, we will demonstrate that when considering the conventional categorization of pairwise error events, the accuracy of the cutoff rate rapidly degrades in the low-SNR region owing to the fact that multi-user SCMA systems typically encounter an extremely large constellation size. Alternatively, by invoking Bergmans’ concept from 1973 in the categorization of pairwise error events, we obtain a more accurate cutoff rate both in the low- SNR regions and the high-SNR regions. Moreover, we provide insights into the cutoff rate derivation process, which reveals some general guidelines for designing a beneficial codebook, capable of improving SCMA with respect to its original low-density signature-based counterpart.

Risk to space sustainability from large constellations of satellites

The number of artificial objects in orbit continues to increase and, with it, a key threat to space sustainability. In response, space agencies have identified a set of mitigation guidelines aimed at enabling space users to reduce the generation of space debris by, for example, limiting the orbital lifetime of their spacecraft and launcher stages after the end of their mission. Planned, large constellations of satellites in low Earth orbit (LEO), though addressing the lack of basic internet coverage in some world regions, may disrupt the sustainability of the space environment enabled by these mitigation practices. We analyse the response of the space object population to the introduction of a large constellation conforming to the post-mission disposal guideline with differing levels of success and with different disposal orbit options. The results show that a high success rate of post-mission disposal by constellation satellites is a key driver for space sustainability.

Optimal Scheduling of a Constellation of Earth-Imaging Satellites, for Maximal Data Throughput and Efficient Human Management

A mixed-integer linear program (MILP) approach to scheduling a large constellation of Earth-imaging satellites is presented. The algorithm optimizes the assignment of imagery collects, image data downlinks, and "health & safety" contacts, generating schedules for all satellites and ground stations in a network. Hardware-driven constraints (e.g., the limited agility of the satellites) and operations-driven constraints (e.g., guaranteeing a minimum contact frequency for each satellite) are both addressed. Of critical importance to the use of this algorithm in real-world operations, it runs fast enough to allow for human operator interaction and repeated rescheduling. This is achieved by a partitioning of the problem into sequential steps for downlink scheduling and image scheduling, with a novel dynamic programming (DP) heuristic providing a stand-in for imaging activity in the MILP when scheduling the downlinks.

Higher Dimensional Meta-State Analysis Reveals Reduced Resting fMRI Connectivity Dynamism in Schizophrenia Patients.

Resting-state functional brain imaging studies of network connectivity have long assumed that functional connections are stationary on the timescale of a typical scan. Interest in moving beyond this simplifying assumption has emerged only recently. The great hope is that training the right lens on time-varying properties of whole-brain network connectivity will shed additional light on previously concealed brain activation patterns characteristic of serious neurological or psychiatric disorders. We present evidence that multiple explicitly dynamical properties of time-varying whole-brain network connectivity are strongly associated with schizophrenia, a complex mental illness whose symptomatic presentation can vary enormously across subjects. As with so much brain-imaging research, a central challenge for dynamic network connectivity lies in determining transformations of the data that both reduce its dimensionality and expose features that are strongly predictive of important population characteristics. Our paper introduces an elegant, simple method of reducing and organizing data around which a large constellation of mutually informative and intuitive dynamical analyses can be performed. This framework combines a discrete multidimensional data-driven representation of connectivity space with four core dynamism measures computed from large-scale properties of each subject’s trajectory, ie., properties not identifiable with any specific moment in time and therefore reasonable to employ in settings lacking inter-subject time-alignment, such as resting-state functional imaging studies. Our analysis exposes pronounced differences between schizophrenia patients (Nsz = 151) and healthy controls (Nhc = 163). Time-varying whole-brain network connectivity patterns are found to be markedly less dynamically active in schizophrenia patients, an effect that is even more pronounced in patients with high levels of hallucinatory behavior. To the best of our knowledge this is the first demonstration that high-level dynamic properties of whole-brain connectivity, generic enough to be commensurable under many decompositions of time-varying connectivity data, exhibit robust and systematic differences between schizophrenia patients and healthy controls.

Non-Uniform Constellations for ATSC 3.0

This paper introduces the concept of a non-uniform constellation (NUC) in contrast to conventional uniform quadrature-amplitude modulation (QAM) constellations. Such constellations provide additional shaping gain, which allows reception at lower signal-to-noise ratios. ATSC3.0 will be the first major broadcasting standard, which completely uses NUCs due to their outstanding properties. We will consider different kinds of NUCs and describe their performance: 2-D NUCs provide more shaping gain at the cost of higher demapping complexity, while 1-D NUCs allow low-complexity demapping at slightly lower shaping gains. These NUCs are well suited for very large constellations sizes, such as 1k and 4k QAM.

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

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.

Self-organising satellite constellation in geostationary earth orbit

This paper presents a novel solution to the problem of autonomous task allocation for a self-organising satellite constellation in Earth orbit. The method allows satellites to cluster themselves above targets on the Earth's surface. This is achieved using coupled selection equations (CSEs), a dynamical systems approach to combinatorial optimisation whose solution tends asymptotically towards a Boolean matrix describing the pairings of satellites and targets, which solves the relevant assignment problems. Satellite manoeuvres are actuated by an artificial potential field method which incorporates the CSE output. Three demonstrations of the method's efficacy are given, first with equal numbers of satellites and targets, then with a satellite surplus, including agent failures, and finally with a fractionated constellation. Finally, a large constellation of 100 satellites is simulated to demonstrate the utility of the method in future swarm mission scenarios. The method provides efficient solutions with quick convergence, is robust to satellite failures, and hence appears suitable for distributed, on-board autonomy.

On the banality of forest governance fragmentation: Exploring ‘‘gecko politics’’ as a bureaucratic behaviour in limited statehood

Governance of tropical forests at both the international and the national level is more and more fragmented because of the large constellation of actors, ideas and interests influencing forestland policies for their conservation or change. In public policy area, fragmentation is to some extent the result of competing interests and objectives that lead to a power play in policy-making between actor groups (both public and private), i.e., state bureaucracies, Intergovernmental organisations, nongovernmental organisations and private companies. In this power relationship for the control of tropical forestlands, the hegemony of state bureaucracies in forest governance processes is increasingly contested because of the growing influence of external actor groups in forest governance. Our findings show that the fragmentation of forest governance, which might be considered specific to pluralistic systems, became banal even in tropical countries with a state-centred system like Cameroon where the state claims hegemonic ownership of forestlands. This fragmentation is particularly reinforced in limited statehood where state bureaucracies are attracted to/dependent on external resources for dealing with weak institutional capacity and to face the government's lack of ability to supply public goods and services. But despite the governance fragmentation constraints, this article reveals that some governments and sectoral bureaucracies in limited statehood are capable of using cunning behaviours to secure their own interests in domestic forestland policies and avoid pressure from external actors. In the case of Cameroon for example, the strategy being used relies on a wait-and-see game, uncertainty and recourse to sudden, ambiguous decisions; this is what we call “gecko politics”. Broadly, this article seeks to understand how a government or state bureaucracy in limited statehood with a state-centred system can try to impose its own interests in the context of forest governance fragmentation?

Distributed Greedy Algorithm for Satellite Assignment Problem with Submodular Utility Function

Recent advances in spacecraft technology allow a large number of smaller and cheaper satellites to fulfill useful Earth monitoring roles. An emerging challenge is to coordinate those satellites to monitor the Earth's surface and atmosphere. In this paper, we study the satellite assignment problem for a large constellation of Earth observing satellites, where a set of satellites assign themselves to observe a set of locations. The objective is to maximize a global utility function that is associated with the assignment profile. Due to the communication constraints of the space system, we develop a distributed assignment algorithm where each satellite makes its choice based on local information and local communication. We show that the efficiency ratio of the distributed solution to the optimal one is lower bounded by 1/2. Moreover, this bound is proven to be tight in the sense that there exist scenarios where the bound 1/2 is approached as close as possible.

Maternal control of the Drosophila dorsal-ventral body axis.

The pathway that generates the dorsal-ventral (DV) axis of the Drosophila embryo has been the subject of intense investigation over the previous three decades. The initial asymmetric signal originates during oogenesis by the movement of the oocyte nucleus to an anterior corner of the oocyte, which establishes DV polarity within the follicle through signaling between Gurken, the Drosophila Transforming Growth Factor (TGF)-α homologue secreted from the oocyte, and the Drosophila Epidermal Growth Factor Receptor (EGFR) that is expressed by the follicular epithelium cells that envelop the oocyte. Follicle cells that are not exposed to Gurken follow a ventral fate and express Pipe, a sulfotransferase that enzymatically modifies components of the inner vitelline membrane layer of the eggshell, thereby transferring DV spatial information from the follicle to the egg. These ventrally sulfated eggshell proteins comprise a localized cue that directs the ventrally restricted formation of the active Spätzle ligand within the perivitelline space between the eggshell and the embryonic membrane. Spätzle activates Toll, a transmembrane receptor in the embryonic membrane. Transmission of the Toll signal into the embryo leads to the formation of a ventral-to-dorsal gradient of the transcription factor Dorsal within the nuclei of the syncytial blastoderm stage embryo. Dorsal controls the spatially specific expression of a large constellation of zygotic target genes, the Dorsal gene regulatory network, along the embryonic DV circumference. This article reviews classic studies and integrates them with the details of more recent work that has advanced our understanding of the complex pathway that establishes Drosophila embryo DV polarity. For further resources related to this article, please visit the WIREs website. The authors have declared no conflicts of interest for this article.

On the relation between asymmetries in the ring current and magnetopause current

Motion of charged particles in the Earth's magnetosphere sets up a system of currents. Current continuity requires that these currents are closed, either locally or via other current systems. In this paper, we have investigated whether magnetopause surface currents can contribute to ring current closure. Using measurements from the Cluster constellation of spacecraft, we calculated thickness and current density of the magnetopause current layer for a large number of flank magnetopause traversals. For each event, we consulted sectorial ring current indices, derived from SuperMAG—a large constellation of ground‐based magnetometer stations. SuperMAG results show a significant and persistent dawn‐dusk asymmetry in ground magnetic perturbations which indicates a more intense ring current on the duskside. The asymmetries become more pronounced during disturbed magnetospheric conditions, indicating an increased divergence of the current and closure through other current systems. A similar response to geomagnetic activity is also observed at the magnetopause. Duskside magnetopause current densities are generally higher than their dawnside counterparts, and also, the magnetopause asymmetry becomes more pronounced during disturbed conditions. Although the two current systems are related to different processes—gradient drift of energetic plasma sheet particles for the ring current and a surface current due to differential motion of ions and electrons inside the magnetopause interface for the magnetopause current—the results demonstrate a mutual relation between the two current systems.

Practical linear precoder design for finite alphabet multiple‐input multiple‐output orthogonal frequency division multiplexing with experiment validation

A low complexity precoding method is proposed for practical multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. Based on the two-step optimal precoder design algorithm that maximises the lower bound of the mutual information with finite-alphabet inputs, the proposed method simplifies the precoder design by fixing the right singular vectors of the precoder matrix, eliminating the iterative optimisation between the two steps, and discretising the search space of the power allocation vector. For a 4 × 4 channel, the computational complexity of the proposed precoder design is reduced to 3 and 6% of that required by the original two-step algorithm with quadrature phase shift keying (QPSK) and 8 phase-shift keying (8PSK), respectively. The proposed method achieves nearly the same mutual information as the two-step iterative algorithm for a large range of signal-to-noise ratio (SNR) region, especially for large MIMO size and/or high constellation systems. The proposed precoding design method is applied to a 2 × 2 MIMO-OFDM system with 2048 subcarriers by designing 1024 precoders for extended channel matrices of size 4 × 4. A transceiver test bed implements these precoding matrices in comparison with other existing precoding schemes. Indoor experiments are conducted for fixed-platform non-line-of-sight channels, and the data processing results show that the proposed precoding method achieves the lowest bit error rate compared with maximum diversity, classic water-filling and channel diagonalisation methods.

On Optimal TCM Encoders

An asymptotically optimal trellis-coded modulation (TCM) encoder requires the joint design of the encoder and the binary labeling of the constellation. Since analytical approaches are unknown, the only available solution is to perform an exhaustive search over the encoder and the labeling. For large constellation sizes and/or many encoder states, however, an exhaustive search is unfeasible. Traditional TCM designs overcome this problem by using a labeling that follows the set-partitioning principle and by performing an exhaustive search over the encoders. In this paper we study binary labelings for TCM and show how they can be grouped into classes, which considerably reduces the search space in a joint design. For 8-ary constellations, the number of different binary labelings that must be tested is reduced from 8!=40320 to 240. For the particular case of an 8-ary pulse amplitude modulation constellation, this number is further reduced to 120 and for 8-ary phase shift keying to only 30. An algorithm to generate one labeling in each class is also introduced. Asymptotically optimal TCM encoders are tabulated which are up to 0.3 dB better than the previously best known encoders.