Arguments for exception in US security discourse

In his influential State of Exception, Giorgio Agamben proposes that, even in apparently liberal western democracies, the state will routinely use the contingency of national emergency to suspend civil liberties and justify expansion of military and police powers. We investigated rhetorical strategies deployed in the web pages of US security agencies, created or reformed in the aftermath of the 9/11 events, to determine whether they present argumentation conforming to Agamben’s model. To expose rhetorical content, we examined strategies operating at two levels within our corpus. Argument schemes and underlying warrants were identified through close examination of systematically selected core documents. Semantic fields establishing themes of threat and danger were also explored, using automatic corpus tools to expose patterns of lexical selection established across the whole corpus. The study recovered evidence of rhetoric broadly consistent with the logic predicted by State of Exception theory, but also presented nuanced findings whose interpretation required careful re-appraisal of core ideas within Agamben’s work

General perturbation method for satellite constellation deployment using nodal precession

The dawn of "New Space" in recent years is changing the landscape of the space industry. In particular, the shift to smaller satellites, requiring shorter development time s and using off-the-shelf-components and standardized buses, has led to a continuing reduction in spacecraft cost. However, launch costs remain extremely high and frequently dominate the total mission cost. Additionally, many small satellites are designed to operate as part of a larger constellation, but traditional launch methods require a difference dedicated launch for each orbit plane to be populated. This need for multiple costly launches can stifle, and even prohibit, some missions requiring numerous orbit planes as the launch cost increases beyond what can be justified for the mission. As of 2014, most smallsats, including CubeSats, have been launched on opportunistic ‘rideshare’ or ‘piggy-back’ launches, in which the spacecraft shares its launch with other craft, often as a secondary payload. This has the advantage of providing a cheaper launch but restricts the operator’s choice of orbit, which will affect the system performance

General perturbation method for satellite constellation reconfiguration using low-thrust maneuvers

A general perturbation solution to a restricted low-thrust Lambert rendezvous problem, considering circular-to-circular in-plane maneuvers using tangential thrust and including a coast arc, is developed. This provides a fully analytical solution to the satellite reconnaissance problem. The solution requires no iteration. Its speed and simplicity allow problems involving numerous spacecraft and maneuvers to be studied; this is demonstrated through two case studies. In the first, a range of maneuvers providing a rapid flyover of Los Angeles is generated, giving an insight to the trade space and allowing the maneuver that best fulfills the mission to be selected. A reduction in flyover time from 13.8 to 1.6 days is possible using a less than 17 m∕s velocity change. A comparison with a numerical propagator including atmospheric friction and an 18th-order tesseral model shows 4 s of difference in the time of flyover. A second study considers a constellation of 24 satellites that can maneuver into repeating ground track orbits to provide persistent coverage of a region. A set of maneuvers for all satellites is generated for four sequential targets, allowing the most suitable maneuver strategy to be selected. Improvements in coverage of greater than 10 times are possible as compared to a static constellation using 35% of the propellant available across the constellation

Analysis of responsive satellite manoeuvres using graph theoretical techniques

Manoeuvrable, responsive satellite constellations that can respond to real-time events could provide critical data on-demand to support, for example, disaster monitoring and relief efforts. The authors demonstrate the feasibil-ity of such a system by expanding on a fully-analytical method for designing responsive spacecraft manoeuvres using low-thrust propulsion. This method enables responsive manoeuvre planning to provide coverage of targets on the Earth, with each manoeuvre option having a different target look angle, and requiring a different manoeuvre time and propellant cost. The trade-space for this analysis rapidly expands when considering multiple space-craft, targets and manoeuvres. To explore the trade-space efficiently, it is perceived as a graph in which connections are rapidly traversed to identify favourable routes to achieve the mission goals. The case study presented considers four satellites required to provide flyovers of two targets, with an associated graph of possible manoeuvres comprising 10726 nodes. The min-imum time solution is 2.59 days to complete both flyovers with 7.037 m/s change in velocity. Investigation of the graph highlights that selecting a good but not minimum time solution can allow the system to perform well but also have alternate options available to deal with possible errors in the manoeuvre execution, or changes in mission priorities. Restricting the prob-lem to consider only two satellites, with a smaller swath and less available propellant, reduces the graph to 510 nodes. In this case, the minimum time solution requires 9.04 m/s velocity change and takes approximately 2.59 days. The analysis also provides non-intuitive solutions, for example, that it is faster for one satellite to perform two targeting manoeuvres than for two satellites to manoeuvre simultaneously

Dynamical influence driven space system design

Complex networks are emerging in low-Earth-orbit, with many thousands of satellites set for launch over the coming decade. These data transfer networks vary based on spacecraft interactions with targets, ground stations, and other spacecraft. While constellations of a few, large, and precisely deployed satellites often produce simple, grid-like, networks. New small-satellite constellations are being deployed on an ad-hoc basis into various orbits, resulting in complex network topologies. By modelling these space systems as flow networks, the dominant eigenvectors of the adjacency matrix identify influential communities of ground stations. This approach provides space system designers with much needed insight into how differing station locations can better achieve alternative mission priorities and how inter-satellite links are set to impact upon constellation design. Maximum flow and consensus-based optimisation methods are used to define system architectures that support the findings of eigenvector-based community detection

Identifying effective sink node combinations in spacecraft data transfer networks

Complex networks are emerging in low-Earth-orbit as the communication architectures of inter-linked space systems. These data transfer networks vary based on spacecraft interaction with targets and ground stations, which respectively represent source and sink nodes for data flowing through the network. We demonstrate how networks can be used to identify effective sink node selections that, in combination, provide source coverage, high data throughput, and low latency connections for intermittently connected, store-and-forward space systems. The challenge in this work is to account for the changing data transfer network that varies significantly depending on the ground stations selected -- given a system where data is downlinked by spacecraft at the first opportunity. Therefore, passed-on networks are created to capture the redistribution of data following a sink node's removal from the system, a problem of relevance to traffic management in a variety of flow network applications. Modelling the system using consensus dynamics, enables sink node selections to be evaluated in terms of their source coverage and data throughput. While restrictions in the depth of propagation when defining passed-on networks, ensures the optimisation implicitly rewards lower latency connections. This is a beneficial by-product for both space system design and store-and-forward data networks in general. The passed-on networks also provide an insight into the relationship between sink nodes, with eigenvector embedding-based communities identifying sink node divisions that correspond with differences in source node coverage

Mapping of shifting tidal estuaries to support inshore rescue

Across the world, many coastal tidal regions are unsafe to navigate due to shifting mud and sand pushed by water currents. Ability to regularly map the current location of a channel will aid safe passage for commercial, leisure and rescue craft. This work investigates the use of synthetic aperture radar data derived from satellites to provide accurate mapping of moving channels in coastal regions. As images must be collected at low tide, data availability is assessed considering the relationship between the orbital motion of the satellites and the tides. Change detection methods are applied to suitable images to map changes in the location of navigable channels. Pixels that undergo similar changes over time (e.g. from water covered to exposed sand) are grouped together by examining the principal component of the covariance matrix, for a vector composed of pixel values from the same location at different times. The Solway Firth in Great Britain is selected as a trial site as it is exposed to some of Europe's fastest tidal movements and ranges, and hence is one of Great Britain's most treacherous stretches of coastline

Towards a satellite system for archaeology? Simulation of an optical satellite mission with ideal spatial and temporal resolution, illustrated by a case study in Scotland

Applications of remote sensing data for archaeology rely heavily on repurposed data, which carry inherent limitations in their suitability to help address archaeological questions. Through a case study framed around archaeological imperatives in a Scottish context, this work investigates the potential for existing satellite systems to provide remote sensing data that meet defined specifications for archaeological prospection, considering both spatial and temporal resolution, concluding that the availability of commercial data is currently insufficient. Tasking a commercial constellation of 12 spacecraft to collect images of a 150 km 2 region in Scotland through the month of July 2020 provided 26 images with less than 50% cloud cover. Following an analysis of existing systems, this paper presents a high-level mission architecture for a bespoke satellite system designed from an archaeological specification. This study focuses on orbit design and the number of spacecraft needed to meet the spatial and temporal resolution requirements for archaeological site detection and monitoring in a case study of Scotland, using existing imaging technology. By exploring what an ideal scenario might look like from a satellite mission planning perspective, this paper presents a simulation analysis that foregrounds archaeological imperatives and specifies a satellite constellation design on that basis. High-level design suggests that a system of eight 100 kg spacecraft in a 581 km altitude orbit could provide coverage at a desired temporal and spatial resolution of two-weekly revisit and <1 m ground sampling distance, respectively. The potential for such a system to be more widely applied in regions of similar latitude and climate is discussed

Extension of the sun-synchronous Orbit

Through careful consideration of the orbit perturbation force due to the oblate nature of the primary body a secular variation of the ascending node angle of a near-polar orbit can be induced without expulsion of propellant. Resultantly, the orbit perturbations can be used to maintain the orbit plane in, for example, a near-perpendicular (or at any other angle) alignment to the Sun-line throughout the full year of the primary body; such orbits are normally termed Sun-synchronous orbits [1, 2]. Sun-synchronous orbits about the Earth are typically near-circular Low-Earth Orbits (LEOs), with an altitude of less than 1500 km. It is normal to design a LEO such that the orbit period is synchronised with the rotation of the Earth‟s surface over a given period, such that a repeating ground-track is established. A repeating ground-track, together with the near-constant illumination conditions of the ground-track when observed from a Sun-synchronous orbit, enables repeat observations of a target over an extended period under similar illumination conditions [1, 2]. For this reason, Sun-synchronous orbits are extensively used by Earth Observation (EO) platforms, including currently the Environmental Satellite (ENVISAT), the second European Remote Sensing satellite (ERS-2) and many more. By definition, a given Sun-synchronous orbit is a finite resource similar to a geostationary orbit. A typical characterising parameter of a Sun-synchronous orbit is the Mean Local Solar Time (MLST) at descending node, with a value of 1030 hours typical. Note that ERS-1 and ERS-2 used a MLST at descending node of 1030 hours ± 5 minutes, while ENVISAT uses a 1000 hours ± 5 minutes MLST at descending node [3]. Following selection of the MLST at descending node and for a given desired repeat ground-track, the orbit period and hence the semi-major axis are fixed, thereafter assuming a circular orbit is desired it is found that only a single orbit inclination will enable a Sun-synchronous orbit [2]. As such, only a few spacecraft can populate a given repeat ground-track Sun-synchronous orbit without compromise, for example on the MLST at descending node. Indeed a notable feature of on-going studies by the ENVISAT Post launch Support Office is the desire to ensure sufficient propellant remains at end-of-mission for re-orbiting to a graveyard orbit to ensure the orbital slot is available for future missions [4]. An extension to the Sun-synchronous orbit is considered using an undefined, non-orientation constrained, low-thrust propulsion system. Initially the low-thrust propulsion system will be considered for the free selection of orbit inclination and altitude while maintaining the Sun-synchronous condition. Subsequently the maintenance of a given Sun-synchronous repeat-ground track will be considered, using the low-thrust propulsion system to enable the free selection of orbit altitude. An analytical expression will be developed to describe these extensions prior to then validating the analytical expressions within a numerical simulation of a spacecraft orbit. Finally, an analysis will be presented on transfer and injection trajectories to these orbits