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

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

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

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

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

Small satellite operations planning for agile disaster response using graph theoretical techniques

Agile, manoeuvrable, satellite constellations have the potential to fundamentally change space mission design by moving away from traditional missions, designed to address predicted demand, and instead providing responsive systems that can react to real-time events, such as natural disasters. The unique advantages of responsive constellations are enhanced by the use of small satellites, whose short development times and low cost can offset the increased risk and shorter mission life inherent in the use of manoeuvrable spacecraft. In addition, newly developed, highly efficient propulsion systems can provide small satellites with agile manoeuvrability. This could enable agile satellite systems where efficient, low-thrust, responsive manoeuvres can be used to ensure rapid flyover of targets on Earth. The authors have previously developed a fully analytical method of designing such manoeuvres, which allows consideration of multiple targeting options, each with different flyover times, view angles, and propellant requirements. However, a long-term, holistic understanding of the concept of operations is required to effectively implement an agile satellite system. To facilitate this, the existing analytical methodology has been combined with graph theoretical techniques to allow the complex trade-space to be perceived as a graph. The connections in the graph represent possible manoeuvres and are rapidly traversed to identify favourable routes to achieve the desired goal. The effect of changes in mission priorities can be assessed by reweighting the graph, avoiding the need to recalculate the manoeuvre options. This work demonstrates that the proposed method can be successfully used to plan sequential flyovers of a moving target; in this case, a tropical storm. For the small spacecraft and low-thrust propulsion system considered, the possible changes in flyover time for each target are small, however, these small adjustments can be used to significantly improve the quality of the obtained data compared to a non-manoeuvring spacecraft

Satellite coverage assessment considering cloud cover

Due to cloud cover, the performance provided by optical or lidar satellite systems may differ from the predicted performance. This work presents a means of rapidly assessing the coverage that can be expected from a satellite constellation considering cloud probability. A latitude-specific pseudo-circumference is defined and calculated using a cloud factor to provide insights into the relative coverage available to different regions. Comparison of the results from the presented method with simulated and historical data collected by the Sentinel-2 constellation shows the potential of the proposed method to calculate expected time between cloud free images when averaged over the long-term. Appropriate choice of cloud model is found to be an important consideration to ensure the validity of the method

Tracking shifting estuaries with remote sensing techniques to aid lifeboat rescue services

In certain areas of the world with large tidal ranges, rain and wind can cause the path of channels and estuaries to shift dramatically. Lifeboat rescue services must navigate through these areas in spite of the dangers it can entail. This work investigates the alternatives to provide regular mapping of the seabed, enabling these teams to reach casualties without becoming casualties themselves. Underwater Vehicles, SONAR systems and Unmanned Aerial Vehicles are all considered viable options but satellites equipped with synthetic aperture radar are proven to be the most advantageous. To map the path of the estuary, images must be taken during low tide, therefore, data availability is assessed by studying the revisit time and matching this to the tidal status of the area of interest. Different satellite options are examined, including commercial and non-commercial but a specific focus is given to Sentinel-1 due to its free accessibility. The periodicity of the satellite coupled with the tidal behaviour causes intervals during the month where no usable images can be taken. The maximum number of days between consecutive useful images is found to be 12, with an average of 6 useful images per month. The periods where these intervals happen are also identified. Therefore, to meet the user needs, an auxiliary system must be implemented to assist the satellite and increase the number of useful images taken per month. The area of interest of this study is the Solway Firth due to its fast tidal movements and ranges