CERES :: Browsing by Author "Hobbs, Stephen"

Browsing by Author "Hobbs, Stephen"

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Open Access
Collision analysis for multiple satellites released from a common dispenser
(ESA Conference Bureau / ATPI Corporate Events, 2023-06-16) D'Anniballe, Antonio; Felicetti, Leonard; Hobbs, Stephen
The number of small spacecraft launched to space has increased dramatically in the past few years, and with the emergence of mega-constellations it is projected to increase even more in the coming decades. Small satellites are usually launched together in rideshare launches and released from a common dispenser when reaching nominal orbit. Due to the lack of available measurement and control capabilities, the release phase is vulnerable to collision risk, as small uncertainties in the initial position can quickly grow causing a high probability of collision. In this paper a framework for analysing the safety of a genric dispenser is proposed and applied to the study of a cylindrical dispenser. Through numerical simulations and linear covariance propagation, the evolution of the spacecraft state is retrieved and used for computing a set of performance metrics, such as the total probability of collision and the number of conjunction events. This method is then applied to a parametric analysis of the dispenser, examining how the performance metrics vary with parameters such as the velocity of release or the time between releases. The results thus obtained will be relevant to the safe design of spacecraft dispensers.
Open Access
CUSTARD (Cranfield University Space Technology Advanced Research Demonstrator) - A Micro-System Technology Demonstrator Nanosatellite. Summary of the Group Design Project MSc in Astronautics and Space Engineering. 1999-2000, Cranfield University
(2003-09-19T00:00:00Z) Hobbs, Stephen; Turner, Ray
CUSTARD (Cranfield University Space Technology And Research Demonstrator) was the group design project for students of the MSc in Astronautics and Space Engineering for the Academic Year 1999/2000 at Cranfield University. The project involved the initial design of a nanosatellite to be used as a technology demonstrator for microsystem technology (MST) in space. The students worked together as one group (organised into several subgroups, e.g. system, mechanical), with each student responsible for a set of work packages. The nanosatellite designed had a mass of 4 kg, lifetime of 3 months in low Earth orbit, coarse 3-axis attitude control (no orbit control), and was capable of carrying up to 1 kg of payload. The electrical power available was 18 W (peak). Assuming a single X-band ground station at RAL (UK), a data rate of up to 1 M bit s-1 for about 3000 s per day is possible. The payloads proposed are a microgravity laboratory and a formation flying experiment. The report summarises the results of the project and includes executive summaries from all team members. Further information and summaries of the full reports are available from the College of Aeronautics, Cranfield University.
Open Access
Debris removal from low earth orbit (DR LEO)
(Cranfield University, 2010-09-13) Hobbs, Stephen
Students of the MSc course in Astronautics and Space Engineering 2009/10 at Cranfield University studied a low Earth orbit (LEO) debris removal mission for their group project. The mission's name was DR LEO (Debris Removal from LEO) and its aim was to develop a credible mission baseline using conventional technology to perform active debris removal from LEO. This report summarises the students' work and their findings. The report consists of an overview and discussion of the technical work of the project and a compilation of the executive summaries which describe the specific contributions of each student. The baseline mission design developed is broadly credible and provides a useful benchmark against which other missions can be compared to evaluate the potential of alternative technologies. The mission costing from this first design iteration sets a benchmark cost per unit mass de-orbited of approximately € 20-30k kg - 1 using conventional chemical propulsion.
Open Access
Development of commercial drag-augmentation systems for small satellites
(ESA Space Debris Office, 2017-06-30) Palla, Chiara; Kingston, Jenny; Hobbs, Stephen
In the framework of the ESA CleanSat programme Cranfield University is developing a family of drag augmentation system (DAS) modules to enable small satellites in Low Earth Orbit (LEO) to comply with space debris mitigation requirements. There are currently two mature Cranfield DAS designs based on deployable Kapton sails using stored energy for deployment. One concept is Icarus and it is currently on-board the UK’s TechDemoSat-1 (launched 8 July 2014) and Carbonite-1 spacecraft (launched 10 July 2015). The second concept is the de-orbit mechanism (DOM) module, which is due to fly as technological demonstrator on the upcoming ESA ESEO mission. The key drivers used during the design process were: low cost, low mass, easy testability, safety, reliability, and avoidance of additional debris production. These drivers matched with top-level requirements, from a potential customers perspective (e.g.: satellite integrators), which were defined during the CleanSat study. Other relevant requirements for the DAS included demisability, performance (in terms of orbital decay), area-to-mass ratio, functionality, lifetime, and environment compatibility. This paper discusses the compliance of the Cranfield DAS designs with the identified requirements, and illustrates the scalability via application to several case study missions (500 kg and 200 kg LEO satellites). The two most challenging aspects to assess were compliance with the lifetime required for storage on ground and pre-deployment on orbit, and the effect of the orbital environment (radiation, ATOX, debris) on the sail. The study has provided useful input to explore new concepts based on the heritage designs; these concepts are evolutions of the DOM unit and hybrid designs. The hybrid design combines aspects of the Icarus and the DOM concepts to reduce the limitations of the respective individual devices and improve scalability, adaptability and manufacturability. In addition, this work is helping to achieve commercial readiness for the technology. This will enable development of a commercial DAS offering that will be an attractive solution for small satellite integrators, allowing them to meet debris mitigation requirements.
Open Access
Disposal orbits for GEO spacecraft: A method for evaluating the orbit height distributions resulting from implementing IADC guidelines
(Elsevier Science B.V., Amsterdam., 2010-04-01T00:00:00Z) Hobbs, Stephen
Geostationary orbit (GEO) is the most commercially valuable Earth orbit. The Inter-Agency Space Debris Coordination Committee (IADC) has produced guidelines to help protect this region from space debris. The guidelines propose moving a satellite at the end of its operational life to a disposal orbit, which is designed so that satellites left there will not infringe the operational GEO region within a period of at least 100 yr. Standards are being developed through the International Organisation for Standardization to translate the IADC guidelines into engineering practice. This article presents an analytical method for calculating the distribution of final orbits assuming the IADC guidelines in GEO are implemented, as a function of distributions of satellite parameters (mass per unit area, solar radiation pressure reaction coefficient), the fuel measurement uncertainty, and the desired reliability of the disposal manoeuvre. Results show that typically the fuel measurement uncertainty dominates the distribution of perigee heights rather than the scatter in satellite properties or desired manoeuvre reliability. The method is simple to implement and allows the effects of changes in system parameters to be evaluated quickly.
Open Access
Drag augmentation systems for space debris mitigation
(Elsevier, 2021-06-01) Serfontein, Zaria; Kingston, Jennifer; Hobbs, Stephen; Holbrough, Ian E.; Beck, James C.
Space debris is a critical threat to future and on-going missions. The commercialisation of the space sector has led to a rapid growth in the number of small satellites in recent years, which are adding to the already high number of objects currently in low-Earth orbit (LEO). Low-cost small satellites operators are under increasing pressure to comply with debris mitigation guidelines as part of the application process for a launch licence. Drag augmentation systems are a potential low-cost and low-impact solution for small satellites. By increasing the effective area of a satellite, and therefore its drag, these sails reduce the de-orbit period of a satellite, subsequently reducing the probability of significant collisions and supporting the sustainable use of space. Cranfield University are developing a family of drag augmentation systems (DAS) to assist in the long-term conservation of the space environment. The DAS are lightweight, cost-effective, reliable sails deployed at end of mission. Currently three of the drag sails designed, manufactured, and tested at Cranfield University are in orbit and two of the devices have successfully deployed their sails. This paper will discuss these sails and will highlight results from recent studies; examining the scalability of the system, the vehicle dynamics after sail deployment, the medium-term impact of the sail on the host satellite's ability to continue operations, and the long-term effect of the sail on the demisability of the satellite. The DAS technology has a strong enabling potential for future space activities, allowing satellites to operate responsibly and sustainably.
Open Access
Drag augmentation systems for space debris mitigation
(International Astronautical Federation, 2020-10) Serfontein, Zaria; Kingston, Jennifer; Hobbs, Stephen; Holbrough, Ian E.; Beck, James C.
Space debris is recognised as a critical threat for the space industry. The proliferation of small satellites has invited commercialisation and subsequently, the growing number of satellites are adding to the already high number of objects currently in low-Earth orbit (LEO). Low-cost small satellites are under increasing pressure to meet debris mitigation guidelines and failure to comply could result in a launch licence being denied. Drag augmentation systems increase the drag area of a spacecraft, minimising the de-orbit period and thus reducing the probability of significant collisions and supporting the sustainable use of space. In response to the growing number of small satellites (10-500 kg) unable to de-orbit from low-Earth orbit within 25 years, Cranfield University has developed a family of drag augmentation systems (DAS). The DAS are lightweight, cost-effective sails deployed at end of mission and are reliable solutions for deorbiting small satellites, assisting in the conservation of the space environment. Three drag sails designed, manufactured and tested at Cranfield University are currently in orbit, with two sails already successfully deployed. This paper details the sails and will discuss findings from recent studies; examining the system’s scalability, the post-deployment vehicle dynamics, the medium-term impact of the sail on the satellite’s ability to conduct science and the long-term effect of the sail on the satellite’s re-entry and demise. The DAS technology have a strong enabling potential for future space activities, allowing satellites to operate responsibly and sustainably.
Open Access
Dynamics and control issues for future multistatic spaceborne radars
(2004-12-18T00:00:00Z) Hobbs, Stephen
Concepts for future spaceborne radar systems are being developed which rely on the transmitter and receiver(s) being carried on separate spacecraft. The potential advantages include lower cost than current spaceborne radars and improved measurement capability. This paper reviews two currently proposed systems: GNSS reflectometry (GNSS-R) and a geosynchronous synthetic aperture radar constellation (GeoSAR). GNSS-R uses reflections of signals from GPS (and Galileo when available) to measure the height and state of the ocean surface. The receiver is typically in a low Earth obit (LEO) and provides global coverage. GeoSAR uses a radar receiver in geosynchronous orbit (slightly displaced from geostationary but still with a period of 1 day). The radar sees a fixed region of the Earth and is able to integrate signals over long periods to obtain a satisfactory signal-to-noise ratio. If several receiver spacecraft are used simultaneously the time to obtain an image can be reduced in proportion to the number of spacecraft used. The principles of these two systems are described and then requirements applying to the system dynamics and control are derived. For GNSS-R the requirements are relatively easy to achieve (coarse pointing and only basic orbit control). GeoSAR’s requirements are more demanding although the environmental disturbances at geosynchronous orbit height are significantly smaller than in LEO. For GeoSAR the most demanding requirement is the need for centimetre-level orbit measurements to allow aperture synthesis to be implemente
Open Access
Effects of long-term exposure to the low-earth orbit environment on drag augmentation systems
(IAF, 2020-10-14) Serfontein, Zaria; Kingston, Jennifer; Hobbs, Stephen; Holbrough, Ian E.; Beck, James C.; Impey, Susan A.; Aria, Adrianus Indrat
Spacecraft in low-Earth orbit are exposed to environmental threats which can lead to material degradation and component failures. The presence of atomic oxygen and collisions from orbital debris have detrimental effects on the structures, thus affecting their performance. Cranfield University has developed a family of drag augmentation systems (DAS), for end-of-life de-orbit of satellites, addressing the space debris challenge and ensuring that satellites operate responsibly and sustainably. Deorbit devices are stowed on-orbit for the duration of the mission lifetime and, once deployed, the devices must withstand this harsh low-Earth environment until re-entry; a process which can take several years. The DAS’ deployable aluminised Kapton sails are particularly susceptible to undercutting by atomic oxygen. In preparation for commercialising the DAS, Cranfield University and Belstead Research Ltd. have submitted several joint proposals to better understand the degradation process of the drag sail materials and to qualify the materials for the specific application of drag sails in low Earth Orbit (LEO). This paper will outline the proposals and the expected benefits from the projects. Additionally, collisions with debris could accelerate the degradation of the system and generate additional debris. This paper will discuss a future ESABASE2 risk assessment study, aiming to quantifying the probability of collisions between the deployed drag sail and orbital debris. The atmospheric models required to simulate the aforementioned risks are complex and often fail to accurately predict performance or degradation observed in the space environment. A previous UKSA Pathfinder project highlighted this issue when different atmospheric models with varying levels of solar activity yielded drastically different re-entry times. Since Cranfield University has two deployed drag sails in orbit, previous de-orbit analysis performed using STELA and DRAMA will be updated and the simulations will be compared to actual data. This paper will conclude in a summation of the different on-going research projects at Cranfield University related to commercialising the DAS family. This research will benefit the wider space community by expanding the understanding of the effects of long-term exposure on certain materials, as well as improving the validity of future low Earth atmospheric models.
Open Access
Effects of long-term exposure to the low-earth orbit environment on drag augmentation systems
(Elsevier, 2021-06-10) Serfontein, Zaria; Kingston, Jennifer; Hobbs, Stephen; Impey, Susan A.; Aria, Adrianus Indrat; Holbrough, Ian E.; Beck, James C.
Spacecraft in low-Earth orbit are exposed to environmental threats which can lead to material degradation and component failures. The presence of atomic oxygen and collisions from orbital debris have detrimental effects on the structures, thus affecting their performance. Cranfield University has developed a family of drag augmentation systems (DAS), for end-of-life de-orbit of satellites, addressing the space debris challenge and ensuring that satellites operate responsibly and sustainably. De-orbit devices are stowed on-orbit for the duration of the mission lifetime and, once deployed, the devices must withstand the harsh low Earth environment until re-entry; a process which can take several years. The DAS’ deployable aluminised Kapton sails are particularly susceptible to undercutting by atomic oxygen. In preparation for commercialising the DAS, Cranfield University are investigating the degradation process of the drag sail materials, with the end goal of qualifying the materials for the specific application of drag sails in low Earth orbit (LEO). This paper will outline the proposed research and the expected benefits from the projects. This paper will conclude in a summation of the different on-going research projects at Cranfield University related to commercialising the DAS family. This research will benefit the wider space community by expanding the understanding of the effects of long-term exposure on certain materials, as well as improving the validity of future low Earth atmospheric models.
Open Access
Feasibility of passive bistatic geosynchronous radar using comsat transmissions
(IEEE, 2018-11-05) Hobbs, Stephen; Convenevole, Carlo; Gashinova, M.; Cherniakov, M.; Cassidy, S.
Communication satellites in geosynchronous orbit are increasingly broadcasting digital signals with high bandwidth and high power. These signals are in principle well-suited to radar imaging and the study presented here is an initial feasibility study for a passive bistatic synthetic aperture radar using satellites in geosynchronous orbit (GEO). The persistent viewing possible from GEO could enable important new applications. The mission concept is outlined and studies of the available signal formats identify digital TV broadcasts in Ku-band as most suitable for radar imaging. The additional space hardware required is a dedicated receive channel, which could be implemented as a hosted payload at modest cost. Our findings so far suggest that the mission concept is feasible for coarse spatial resolution images and that it could therefore provide a low-cost technology demonstration of geosynchronous radar.
Open Access
GeoSAR Feasibility Study: Summary of the Group Design Project. MSc in Astronautics and Space Engineering 2012/13
(Cranfield University, 2013-06-28) Hobbs, Stephen
Students of the MSc course in Astronautics and Space Engineering 2012–13 at Cranfield University performed a feasibility study of a geosynchronous radar mission for their group project. This report summarises the students’ work and their findings. The report consists of an overview and discussion of the technical work of the project and a compilation of the executive summaries which describe the specific contributions of each student. The mission studied is a Ku-band monostatic SAR in a small inclination (80 km relative orbit diameter) geosynchronous orbit. The total launch mass is 2 tonnes, and images with resolution 40 m to monitor land subsidence in urban areas are produced. The mission appears to be feasible within the scope of an ESA Earth Explorer proposal.
Open Access
GeoSAR: Summary of the Group Design Project MSc in Astronautics and Space Engineering 2005/06 Cranfield University
(2006-08-14T00:00:00Z) Hobbs, Stephen
Students of the MSc course in Astronautics and Space Engineering 2005/06 at Cranfield University took GeoSAR as one of their group projects. This report summarises their findings. GeoSAR is an initial feasibility study for a satellite carrying a passive bistatic radar receiver based in geosynchronous orbit. The feasibility of the radar concept has already been established (e.g. through the work of Prati et al. of Politecnico di Milano) but no designs have yet been published for a satellite to support the mission. This project develops an outline design of the spacecraft and confirms its feasibility within a (conservative) mass budget of approximately 300 kg. Mission drivers are the radar antenna diameter and the station-keeping propulsion required for a design life of 15 years. Technologies such as inflatable structures and (field- emission) electric propulsion are used to reduce the spacecraft's mass. An outline cost estimate for the mission suggests that a GeoSAR mission would be significantly cheaper than conventional low-Earth orbit radar satellites to achieve similar capability in terms of rapid-response imaging and interferometry. In several areas the GeoSAR design is conservative and it is plausible that on further iterations of the design the cost and mass can be reduced. This suggests that among options for future Earth observation missions, GeoSAR deserves serious consideration.
Open Access
Geostare system performance assessment methodology
(IEEE, 2016-11-03) Hobbs, Stephen; Convenevole, Carlo; Guarnieri, A. M.; Wadge, G.
Geosynchronous synthetic aperture radar (GEO SAR) is attracting growing interest due to its potential for flexible and frequent imaging over continental areas. Studies are underway to evaluate mission design options and to investigate enabling technologies. This article outlines a method for assessing mission performance which accounts for actual land-cover distributions and weather statistics to derive statistical estimates of mission performance relative to user-defined requirements. A technical challenge is to account accurately for the effects of surface “clutter”: several methods for this are available and will be evaluated. The methodology is outlined. The performance estimates will be used to refine and validate mission design options and to build evidence for the expected benefits of GEO SAR
Open Access
HySim: a tool for space-to-space hyperspectral resolved imagery
(International Astronautical Federation (IAF), 2023-10-06) Felicatti, Leonard; Hobbs, Stephen; Leslie, Cameron; Rowling, Samuel; Dhesi, Mekhi; Harris, Toby; Brydon, George; Chermak, Lounis; Soori, Umair; Allworth, James; Balson, David
This paper introduces HySim, a novel tool addressing the need for hyperspectral space-to-space imaging simulations, vital for in-orbit spacecraft inspection missions. This tool fills the gap by enabling the generation of hyperspectral space-to-space images across various scenarios, including fly-bys, inspections, rendezvous, and proximity operations. HySim combines open-source tools to handle complex scenarios, providing versatile configuration options for imaging scenarios, camera specifications, and material properties. It accurately simulates hyperspectral images of the target scene. This paper outlines HySim's features, validation against real space-borne images, and discusses its potential applications in space missions, emphasising its role in advancing space-to-space inspection and in-orbit servicing planning.
Open Access
Improving 2D resolution in geosynchronous SAR via spatial spectrum synthesis: method and verification
(IEEE, 2024-02-01) Chen, Xinyan; Chen, Zhiyang; Li, Yuanhao; Hu, Cheng; Dong, Xichao; Hobbs, Stephen
Geosynchronous synthetic aperture radar (GEO SAR) has the advantages of a short revisit time and a large beam footprint. Enhancing spatial resolution has become a hotspot in spaceborne SAR research areas. The existing designs of GEO SAR suffer from the problem of poor resolution (greater than 10 m) and cause low-precision observation in urban areas. First time, this article points out that GEO SAR has the ability to improve the two-dimensional (2-D) resolution of SAR images by spatial spectrum synthesis based on 2-D baselines, and proposes models and a signal processing method involved in GEO SAR spectrum synthesis. The 2-D spatial baselines of GEO SAR are analyzed and evaluated first based on satellite software and real ephemeris from Beidou Inclined Geosynchronous Orbit (IGSO) navigation satellites to demonstrate the potential of 2-D resolution improvement. Then the analytical models of the spectral shape and relative spectral shift between GEO SAR images are derived. Furthermore, a 2-D spectrum synthesis algorithm suitable for GEO SAR is also proposed, where we use deramping operation instead of the traditional spectrum shifting process. Nonideal factors are considered in the processing of synthesized. In addition, performances of the proposed algorithm, including the resolution improvement factor, the amplitude fluctuation, and the critical baseline, are constructed. Finally, computer simulations and equivalent experiments based on Beidou IGSO navigation satellites verify the proposed algorithm.
Open Access
Insect ventral radar cross-section polarisation dependence measurements for radar entomology
(The Institution of Electrical Engineers, 2006-12-31T00:00:00Z) Hobbs, Stephen; Aldhous, Anthony C.
Radar entomology has developed such that routine long-term monitoring of insect flight through the atmospheric boundary layer is now practical. Typical entomological radars use X-band (9.4 GHz) marine transceivers with a vertical pencil beam and rotate the plane of polarisation about the beam axis. Ideally, insect species and other parameters (mass etc.) should be estimated from the measured radar cross-section variation with polarisation angle. For this, a library of known insect cross-section polarisation signatures is required. Two models are currently used to parameterise the polarisation signature: the harmonic model and a model using the scattering matrix for symmetric targets (SM3). Data from the literature and a doctoral research project are presented and analysed to obtain parameters for both the harmonic and SM3 models. Knowledge of the measurement errors allows SM3 parameter uncertainties to be quantified in most cases using a maximum likelihood approach. Results for 68 insects representing 24 species are presented. These include several economically significant species (e.g. bees and locusts), with individual insect masses ranging from 9 mg to 3 g
Open Access
Laplace plane and low inclination geosynchronous radar mission design
(Springer, 2017-05-17) Hobbs, Stephen; Sanchez, Joan-Pau
This study is inspired by the Laplace orbit plane property of requiring minimal station-keeping and therefore its potential use for long-term geosynchronous synthetic aperture radar (GEOSAR) imaging. A set of GEOSAR user requirements is presented and analysed to identify significant mission requirements. Imaging geometry and power demand are assessed as a function of relative satellite speed (which is determined largely by choice of orbit inclination). Estimates of the cost of station-keeping as a function of orbit inclination and right ascension are presented to compare the benefits of different orbit choices. The conclusion is that the Laplace plane (and more generally, orbits with inclinations up to 15°) are attractive choices for GEOSAR.
Open Access
Laplace plane GeoSAR feasibility study: summary of the group design project MSc in astronautics and space engineering 2014-15, Cranfield University
(Cranfield University, 2015-10-07) Hobbs, Stephen
Students of the MSc course in Astronautics and Space Engineering 2014-15 at Cranfield University performed a feasibility study of a geosynchronous radar mission for their group project. This report summarises the students' work and their findings. The report consists of an overview and discussion of the technical work of the project and a compilation of the executive summaries which describe the special contributions of each student. The mission studied is a geosynchronous synthetic aperture radar Earth observation mission using the Laplace orbit plane to reduce station-keeping propulsion demand. User applications are drawn from a wide range of sectors (agriculture, meteorology, geohazards, etc.) and are translated into system design requirements. The proposed mission design uses satellites with 13 m diameter antennas and a total electrical power demand of 6 kW. The mission seems feasible, although further study is recommended especially for the areas of _ orbit selection with respect to user requirements, imaging performance and orbit maintenance, _ mass budget (driven largely by the propulsion system), _ user requirements, imaging performance and operational imaging modes, _ opportunities for improved imaging with a constellation.
Open Access
Linear mixture modelling solution methods for satellite remote sensing
(2003-11-04T00:00:00Z) Hobbs, Stephen
This report documents the algorithms used in the program MIXMOD to analyse mixed pixel data (assuming linear mixing). The report describes the mathematical algorithms rather than acting as a user’s manual for MIXMOD. The algorithms described obtain the desired solutions, quantify the quality of the solution, and estimate error bounds, using a variety of methods. A novel feature is the ability to handle uncertainty in the assumed end-member spectra, which in practice may be the dominant source of error. The report includes a brief literature review to place the work in its broader context (with references to the mathematics of linear systems and other applications of mixture modelling in remote sensing