Comparative Assessment Of Different Constellation Geometries For Space-Based Application

As services from space are becoming an asset for life on Earth and the demand for data from space increases, the international interest in satellite constellations is increasingly growing. GPS (Global Positioning System) provides positioning and navigation. Iridium contains a relatively larger number of satellites for communication purpose. Molniya is a high elliptical orbits constellation providing high latitude coverage. Disaster Monitoring constellation consists of remote sensing satellites and brings responsiveness needed for emergencies. Recently, some companies, such as OneWeb, Samsung and Space-X, have made public their plan to deploy mega constellations of nanosatellites for global internet. Different constellation geometries have been proposed to meet various mission requirements, each one having specific advantages in terms of coverage, responsiveness, cost, etc. Thus, designing a constellation is a trade-off choice. The choice for a constellation is highly influenced by many factors, such as the system cost, the interaction with space environment (radiation and space debris), and the targeted terrestrial coverage. The design of a constellation requires selecting the parameters that best meet the mission requirements. To accomplish this, several studies on the comparison of satellite constellations proposed detailed analysis, e.g. the multi-criteria comparison for responsive constellations, the coverage assessment of elliptical constellations. However, most of them only focused on one or few performances, lacking of generalisation. A general study of constellation geometry can provide a basis for understanding the constellation design. This will allow the process of constellation design to be expedited by offering a proposal of an existing constellation style. This paper comparatively assesses different constellation geometries, including the classical proposed geometries and some less used configurations, and chooses the constellation geometry best suitable for a given mission (e.g. remote sensing, global internet). In this work, several parameters of constellation design will be considered to make a quantitative assessment: coverage (global or local), frequency of ground track repetition, responsiveness (i.e., how fast a satellite can be launched and the data return to Earth after launch), robustness to failure and speed of replenishment, end of life disposal, number of satellites and orbital altitude. The assessment will be conducted in a parametric approach. Each factor will be quantitatively evaluated by deriving a fitness function. Then, a series of weighting coefficients adapted to the given mission requirements will be chosen for the global fitness functions. Through multi objective optimisation, the constellation geometry best suitable for the given mission requirements will be derived

Low-Thrust Minimum-Fuel Optimization in the Circular Restricted Three-Body Problem

A study was conducted to demonstrate low-thrust minimum-fuel optimization in the circular restricted three-body problem. The study solved the problem of geostationary transfer orbit (GTO)-to-halo transfer for the first time. This result was achieved with an indirect approach and constant specific impulse engine. Thrust-to-mass ratios in agreement with currently available technology were considered. Some effective techniques were applied to cope with problem complexity. These methods involved solving the minimum-fuel, minimum energy, and minimum-time problems, implementing energy-to-fuel homotopy, continuing the maximum thrust magnitude, and computing the analytic Jacobians

Orbit Raising and De-Orbit for Coplanar Satellite Constellations with Low-Thrust Propulsion

This paper deals with the planar transfer problem (i.e. orbit raising and deorbiting phases) for low Earth orbit coplanar satellites constellation. The objectives are to minimize the total time of transfer and to maximize the miss distance during these phases so as to minimize the collision hazard. A Blended Error- Correction (BEC) steering law, consisting of tangential thrust and inertial thrust based on the offset in mean orbital parameters, is developed to design the transfer trajectory for a single satellite. The semi-analytical technique is used to evaluate the variation in orbital parameters over one orbit revolution to reduce the computation load. The numerical results show that the BEC steering law is able to identify near time-optimal solutions and the semi-analytical results have good accuracy. For multiple satellites transfer, the orbit transfer trajectory designed for a single satellite is used as a baseline for a global multi-satellite analysis of the miss distance among pair satellites during the orbit raising and de-orbiting phases. Considering limits on the transfer starting time for de-orbit mission, multi-objective optimization is used to find out the optimal transfer starting time for each satellite

Active space debris removal by a hybrid propulsion module

During the last 40 years, the mass of the artificial objects in orbit increased quite steadily at the rate of about 145 metric tons annually, leading to a total tally of approximately 7000 metric tons. Now, most of the cross-sectional area and mass (97% in LEO) is concentrated in about 4600 intact objects, i.e. abandoned spacecraft and rocket bodies, plus a further 1000 operational spacecraft. Simulations and parametric analyses have shown that the most efficient and effective way to prevent the outbreak of a long-term exponential growth of the catalogued debris population would be to remove enough cross-sectional area and mass from densely populated orbits. In practice, according to the most recent NASA results, the active yearly removal of approximately 0.1% of the abandoned intact objects would be sufficient to stabilize the catalogued debris in low Earth orbit, together with the worldwide adoption of mitigation measures. The candidate targets for removal would have typical masses between 500 and 1000 kg, in the case of spacecraft, and of more than 1000 kg, in the case of rocket upper stages. Current data suggest that optimal active debris removal missions should be carried out in a few critical altitude-inclination bands. This paper deals with the feasibility study of a mission in which the debris is removed by using a hybrid propulsion module as propulsion unit. Specifically, the engine is transferred from a servicing platform to the debris target by a robotic arm so to perform a controlled disposal. Hybrid rocket technology for de-orbiting applications is considered a valuable option due to high specific impulse, intrinsic safety, thrust throttle ability, low environmental impact and reduced operating costs. Typically, in hybrid rockets a gaseous or liquid oxidizer is injected into the combustion chamber along the axial direction to burn a solid fuel. However, the use of tangential injection on a solid grain Pancake Geometry allows for more compact design of the propulsion unit. Only explorative tests were performed in the past on this rocket configuration, which appears to be suitable as de-orbiting system of new satellites as well as for direct application on large debris in the framework of a mission for debris removal. The paper describes some critical aspects of the mission with particular concern to the target selection, the hybrid propulsion module, the operations as well as the systems needed to rendezvous and dock with the target, and the disposal strateg

Modelling the impact of atherosclerosis on drug release and distribution from coronary stents

Although drug-eluting stents (DES) are now widely used for the treatment of coronary heart disease, there remains considerable scope for the development of enhanced designs which address some of the limitations of existing devices. The drug release profile is a key element governing the overall performance of DES. The use of in vitro, in vivo, ex vivo, in silico and mathematical models has enhanced understanding of the factors which govern drug uptake and distribution from DES. Such work has identified the physical phenomena determining the transport of drug from the stent and through tissue, and has highlighted the importance of stent coatings and drug physical properties to this process. However, there is limited information regarding the precise role that the atherosclerotic lesion has in determining the uptake and distribution of drug. In this review, we start by discussing the various models that have been used in this research area, highlighting the different types of information they can provide. We then go on to describe more recent methods that incorporate the impact of atherosclerotic lesions

Multi-criteria design of continuous global coverage Walker and Street-of-Coverage constellations through property assessment

No general rules exist for constellation design; instead, constellation designers have to consider various cost drivers in a trade-off way. This paper presents a systematic method for the design of continuous global coverage Walker and Street-of-Coverage constellations, by taking seven critical constellation properties (coverage, robustness, self-induced collision avoidance, launch, build-up, station-keeping, and end-of-life disposal) as design criteria. In this method, a set of characteristic parameters, which can determine the constellation configuration, are first identified based on the review of Walker and Street-of-Coverage constellations. Then a series of indexes are proposed and modelled as functions of the characteristic parameters, to quantitatively assess the constellation properties. Through the quantitative assessment, the influence of constellation configuration on constellation properties are revealed, and the trade-offs between constellation properties are analysed. Finally, taking the characteristic parameters and constellation properties as the design variables and objectives, a multi-objective optimisation problem is formulated to find the globally optimal constellations for given missions

Low-thrust planar transfer for co-planar low Earth orbit satellites considering self-induced collision avoidance

This paper deals with the planar transfer problems (orbit raising and de-orbiting) for co-planar satellites with low-thrust propulsion, taking the self-induced collision avoidance into consideration at the mission design stage. A Blended Error-Correction steering law, with which the thrust direction changes in a self-adaptive way, is developed by blending two types of efficient steering laws and offsetting the errors of the instantaneous orbit with respect to the target orbit. The semi-analytical solutions for orbital elements, which reduce the computational load of propagating long-duration trajectories, are derived by computing the analytical incremental changes in orbital elements after every revolution with an orbital averaging technique. Based on the analytical Blended Error-Correction steering law and semi-analytical solutions, transfers can be computed quickly for any starting times. Finally, the self-induced collision, which is modeled by miss distance, is avoided by scheduling properly the timing to start transfer for every satellite

Orbit-Attitude Predictive Control in the Vicinity of Asteroids with In Situ Gravity Estimation

This paper presents an integrated model-learning predictive control scheme for spacecraft orbit-attitude stationkeeping in the vicinity of asteroids. The orbiting probe relies on optical and laser navigation while attitude measurements are provided by star trackers and gyroscopes. The asteroid gravity field inhomogeneities are assumed to be unknown a priori. The state and gravity model parameters are estimated simultaneously using an unscented Kalman filter. The proposed gravity model identification enables the application of a learning-based predictive control methodology. The predictive control allows for a high degree of accuracy because the predicted model is progressively identified in situ. Consequently, the tracking errors decrease over time as the model accuracy increases. Finally, a constellation mission concept is analyzed in order to speed up the model identification process. Numerical results are shown and discussed