The Experts below are selected from a list of 177 Experts worldwide ranked by ideXlab platform
Xibin Cao - One of the best experts on this subject based on the ideXlab platform.
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Tangent-impulse transfer from elliptic orbit to an excess velocity vector
Chinese Journal of Aeronautics, 2014Co-Authors: Gang Zhang, Xiangyu Zhang, Xibin CaoAbstract:Abstract The two-body orbital transfer problem from an elliptic parking orbit to an excess velocity vector with the tangent impulse is studied. The direction of the impulse is constrained to be aligned with the velocity vector, then speed changes are enough to nullify the relative velocity. First, if one tangent impulse is used, the transfer orbit is obtained by solving a single-variable function about the True Anomaly of the initial orbit. For the initial circular orbit, the closed-form solution is derived. For the initial elliptic orbit, the discontinuous point is solved, then the initial True Anomaly is obtained by a numerical iterative approach; moreover, an alternative method is proposed to avoid the singularity. There is only one solution for one-tangent-impulse escape trajectory. Then, based on the one-tangent-impulse solution, the minimum-energy multi-tangent-impulse escape trajectory is obtained by a numerical optimization algorithm, e.g., the genetic method. Finally, several examples are provided to validate the proposed method. The numerical results show that the minimum-energy multi-tangent-impulse escape trajectory is the same as the one-tangent-impulse trajectory.
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Tangent-Impulse Interception for a Hyperbolic Target
Hindawi Limited, 2014Co-Authors: Dongzhe Wang, Gang Zhang, Xibin CaoAbstract:The two-body interception problem with an upper-bounded tangent impulse for the interceptor on an elliptic parking orbit to collide with a nonmaneuvering target on a hyperbolic orbit is studied. Firstly, four special initial True anomalies whose velocity vectors are parallel to either of the lines of asymptotes for the target hyperbolic orbit are obtained by using Newton-Raphson method. For different impulse points, the solution-existence ranges of the target True Anomaly for any conic transfer are discussed in detail. Then, the time-of-flight equation is solved by the secant method for a single-variable piecewise function about the target True Anomaly. Considering the sphere of influence of the Earth and the upper bound on the fuel, all feasible solutions are obtained for different impulse points. Finally, a numerical example is provided to apply the proposed technique for all feasible solutions and the global minimum-time solution with initial coasting time
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reachable domain of spacecraft with a single tangent impulse considering trajectory safety
Acta Astronautica, 2013Co-Authors: Gang Zhang, Xibin CaoAbstract:Abstract The reachable domain of the two-body transfer orbit with a single upper-bounded tangent impulse is studied. Three cases are analyzed for either the magnitude of the tangent impulse or the initial impulse point being free, or both being free. For a fixed impulse magnitude and a free initial impulse point, the initial orbit is proved to be one of the envelopes of the reachable domain. Moreover, the trajectory safety for the transfer orbit requires a lower bound on the perigee altitude and an upper bound on the apogee altitude. Then the ranges of the impulse magnitude and the initial True Anomaly can be obtained by solving quadratic and cubic inequalities, respectively. If both constraints are required for an arbitrary impulse point, the range of the impulse magnitude is obtained with impulses at the perigee and the apogee. Several numerical examples with different eccentricities are provided to show the geometry of the reachable domain and to verify the proposed method.
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Optimal Two-Impulse Cotangent Rendezvous Between Coplanar Elliptical Orbits
Journal of Guidance Control and Dynamics, 2013Co-Authors: Gang Zhang, Di Zhou, Zhaowei Sun, Xibin CaoAbstract:This paper studies the two-impulse cotangent rendezvous problem between two coplanar elliptical orbits. This problem requires the same flight time for two spacecraft and a cotangent transfer between the initial and final orbits. For two coplanar circular orbits, the closed-form solution is obtained and its total cost is equal to that of the Hohmann transfer. However, for two coplanar elliptical orbits, the solutions are obtained only by a numerical iterative algorithm. There are many solutions for the multiple-revolution case. Moreover, the minimum-fuel two-impulse cotangent transfer can be expressed as the True Anomaly of final orbit. With the minimum-fuel transfer, a simple method for the optimal revolution numbers is proposed based on the first-order Taylor series expansion of the flight-time equation. Then, the minimum-fuel two-impulse cotangent rendezvous is obtained by calculating and comparing two or four candidates. Two numerical examples are provided to apply the proposed technique for all soluti...
Gang Zhang - One of the best experts on this subject based on the ideXlab platform.
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Tangent-impulse transfer from elliptic orbit to an excess velocity vector
Chinese Journal of Aeronautics, 2014Co-Authors: Gang Zhang, Xiangyu Zhang, Xibin CaoAbstract:Abstract The two-body orbital transfer problem from an elliptic parking orbit to an excess velocity vector with the tangent impulse is studied. The direction of the impulse is constrained to be aligned with the velocity vector, then speed changes are enough to nullify the relative velocity. First, if one tangent impulse is used, the transfer orbit is obtained by solving a single-variable function about the True Anomaly of the initial orbit. For the initial circular orbit, the closed-form solution is derived. For the initial elliptic orbit, the discontinuous point is solved, then the initial True Anomaly is obtained by a numerical iterative approach; moreover, an alternative method is proposed to avoid the singularity. There is only one solution for one-tangent-impulse escape trajectory. Then, based on the one-tangent-impulse solution, the minimum-energy multi-tangent-impulse escape trajectory is obtained by a numerical optimization algorithm, e.g., the genetic method. Finally, several examples are provided to validate the proposed method. The numerical results show that the minimum-energy multi-tangent-impulse escape trajectory is the same as the one-tangent-impulse trajectory.
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Tangent-Impulse Interception for a Hyperbolic Target
Hindawi Limited, 2014Co-Authors: Dongzhe Wang, Gang Zhang, Xibin CaoAbstract:The two-body interception problem with an upper-bounded tangent impulse for the interceptor on an elliptic parking orbit to collide with a nonmaneuvering target on a hyperbolic orbit is studied. Firstly, four special initial True anomalies whose velocity vectors are parallel to either of the lines of asymptotes for the target hyperbolic orbit are obtained by using Newton-Raphson method. For different impulse points, the solution-existence ranges of the target True Anomaly for any conic transfer are discussed in detail. Then, the time-of-flight equation is solved by the secant method for a single-variable piecewise function about the target True Anomaly. Considering the sphere of influence of the Earth and the upper bound on the fuel, all feasible solutions are obtained for different impulse points. Finally, a numerical example is provided to apply the proposed technique for all feasible solutions and the global minimum-time solution with initial coasting time
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reachable domain of spacecraft with a single tangent impulse considering trajectory safety
Acta Astronautica, 2013Co-Authors: Gang Zhang, Xibin CaoAbstract:Abstract The reachable domain of the two-body transfer orbit with a single upper-bounded tangent impulse is studied. Three cases are analyzed for either the magnitude of the tangent impulse or the initial impulse point being free, or both being free. For a fixed impulse magnitude and a free initial impulse point, the initial orbit is proved to be one of the envelopes of the reachable domain. Moreover, the trajectory safety for the transfer orbit requires a lower bound on the perigee altitude and an upper bound on the apogee altitude. Then the ranges of the impulse magnitude and the initial True Anomaly can be obtained by solving quadratic and cubic inequalities, respectively. If both constraints are required for an arbitrary impulse point, the range of the impulse magnitude is obtained with impulses at the perigee and the apogee. Several numerical examples with different eccentricities are provided to show the geometry of the reachable domain and to verify the proposed method.
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Optimal Two-Impulse Cotangent Rendezvous Between Coplanar Elliptical Orbits
Journal of Guidance Control and Dynamics, 2013Co-Authors: Gang Zhang, Di Zhou, Zhaowei Sun, Xibin CaoAbstract:This paper studies the two-impulse cotangent rendezvous problem between two coplanar elliptical orbits. This problem requires the same flight time for two spacecraft and a cotangent transfer between the initial and final orbits. For two coplanar circular orbits, the closed-form solution is obtained and its total cost is equal to that of the Hohmann transfer. However, for two coplanar elliptical orbits, the solutions are obtained only by a numerical iterative algorithm. There are many solutions for the multiple-revolution case. Moreover, the minimum-fuel two-impulse cotangent transfer can be expressed as the True Anomaly of final orbit. With the minimum-fuel transfer, a simple method for the optimal revolution numbers is proposed based on the first-order Taylor series expansion of the flight-time equation. Then, the minimum-fuel two-impulse cotangent rendezvous is obtained by calculating and comparing two or four candidates. Two numerical examples are provided to apply the proposed technique for all soluti...
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analytical study of tangent orbit and conditions for its solution existence
Journal of Guidance Control and Dynamics, 2012Co-Authors: Gang Zhang, Di Zhou, Daniele Mortari, Troy A HendersonAbstract:This paper presents an analytical study of the two-body tangent orbit technique by providing solution-existence conditions. The flight-direction angle is used to describe and solve this problem. Closed-form solutions are obtained for three classic problems: specified arrival flight-direction angle, specified departure flight-direction angle, and cotangent transfers. Not all of the problems admit solutions; thus, closed-form conditions for solution existence are provided by imposing a positive semilatus rectum constraint and a negative transfer-orbit energy (elliptic orbit transfer) constraint. The final solution-existence condition is then provided in terms of the True Anomaly range for initial or final orbit. The singularity problem of 180 deg orbit transfer is also analyzed. Several examples are provided to verify the proposed analytical methods.
R M Killen - One of the best experts on this subject based on the ideXlab platform.
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the meteoroid stream of comet encke at mercury implications for mercury surface space environment geochemistry and ranging observations of the exosphere
Geophysical Research Letters, 2015Co-Authors: Apostolos A Christou, R M Killen, M H BurgerAbstract:We test the hypothesis that an annually repeatable Ca emission excess in Mercury's exosphere at a True Anomaly Angle (TAA) of 25° ± 5° is due to particles from comet 2P/Encke impacting the surface. By simulating the dynamical evolution of Encke particles under planetary perturbations and Poynting-Robertson drag, we find that millimeter-sized grains ejected 1–2 ×104 years ago encounter Mercury at TAA = 350°–30°. The timing of the excess emission is consistent with a major dust release episode ≲20 kyr ago, possibly due to Encke progenitor breakup. The emission mechanism is likely the direct injection of impact-liberated Ca into sunlight rather than nightside surface adsorption for subsequent release at dawn. The timing of dust release from the comet depends on this mechanism; a 10 kyr age is implied by the direct-injection scenario.
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impact vaporization as a possible source of mercury s calcium exosphere
Icarus, 2015Co-Authors: R M Killen, Joseph M HahnAbstract:Abstract Mercury’s calcium exosphere varies in a periodic way with that planet’s True Anomaly. We show that this pattern can be explained by impact vaporization from interplanetary dust with variations being due to Mercury’s radial and vertical excursions through an interplanetary dust disk having an inclination within 5 degrees of the plane of Mercury’s orbit. Both a highly inclined dust disk and a two-disk model (where the two disks have a mutual inclination) fail to reproduce the observed variation in calcium exospheric abundance with Mercury True Anomaly angle. However, an additional source of impacting dust beyond the nominal dust disk is required near Mercury’s True Anomaly ( ν ) 25° ± 5°. This is close to but not coincident with Mercury’s True Anomaly ( ν = 45°) when it crosses Comet 2P/Encke’s present day orbital plane. Interestingly, the Taurid meteor storms at Earth, which are also due to Comet Encke, are observed to occur when Earth’s True Anomaly is ±20 or so degrees before and after the position where Earth and Encke orbital planes cross. The lack of exact correspondence with the present day orbit of Encke may indicate the width of the potential stream along Mercury’s orbit or a previous cometary orbit. The extreme energy of the escaping calcium, estimated to have a temperature >50,000 K if the source is thermal, cannot be due to the impact process itself but must be imparted by an additional mechanism such as dissociation of a calcium-bearing molecule or ionization followed by recombination.
Joseph M Hahn - One of the best experts on this subject based on the ideXlab platform.
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impact vaporization as a possible source of mercury s calcium exosphere
Icarus, 2015Co-Authors: R M Killen, Joseph M HahnAbstract:Abstract Mercury’s calcium exosphere varies in a periodic way with that planet’s True Anomaly. We show that this pattern can be explained by impact vaporization from interplanetary dust with variations being due to Mercury’s radial and vertical excursions through an interplanetary dust disk having an inclination within 5 degrees of the plane of Mercury’s orbit. Both a highly inclined dust disk and a two-disk model (where the two disks have a mutual inclination) fail to reproduce the observed variation in calcium exospheric abundance with Mercury True Anomaly angle. However, an additional source of impacting dust beyond the nominal dust disk is required near Mercury’s True Anomaly ( ν ) 25° ± 5°. This is close to but not coincident with Mercury’s True Anomaly ( ν = 45°) when it crosses Comet 2P/Encke’s present day orbital plane. Interestingly, the Taurid meteor storms at Earth, which are also due to Comet Encke, are observed to occur when Earth’s True Anomaly is ±20 or so degrees before and after the position where Earth and Encke orbital planes cross. The lack of exact correspondence with the present day orbit of Encke may indicate the width of the potential stream along Mercury’s orbit or a previous cometary orbit. The extreme energy of the escaping calcium, estimated to have a temperature >50,000 K if the source is thermal, cannot be due to the impact process itself but must be imparted by an additional mechanism such as dissociation of a calcium-bearing molecule or ionization followed by recombination.
D V Heinbuch - One of the best experts on this subject based on the ideXlab platform.
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training a neural network based intrusion detector to recognize novel attacks
Systems Man and Cybernetics, 2001Co-Authors: D V HeinbuchAbstract:While many commercial intrusion detection systems (IDS) are deployed, the protection they afford is modest. State-of-the-art IDS produce voluminous alerts, most false alarms, and function mainly by recognizing the signatures of known attacks so that novel attacks slip past them. Attempts have been made to create systems that recognize the signature of "normal," in the hope that they will then detect attacks, known or novel. These systems are often confounded by the extreme variability of nominal behavior. The paper describes an experiment with an IDS composed of a hierarchy of neural networks (NN) that functions as a True Anomaly detector. This result is achieved by monitoring selected areas of network behavior, such as protocols, that are predictable in advance. While this does not cover the entire attack space, a considerable number of attacks are carried out by violating the expectations of the protocol/operating system designer. Within this focus, the NNs are trained using data that spans the entire normal space. These detectors are able to recognize attacks that were not specifically presented during training. We show that using small detectors in a hierarchy gives a better result than a single large detector. Some techniques can be used not only to detect anomalies, but to distinguish among them.
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building a True Anomaly detector for intrusion detection
Military Communications Conference, 2000Co-Authors: S C Lee, D V HeinbuchAbstract:While many commercial intrusion defection systems (IDS) are deployed, the protection they afford is modest. At the state-of-the-art, IDS produce voluminous alerts, most false alarms, and function mainly by recognizing the signatures of known attacks so that novel attacks slip past them. Attempts have been made to create systems that recognize the signature of "normal", in the hope that they will then detect attacks, known or novel. These systems are often confounded by the extreme variability of nominal behavior. This paper describes an experiment with an IDS composed of a hierarchy of neural networks (NN) that functions as a True Anomaly detector. This result is achieved by monitoring selected areas of network behavior, such as protocols, that are predictable in advance. While this does not cover the entire attack space, a considerable number of attacks are carried out by violating the expectations of the protocol/operating system designer. Within this focus, the NNs are trained using data that spans the entire normal space. These detectors are able to recognize attacks that were not specifically presented during training. We show that using small detectors in a hierarchy gives a better result than a single large detector. Some techniques can be used not only to detect anomalies, but to distinguish among them.