International Journal of Space Science and Engineering (5 papers in press)
Optimal Trajectory Design for Global Exploration of an Asteroid via Bi-Impulsive Transfers
by Yu Shi, Hao Peng, Yue Wang, Shijie Xu
Abstract: The trajectory for a global exploration of an asteroid is designed to make a comprehensive investigation of different areas. The areas to be visited are considered as target points scattered on the asteroids surface and all the target points are supposed to be visited by the spacecraft propelled by impulsive thrusts. The trajectory of the spacecraft is optimized for fuel saving in two parts: the transfer orbit optimization and the exploration sequence optimization. Firstly, transfer orbits between any two target points via two impulses are optimized for fuel saving by solving a nonlinear programing problem. The solution of the Lambert problem in the gravitational field of a point mass is used as initial guesses in the optimization. Then, with all the optimal transfer orbits determined, the exploration sequence is processed as a Traveling Salesman Problem (TSP). Branch and bound method and greedy algorithm for solving this problem are compared. Finally, the trajectory for a global exploration of the asteroid 433 Eros is designed for a demonstration purpose.
Keywords: Global exploration; Asteroid; Bi-impulsive transfer orbit; Exploration sequence; Lambert problem; Traveling salesman problem.
GNSS Signals Ionospheric Propagation Characteristics in Space Service Volume
by Kui Lin, Xingqun Zhan, Jihong Huang
Abstract: In recent years, Global Navigation Satellite System (GNSS) showed a remarkable capability to serve for medium and high earth orbit spacecraft, called Space Service Volume (SSV), such as NASAs Magnetospheric Multi-Scale mission, GOES-16 mission, and ESAs Proba-3 mission. However, SSV users will face a critical problem of insufficient GNSS signal visibility. Therefore, it is essential to take full advantage of the main lobe signals of the GNSS antenna, even though part of them are being influenced by the earths atmosphere. In previous studies, signals which pass through the ionosphere are usually discarded, which results in a large discrepancy in mission design and system optimization. If handled properly, the SSV user could see more GNSS satellites, sometimes even making a life-and-death difference. In this paper, a three-dimensional ray-tracing method is adopted to analyze signal propagation through the ionosphere. Based on the International Reference Ionosphere (IRI) module, propagation characteristics of the signals main lobe is analyzed, and the bending angle of the path, ionospheric attenuation, and ionospheric delay are calculated accordingly. Taking GPS constellation and a GEO user as an example, the principles are validated with a careful simulation. The results show an interesting phenomenon that the bending angle of the signal path and the ionospheric attenuation are very small, and the ionospheric delay is large but eliminable. If the ionospheric signals are considered, the signal visibility and availability will significantly increase, which leads to a shortened maximum outage time for SSV users.
Keywords: Space Service Volume; Ray-tracing; Ionospheric Effect; Signal Propagating Characteristic.
Rapid, Automated, Test, Verification and Validation (V&V) for the CubeSats
by Yaseen Zaidi, Norman Fitz-Coy, Robert Van Zyl
Abstract: Bringing up of a small-scale mission assurance and engineering workflow is described. The experiences learned in the ZACUBE-1 mission prompted the development of an automated systems engineering platform leading to conformity in system design, test, and verification. The platform implements the methodology of systems engineering by coordinating diverse elements of the lifecycle and by incorporating the tools involved. The phase B/C activities of system modelling, simulations, prototyping, and design may be unified to a compounding effect and raising the level of the system view. The Verification and Validation (V\&V) is achieved by integrating a test and measurement facility to the platform. With the platform, we accomplish rapid electrical and functional test and verification of the CubeSat subsystems and thermal validation in −20 ◦C to +50 ◦C cycle. The platform is automated by an application software which executes functional and thermal environment tests and provides support for requirements flow, system definition, embedded development, and simulations by integrating real-time target hardware. The platform is exploited in validating an S-band communications subsystem while economizing time and obtaining valuable insight into transmission performance under thermal loading.
Keywords: Automated Test Equipment (ATE); CubeSat; Measurement; Satellite; Space; Systems Engineering; Test; Verification and Validation (V&V).
Motion around the out-of-plane equilibrium points in the photogravitational Copenhagen elliptic restricted three-body problem with oblateness
by Aguda Ekele Vincent
Abstract: This paper studies the motion of an infinitesimal mass near the out-of-plane equilibrium points (OEPs) in the elliptic restricted three-body problem (ER3BP) in the case of two equally heavy bodies (Copenhagen problem) where one of the two primaries is a radiation source and the other an oblate spheroid. We found, as in the photogravitational circular restricted three body problem, that the equations of motion of the three dimensional photogravitational ER3BP allow the existence of OEPs if the radiation parameter has negative values. There are two out of plane equilibria that lie in the (, ) plane in symmetrical positions with respect to the (, ) plane. The positions of the OEPs are affected by the parameters involved in the systems dynamics. In particular, the positions change with increase in the radiation pressure, oblateness, eccentricity and semi-major axis of the orbits. As an application, the positions and linear stability of the problem are investigated numerically for the binary system B1534+12. The OEPs are found unstable.
Keywords: celestial mechanics; elliptic restricted three-body problem; ER3BP; radiation pressure; oblateness; Copenhagen problem; out-of-plane equilibrium points; OEPs; binary system; stability.
LEO Satellite Formation Flying via Differential Atmospheric Drag
by Andrew Tang, Xiaofeng Wu
Abstract: Formation flying involves multiple spacecraft flying with pre-defined relation to each other. This allows a number of individual, smaller satellites to work together and accomplish tasks extraneous to single satellite systems. However, the required precision of orbital positioning and control makes the maintenance of such formations quite challenging. This is particularly true for space systems without propulsive controls; even for systems equipped with active control, propellant consumption can be quite high.
To facilitate orbital control, this study investigates formation flying in low earth orbit (LEO), focusing primarily on propulsion-free methods of control for micro and nano- class satellites such as aerodynamic differential drag. A fuzzy logic control algorithm was developed to control the satellites position by manipulating the drag configuration of each satellite in the formation. The outcome of this study shows that successful formation control can be achieved using drag forces alone. The time taken for each formation control and by-products, including altitude loss are evaluated. The orbital modelling presented here can be used as the baseline for a control algorithm developed for station keeping of satellites in low earth orbit.
Keywords: formation flying; satellite orbit; atmospheric drag; low earth orbit; LEO; Cube Satellite; leader-follower formation.