International Journal of Space Science and Engineering (3 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).