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Bus saturation is deemed as one of the primary causes of delays in the data propagation between spacecraft components. However, the conditions under which bus congestion can affect measurements variability were not well characterized before. This paper presents a bus saturation model and a set of experiments to characterize the bus performance of satellite missions for different traffic load, data rate, and synchronization periods. The results showed an increase of measurements variance of up to 18% caused by bus saturation. Additionally, an algorithm was proposed to reduce the data delay by controlling the saturation on the communication channel at the operational level. [ABSTRACT FROM AUTHOR]

Photovoltaic solar array (PVSA) systems are the most widely used method for spacecraft power generation. However, in many satellite missions, the optimum orientation of the PVSA system is not always compatible with that of the payload orientation. Many methods, have been examined in the past to overcome this problem. Up to date, the most widely used active method for large costly satellites is the Solar Array Drive Mechanism (SADM). The SADM serves as the interface between the satellite body and the PVSA subsystem, enabling the decoupling of their spatial orientation. Nonetheless, there exists a research and development gap for such systems regarding low cost micro-satellites. During the literature study of this paper, individual orbital parameters of various micro-satellites have been extracted and compared to the rotational freedom of the corresponding SADMs used. The findings demonstrated that the implemented SADMs are over designed. It is therefore concluded that these components are not tailored made for each spacecraft mission individually, but rather, exhibit a generic design to full fill a majority of mission profiles and requirements. Motivated by the above analysis, the cardinal objective of the current research is to develop a low cost mechanism that will be precisely tailored for the use of a low Earth orbit (LEO) micro-satellite platform orbiting in altitudes of 500 − 1000 km . The design of the mechanism may vary from the existing miniaturized SADMs. For example, the preliminary analysis of the current research suggests, that the conventional use of the slip ring system as the electronic transfer unit can be replaced by a seMI Orientation Unit (MIOU). Systems engineering tools for concept generation and selection have been used. In addition, simulation and mathematical modelling have been implemented on component and system level, to accurately predict the behaviour of the system under various modes of operation. The production and system testing of the prototype has taken place and it has verified that the development of such a system, will aid the power generation of the solar arrays, while having a positive impact on the cost reduction of such satellites. [ABSTRACT FROM AUTHOR]

With respect to spacecraft attitude control, reaction spheres are promising alternatives to conventional momentum exchange devices for the benefits brought by their 4 π rotation. Many design concepts of reaction spheres have been proposed in the past decades, however, developments of the driving unit and the bearing, as well as their combination remain great challenges. To facilitate research and push developments in this field, this paper provides a comprehensive review of reaction spheres. To some extent, an in-depth survey of multi-DOF (degree of freedom) spherical motors and possible bearings is provided, along with their advantages and weaknesses addressed. Some multi-DOF actuators for different applications, such as robotic joints, are investigated since they share many similar challenges and techniques with reaction spheres. The experimental performances of realized reaction spheres are listed and compared. Limits of current designs are identified and their causes are analyzed and discussed. Compared with existing summaries on multi-DOF actuators and some surveys done for specific reaction spheres' design, this paper provides the first thorough review on reaction spheres, considering approaches to excite and support the free 4 π rotation. [ABSTRACT FROM AUTHOR]

Underactuated attitude control is supposed to be used on spacecraft when failure happens with onboard actuators. One main problem with existing underactuated attitude control designs is their limited capabilities against disturbances. In order to solve this problem, an approach based on the theory of H ∞ is proposed in this paper. Two propositions are derived from the H ∞ theory to improve the robustness of one popular underactuated attitude control design, which was presented by Tsiotras et al. It is proved mathematically that the controller satisfying these two propositions respectively can stabilize the underactuated attitude system locally or globally. The numerical simulations show that the improved controllers based on the H ∞ theory could provide higher pointing accuracy for small satellites against disturbances. This validates the effectiveness of the proposed H ∞ based approach to improve existing underactuated attitude control designs. [ABSTRACT FROM AUTHOR]

The autonomous maneuverable space net, which consists of a flexible net and several maneuverable units, is a promising solution for the active removal of space debris. A novel dynamics model and a corresponding controller are proposed in this paper to resolve the approach control problem. Given that the net tethers cannot be elongated, interval functions and corresponding constraint forces are exploited to model the unilaterally constrained tethers. The proposed approach dynamics model, which is based on the Hamilton principle, includes distinctive velocity jump phenomena. A dual-loop control scheme with double optimization pseudo-dynamics inversion and sliding mode control is also established. Simulation results validate the feasibility of the proposed control scheme. The net can fly along the expected trajectory without suffering unexpected net deformation or orbital radial movement. [ABSTRACT FROM AUTHOR]

Abstract: The pulsar is a high-speed rotationing neutron star with a stable rotational period, being not disturbed and destroyed artificially, and can be taken as the reference quantity of the absolute time. In this article a kind of pulsar time service method based on the Kalman filtering algorithm is proposed, and the simulation analysis of the clock error control based on the Kalman filtering and of the effect of the pulsar catalogue error and the measuring accuracy of the pulsar time of arrival (TOA) on the accuracy of time service is made by taking a certain solar synchronous orbit as an example. The result shows that by utilizing this method the clock error of the satellite-borne clock can be effectively eliminated and its time-dependent increase is restrained, thereby solving the problem that the accuracy of the spacecraft-borne low cost clock can not meet the needs. [Copyright &y& Elsevier]

Most of the CubeSats have a volume range from 1U to 3U, which limits their applications due to the difficulty of miniaturizing payloads. To facilitate the needs on a larger but low-cost satellite platform, the AOXiang (AOX) project has been developed by Northwestern Polytechnical University (NPU). The primary objectives of AOX project are four-folds: 1) To demonstrate the world first 12U CubeSat Star of AOXiang and 12U orbit deployer which uses an innovative electromagnetic unlocking technology. 2) To investigate the feasibility of using polarized sunlight for spacecraft attitude determination and navigation, and perform microgravity research using a miniaturized gravimeter. 3) To test a fault tolerant on-board computer using the System On the Programmable Chip (SOPC) technology, and 4) To gain the experience from developing the CubeSat and the subsystems. The CubeSat was launched in June 2016. Now, the mission has achieved all the goals. This paper provides the detail information of the AOX project, with a focus on the introduction of the subsystems of the 12U CubeSat, the orbit deployer and the payloads. The recent in-orbit results of the first NPU are also presented. In addition to the educational objective that has been reached with more than 50 young scholars and students participated in the project. [ABSTRACT FROM AUTHOR]

In this paper, we systematically introduce a novel geostationary orbit (GEO) space debris removal system called dexterous tethered space robot (DTSR). The DTSR has three notable characteristics: dexterity, lightweight, and cost effectiveness. We first describe the system's design and a typical mission scenario, and then present its two core technologies (vision-based pose measurement and coordinated controller design) in detail. Finally, we present the extensive simulations and ground semiphysical experiments to verify that the DTSR is a feasible solution to effectively remove the GEO space debris. [ABSTRACT FROM PUBLISHER]

ASTROD I is a planned interplanetary space mission with multiple goals. The primary aims are: to test General Relativity with an improvement in sensitivity of over 3 orders of magnitude, improving our understanding of gravity and aiding the development of a new quantum gravity theory; to measure key solar system parameters with increased accuracy, advancing solar physics and our knowledge of the solar system; and to measure the time rate of change of the gravitational constant with an order of magnitude improvement and the anomalous Pioneer acceleration, thereby probing dark matter and dark energy gravitationally. It is envisaged as the first in a series of ASTROD missions. ASTROD I will consist of one spacecraft carrying a telescope, four lasers, two event timers and a clock. Two-way, two-wavelength laser pulse ranging will be used between the spacecraft in a solar orbit and deep space laser stations on Earth, to achieve the ASTROD I goals.For this mission, accurate pulse timing with an ultra-stable clock, and a drag-free spacecraft with reliable inertial sensor are required. T2L2 has demonstrated the required accurate pulse timing; rubidium clock on board Galileo has mostly demonstrated the required clock stability; the accelerometer on board GOCE has paved the way for achieving the reliable inertial sensor; the demonstration of LISA Pathfinder will provide an excellent platform for the implementation of the ASTROD I drag-free spacecraft. These European activities comprise the pillars for building up the mission and make the technologies needed ready. A second mission, ASTROD or ASTROD-GW (depending on the results of ASTROD I), is envisaged as a three-spacecraft mission which, in the case of ASTROD, would test General Relativity to one part per billion, enable detection of solar g-modes, measure the solar Lense-Thirring effect to 10 parts per million, and probe gravitational waves at frequencies below the LISA bandwidth, or in the case of ASTROD-GW, would be dedicated to probe gravitational waves at frequencies below the LISA bandwidth to 100 nHz and to detect solar g-mode oscillations. In the third phase (Super-ASTROD), larger orbits could be implemented to map the outer solar system and to probe primordial gravitational-waves at frequencies below the ASTROD bandwidth. This paper on ASTROD I is based on our 2010 proposal submitted for the ESA call for class-M mission proposals, and is a sequel and an update to our previous paper (Appouchaux et al., Exp Astron 23:491-527, ; designated as Paper I) which was based on our last proposal submitted for the 2007 ESA call. In this paper, we present our orbit selection with one Venus swing-by together with orbit simulation. In Paper I, our orbit choice is with two Venus swing-bys. The present choice takes shorter time (about 250 days) to reach the opposite side of the Sun. We also present a preliminary design of the optical bench, and elaborate on the solar physics goals with the radiation monitor payload. We discuss telescope size, trade-offs of drag-free sensitivities, thermal issues and present an outlook. [ABSTRACT FROM AUTHOR]