Constellation configuration design is a prerequisite and critical step in the construction of a mega-constellation system in low Earth orbit. However, the huge number of satellites and the intricate changes in relative positions among them make the configuration design the most challenging part. In this paper, we propose a configuration design scheme for mega-constellations considering collision-avoidance constraints with the objective of uniform global coverage. In this design scheme, the constellation is made up of multiple Walker constellations with the same orbital altitude and different orbital inclination. Moreover, the analytical expression for the minimum distance between any two satellites in the same orbital altitude is derived, and the constellation internal collision-avoidance constraint is established accordingly. Finally, a permanent inter-satellite link design scheme without dynamic reconstruction is presented based on the mega-constellation configuration. Simulation results show that the mega-constellation design scheme introduced in this paper can achieve relatively uniform global coverage (its N Asset Coverage ranges from 18 to 25). The mixed Walker constellation is capable of providing a greater number of N Asset Coverage for most of the world than the Walker constellation of the same satellite order of magnitude. In addition, the inter-satellite link scheme designed in this paper can ensure continuous and stable communication between any satellite nodes.

The economic problem is a primary consideration in mega-constellations design. This work aims to quantify the cost of mega-constellations and analyze the contribution of reusable launch vehicles to saving the mega-constellations cost. In this paper, the cost estimation model of mega-constellation deployment mission is investigated, consisting of the launch cost and satellite cost. Simulation examples demonstrate the high applicability of the cost estimation model and the considerable cost-effectiveness of the reusable launch vehicle in mega-constellation deployment mission.

A switching neural network control scheme, consisting of the adaptive neural network controller and sliding mode controller, is proposed for underactuated formation reconfiguration in elliptic orbits with the loss of either the radial or in-track thrust. By using the inherent coupling of system states, the switching neural network technique is then adopted to estimate the unmatched disturbances and design the underactuated controller to achieve underactuated formation reconfiguration with high precision. The adaptive neural network controller works in the active region, and the disturbances composed of linearization errors and external perturbations are approximated by radial basis function neural networks. The adaptive sliding mode controller works outside the active region, and the upper bound of the approximation errors is estimated by the adaptation law. The stability of the closed-loop control system is proved via the Lyapunov-based approach. The numerical simulation results have demonstrated the rapid, high-precision and robust performance of the proposed controller compared with the linear sliding mode controller.

Aiming at the practical problems of external unknown disturbance and internal modeling uncertainty, when space tumbling target flies around in close range, a sliding mode controller (SMC) based on active disturbance rejection control (ADRC) technology is proposed to realize real-time estimation and compensation of “total disturbance.” Firstly, according to the motion characteristics of space tumbling target, the relative motion equation in rotating line of sight (RLOS) coordinate system is established; Secondly, the compound controller is designed, and the convergence of the nonlinear state expansion observer and the stability of the closed-loop system are analyzed based on the root locus method and Lyapunov function method respectively; Finally, the simulation results show that the SMC based on ADRC technology can effectively suppress the disturbance and overcome the chattering problem of traditional sliding mode controller. It has a good control quality, and strong robustness is an easy method for engineering practice.

A series of coke samples with loading alkali and alkaline earth metals were prepared by the impregnation method, the NOx emission were investigated in a silica fixed bed reactor in the combustion process of raw coke and coke modified by Na, K, Ca and Mg. The results show that Na, K, Ca and Mg play in-situ catalytic effects on the NOx reduction reactions. When the loading amount of Na2CO3 is 2.0%, the NOx reduction ratio was around 17.4%, when the loading of K2CO3 is 2.0%, the amount of NOx emission is reduced by 26.5%. When the loading of CaCl2 is 2.0%, the amount of NOx emission is reduced by 22.3%. When the loading of MgCl2 is 2.0%, the NOx reduction ratio is about 10.9%.