Analytic optimal pose tracking control in close-range proximity operations with a non-cooperative target

This paper investigates an analytical optimal pose tracking control problem for chaser spacecraft during the close-range proximity operations with a non-cooperative space target subject to attitude tumbling and unknown orbital maneuvering.Firstly,the relative translational motion between the orbital target and the chaser spacecraft is described in the Line-of-Sight(LOS)coordinate frame along with attitude quaternion dynamics.Then,based on the coupled 6-Degree of Freedom(DOF)pose dynamic model,an analytical optimal control action consisting of constrained optimal control value,application time and its duration are proposed via exploring the iterative sequential action control algorithm.Meanwhile,the global closed-loop asymptotic stability of the proposed predictive control action is presented and discussed.Compared with traditional proximity control schemes,the highlighting advantages are that the application time and duration of the devised controller is applied discretely in light of the influence of the instantaneous pose configuration on the pose tracking performance with less energy consumptions rather than at each sample time.Finally,three groups of illustrative examples are organized to validate the effectiveness of the proposed analytical optimal pose tracking control scheme.

Soil Properties and Plant Growth Response to Litter in a Prolonged Enclosed Grassland of Loess Plateau, China

The enclosure and ungrazing practices for grassland management result in accumulation of plant litter on soil surface thus affecting the available soil water and nutrients for plant production. We experimentally investigated the effects of litter on soil properties and plant growth in a prolonged enclosure grassland of Loess Plateau, China. Three different litter manipulations were conducted including removal of all litter, an untreated in-situ control with original litter levels, and a double litter treatment. Litter treatment experiments demonstrated that plant litter affected the superficial soil water. Soil water content in plots with in-situ or double litter is generally higher than that with litter removal. The depletion of soil water up to five days post rainfall is fastest in litter removal plots for the top soil, but no evident difference for the deep ones. Different litter treatments have no significant impact on soil total carbon, nitrogen as well as carbon/nitrogen ratio for consecutive two years experiments. Both above-and below-ground biomasses in plots of litter removal were less than those in the plots of in-situ and double litter treatment. Litter affects plant production mainly through the mechanical barrier regulating root zone soil moisture. Therefore, prolonged litter manipulation experiments are desirable to understand the long-term response of plant growth on litter from nutrient aspect.

Impact of short-term organic amendments incorporation on soil structure and hydrology in semiarid agricultural lands

Soil structure plays an important role in edaphic conditions and the environment. In this study, we investigated the effects of organic amendment on soil structure and hydraulic properties. A corn field in a semiarid land was separately amended with sheep manure compost at five different rates (2, 4, 6, 8 and 10 t/ha) and corn stover (6 t/ha) in combination with two decomposing agents. The soil structure of different amended soils was analyzed from the aggregate and pore domain perspectives. The internal pore structure of the soil was visualized through X-ray computed tomography and quantified using a pore-network model. Soil aggregate-size distribution and stability, saturated hydraulic conductivity, and water-retention curves were measured by sampling or in situ. The gas permeability and diffusivity of different amended soils were simulated based on the extracted pore networks. The aggregate stability of the amended soils was improved compared with the control, that is, the mean weight diameter increased and the percentage of aggregate destruction decreased. The stability of soil aggregates varied non-monotonically with the application rate of compost and decreased after treatment with corn stover and decomposing agents. The pore-network parameters including air-filled porosity, pore radius, throat length, and coordinate number increased for the amended soils compared with the control. The mean pore size increased with increasing compost incorporation rate. The saturated hydraulic conductivity of the compost-amended soils was higher than that of the control but varied quadratically with the application rate. The saturated hydraulic conductivity of soil treated with corn stover and decomposing agents was clearly higher than that without the agent and the control. The greater gas diffusivity and air permeability indicate that soil aeration improved following the incorporation of organic amendments. The air permeability versus air-filled porosity relationship followed a power law, and the gas diffusivity versus air-filled porosity relationship was characterized by a generalized density-corrected model regardless of amendment. The findings of this study can help improve the understanding of soil structure and hydrological function to organic fertilizer incorporation and further monitor the quality of soil structure through the pore space perspective.