Approach and landing guidance design for reusable launch vehicle using multiple sliding surfaces technique

An autonomous approach and landing（A＆L） guidance law is presented in this paper for landing an unpowered reusable launch vehicle（RLV） at the designated runway touchdown. Considering the full nonlinear point-mass dynamics, a guidance scheme is developed in threedimensional space. In order to guarantee a successful A＆L movement, the multiple sliding surfaces guidance（MSSG） technique is applied to derive the closed-loop guidance law, which stems from higher order sliding mode control theory and has advantage in the finite time reaching property.The global stability of the proposed guidance approach is proved by the Lyapunov-based method.The designed guidance law can generate new trajectories on-line without any specific requirement on off-line analysis except for the information on the boundary conditions of the A＆L phase and instantaneous states of the RLV. Therefore, the designed guidance law is flexible enough to target different touchdown points on the runway and is capable of dealing with large initial condition errors resulted from the previous flight phase. Finally, simulation results show the effectiveness of the proposed guidance law in different scenarios.

UAV-based spatial pattern of three-dimensional green volume and its influencing factors in Lingang New City in Shanghai,China

Three-dimensional green volume(TDGV)reflects the quality and quantity of urban green space and its provision of ecosystem services;therefore,its spatial pattern and the underlying influential factors play important roles in urban planning and management.However,little is known about the factors contributing to the spatial pattern of TDGV.In this paper,TDGV and land use intensity(LUI)extracted from high spatial resolution(0.05 m)remotely sensed data acquired by an unmanned aerial vehicle(UAV),anthropogenic factors^(1))and natural factors^(2))were utilized to identify the spatial pattern of TDGV and the potential influencing factors in Lingang New City,a rapidly developed coastal town in Shanghai.The results showed that most of the TDGV was distributed in the western part of this new city and that its spatial variations were significantly axial.TDGV corresponded well with the chronologies of land formation,urban planning,and construction in the city.Generalized least squares(GLS)analysis of TDGV(grid cell size:100×100 m)and its influencing factors showed that the TDGV in this new city was significantly negatively correlated with both LUI and distance from roads and significantly positively correlated with land formation time and distance from water.Distance from buildings did not affect TDGV.Additionally,the degree of influence decreased in the following order:distance from water>land formation time>distance from roads>LUI.These results indicate that the spatial pattern of TDGV in this new town was mainly affected by natural factors(i.e.,the distance from water and land formation time)and that the artificial disturbances caused by rapid urbanization did not decrease the regional TDGV.The main factors shaping the spatial distribution of TDGV in this city were local natural factors.Our findings suggest that the improvement in local soil and water conditions should be emphasized in the construction of new cities in coastal areas to ensure the efficient provision of ecological services by urban green spaces.