GNSS signals are weak and can easily be affected by radio interference. Also, since the signal format is open to the public, the signal can be created easily. Thus, a fake signal (spoofing signal) imitating a satellite signal may be generated and transmitted. As the signal has the same structure as the satellite signal, it is difficult for general receivers to distinguish them. However, the direction in which a spoofing signal arrives is usually different from that in which the satellite signal does. The purpose of this research is to calculate the direction of arrival (DOA) by using an array antenna and to determine whether the received signals are spoofing signals or not. The MUSIC algorithm can estimate the DOA with high accuracy, but this algorithm can be applied only when the number of arriving signals is less than that of antenna elements. Consequently, it is usually difficult to apply it to the satellite signals. Therefore, in this research, we proposed the MUSIC algorithm based on “despreading” of the satellite signals and verified the effectiveness of this method. As a results of experiment, the spoofing signal was detected by estimating the DOA of the received signals.
While the precise ephemeris used for precise positioning by the satellite navigation system is generally referencing navigation satellite's center of mass, fundamentally satellite's antenna phase center should be used and they have difference up to a few meters. The antenna phase center offsets of GPS Block III satellites are investigated and reported. The variation of yaw attitude of GPS Block III satellite is also investigated, which is necessary to apply the antenna phase center offset and useful for the correction of phase wind-up effects due to relative rotation of transmitting and receiving antennas. It is shown that the yaw angle of Block III satellite is controlled in the way different from older satellites during eclipse periods when satellite attitude variates largely.
This study proposes a computational grid generation method to generate prismatic grids suitable for shock adaptation around a winged re-entry vehicle. The grid generation method proposed here is based on an existing method (Takanashi, S., Computers & Fluids, 19 (1991), pp. 393--399), which uses the theory of electric force lines to determine the computational grid lines. In order to control the distortion of the prismatic grids, the proposed method introduces three modifications to the existing method: 1) the grid generation process is divided into two stages, 2) the height of the prism layers is adjusted by considering the curvature of geometries, and 3) the cell width is optimized using the spring analogy. Both the proposed and existing methods are applied to generation of computational grids for a winged re-entry vehicle, HYFLEX. It is found that the existing method gives a computational grid containing cells not suitable for flow computations. On the other hand, the proposed method successfully gives a computational grid with which the flow computation is stably performed. By applying an adaptive refinement of prismatic layers to the detached shock wave, a smooth flow field is obtained behind the sharply resolved shock wave.
GLA (= Gust Load Alleviation) control is an important active safety technology, especially for light-weight flexible wing aircraft with high aspect ratio wings. This paper proposes an adaptive aero-servo-elastic model for control design, including an aerodynamic load distribution estimator using in-flight partial pressure sensing. The GLA control law obtained based on the proposed model consists of pressure control and mode estimation. In general, aero-servo-elastic models require an wing structure dynamics and an aerodynamic model around the wing. However, it is difficult to construct a generic model in advance for complex unsteady aerodynamic phenomena where turbulence and vibration of flexible wings interact. In this case, the optimality and the reliability of a model-based GLA control is significantly reduced. To minimize the uncertainty of the model for unsteady aerodynamics, this method proposes the use of an autoregressive model with time-varying coefficients, which are updated in real-time based on in-flight pressure sensing. The feasibility of the proposed GLA scheme is demonstrated in terms of both pressure control and wing tip displacement estimation through wind tunnel experiments.
A passive blade twist control concept is expected to improve aerodynamic performance of tilt rotor blades. According to previous studies, rotor efficiency can be enhanced in both hover and forward flight modes by elastically changing the blade twist angle as a function of rotational speed. Since the magnitude of the centrifugal force varies with the rotational speed, this elastic twist deformation is obtained by use of extension-twist coupled composite rotor blades. It is known that this extension-twist coupling deformation is greatly affected by geometrical nonlinearity. In this study, an analysis and design method of blade structure considering geometrical nonlinearity is presented for this concept. The extension-twist coupling deformation results of the present method are validated by comparing with those of the finite element analysis. Finally, using this analysis method, research to further improve the efficiency of tilt rotor blades is also conducted.