The Global Position System (GPS) may have poor performance in metropolitan areas where there is a terrible blocking effect for satellite signals. A different positioning system combination will increase the number of visible satellites and enhance the strength of the satellite geometry. The Japanese Aerospace Exploration Agency (JAXA) confirmed that the first quasi-zenith satellite (QZS) ''MICHIBIKI'' began providing positioning signals on June 22, 2011 (JST). The QZS system (QZSS) can provide compatible signals and has a better interoperability with GPS. Therefore, we used integration of the two systems to verify the changes in positioning performance. To demonstrate the positioning accuracy of the proposed methods, evaluations of the positioning performance using only the GPS system and the GPS + QZSS system are conducted using real observation data in Shanghai. Three indicators are used to evaluate the positioning performance: the number of visible satellites (NVS), position dilution of precision (PDOP), and root mean square (RMS). Meanwhile, the results show that GPS augmentation using the QZSS improves the positioning performance for high elevation mask angles. Positioning availability increases more than 30% at 45 degrees, and by as much as 64% on day 3. The performance is more stable using the GPS + QZSS.
This paper deals with a nonlinear adaptive autopilot design for agile missile systems. During the agile turn, there exist highly nonlinear, rapidly changing dynamics and aerodynamic uncertainties. To handle these difficulties, we propose a longitudinal autopilot for angle-of-attack tracking based on backstepping control methodology in conjunction with the time-delay adaptation scheme. The performance of the proposed method is investigated through nonlinear 6-DOF simulations using a reference angle-of-attack profile of agile turn that is obtained from trajectory optimization. An intercept scenario is performed to explore the applicability of the proposed control methodology to agile missile systems.
In this paper, the effect of flexibility of the caudal fin on the propulsive performance of bottlenose dolphins is studied using the numerical simulation technique. The fluid-structure interactions are computed using an assumed mode method together with the 3D modified doublet lattice method (MDLM) and the 3D Navier-Stokes (NS) code. As the first step, the necessary power for the standing swimming condition is determined via numerical simulation using the 3D NS code. With this necessary power, the propulsive performance of horizontal swimming is estimated using the 3D MDLM coupled with an optimum design technique and the 3D NS code. The results show that the power-mass-ratio of the standing swimming is 62.2 W/kg which is approximately 2.6 times larger than that of human athlete, and it is an 11% decrease compared with that obtained under the rigid fin assumption. As to the horizontal swimming, the propulsive efficiency increases approximately 2–4% compared with that of the rigid fin to attain thesame amount of thrust according to the analysis using the 3D MDLM, while the analysis using the 3D NS code predicted an approximate 12% decrease in efficiency due to flow separation observed around the tip region. As the result of these analyses, the maximum speed of horizontal swimming is predicted to be 12 m/s which is a 1 m/s decrease from the 13 m/s estimated under the rigid fin assumption.
Spacecraft can observe multiple sources to achieve the goal of absolute position determination in pulsar-based navigation. In this paper, we draw attention to the problems associated with synchronizing of multiple time-of-arrival (TOA) measurements. The importance of this for navigation performance is not fully appreciated. Based on high-precision pulsar timing models, we establish the linear pulse phase measurement equation for near-Earth spacecraft and deduce the formulae of apparent pulse frequency (APF). We also develop the TOA synchronization model that uses APF to propagate pulse phases. We apply this model in numerical simulations that implement the extended Kalman filter (EKF) to estimate navigation states for geostationary (GEO) satellites. The results show that our model can effectively control navigation errors after TOAs are synchronized properly. We expect that the TOA synchronization technique presented in this paper may be useful for performance improvement of pulsar-based navigation.
This paper proposes a new formation guidance law for multiple UAVs in a three-dimensional space. The proposed formation guidance law produces a velocity command vector of a wingman to form a prescribed formation shape at a specified formation time. The guidance command ensures that the time derivative of a Lyapunov function defined as the square of zero-effort-miss between a wingman and a desired formation position is negative definite during the flight. The velocity command vector is then transformed to typical autopilot inputs of a UAV: speed, flight path angle and heading angle commands. The performance of the proposed law is evaluated using a full nonlinear 6-DOF UAV model in the cases of formation shaping, formation keeping, and formation re-shaping when the leader is accelerating. Simulation results show that the proposed law maintains the formation keeping well and precisely achieves the given shape of formation even when the leader is accelerating.
A Lyapunov-based adaptive controller using a command filtered approach is proposed for the nonlinear longitudinal dynamics of an air-breathing reusable space vehicle (RSV). This paper also proposes the RSV's longitudinal dynamics including thrust misalignment, and the misalignment terms are considered as disturbances during the numerical simulation. The RSV's longitudinal dynamics are highly unstable due to model uncertainty and external disturbance. Model uncertainty includes the variation of aerodynamic coefficients, for which high fidelity modeling is not feasible in a hypersonic region. A minimized online parametric adaptation law using a tuning function is designed to compensate for the tracking errors by the variation of model uncertainties and external disturbance. A first-order constrained command filter is designed to handle the physical constraints of command signals and estimate the partial derivatives of stabilizing functions. Lyapunov-based stability analysis and numerical simulation results are presented to verify that the control and adaptation law can guarantee system stability and track a reference trajectory.