Abstract This paper focuses on robust optimal adaptive control strategy to deal with tracking problem of a quadrotor unmanned aerial vehicle (UAV) in presence of parametric uncertainties, actuator amplitude constraints, and unknown time-varying external disturbances. First, Lyapunov-based indirect adaptive controller optimized by particle swarm optimization (PSO) is developed for multi-input multi-output (MIMO) nonlinear quadrotor to prevent input constraints violation, and then disturbance observer-based control (DOBC) technique is aggregated with the control system to attenuate the effects of disturbance generated by an exogenous system. The performance of synthesis control method is evaluated by a new performance index function in time-domain, and the stability analysis is carried out using Lyapunov theory. Finally, illustrative numerical simulations are conducted to demonstrate the effectiveness of the presented approach in altitude and attitude tracking under several conditions, including large time-varying uncertainty, exogenous disturbance, and control input constraints.

Abstract In recent years, dynamics and control of satellite formation flying have been active areas of research. From the mission planning perspective, three main areas namely formation establishment, maintenance and reconfiguration have been discussed. In this paper, a study of formation establishment under initial uncertainty is presented. In this regard, dynamics of low Earth orbit satellite formation is discussed. Control Lyapunov function approach is adopted to bring a deputy satellite, with perturbed initial conditions into formation with a chief satellite. In order to take account of the initial orbit insertion error, uncertainty in initial conditions of the deputy satellite is considered. For a case study, a relatively small formation is adopted, with air-launched Pegasus as the launch vehicle. For several initial conditions, control function and required time to achieve a given mission accuracy are determined, and results are provided as illustration.

Abstract Accessibility gaps are inherent properties of Low Earth Orbit (LEO) sun-synchronous satellite missions. Long accessibility gaps in satellite missions imply strict in-orbit autonomy requirement, met by expensive solutions. Thus, methods to shorten accessibility gaps in satellite missions are appreciated by space mission designers. For that purpose, in this paper, ground segment site location is employed as a mechanism to reduce the longest accessibility gaps in LEO sun-synchronous missions. For a given repeatability cycle, it is shown that longitude of the ground segment does not affect the access gaps. Simulation results show that increasing the latitude of ground segment reduces the longest accessibility gaps, especially in extreme latitudes near Polar Regions. To avoid polar ground segments due to their practical difficulties, mission architectures with two co-high-latitude ground segments are proposed. Selection of longitude distance between the two co-high-latitude ground segments is discussed to further reduce the longest accessibility gap in LEO sun-synchronous missions. To show the feasibility of the proposed approach, simulation results are included for illustration.

Abstract: The classical metrics of ground segment site location do not take account of the pattern of ground segment access to the satellite. In this paper, based on the pattern of access between the ground segment and the satellite, two metrics for site location of ground segments in Low Earth Orbits (LEO) missions were developed. The two developed access-based metrics are total accessibility duration and longest accessibility gap in a given period of time. It is shown that repeatability cycle is the minimum necessary time interval to study the steady behavior of the two proposed metrics. System and subsystem characteristics of the satellite represented by each of the metrics are discussed. Incorporation of the two proposed metrics, along with the classical ones, in the ground segment site location process results in financial saving in satellite development phase and reduces the minimum required level of in-orbit autonomy of the satellite. To show the effectiveness of the proposed metrics, simulation results are included for illustration.