We present a framework for on-board trajectory planning and guidance for a large class of autonomously guided parafoils. The problem is for the parafoil to reach a given location at a specified altitude with a specified final heading. Through appropriate change of the independent variable, the trajectory planning problem is converted from a three-dimensional free-final time problem to a two-dimensional fixed-final time problem. Using the well-known Dubins path synthesis and known parafoil performance parameters a concept of altitude margin is developed as a quantitative measure of the available maneuvering energy for use in trajectory planning. A hybrid strategy using two methods to generate kinematically feasible fixed-time trajectories is presented, each targeting different range of initial values of the altitude margin. The trajectory can be re-planned on-board in every guidance cycle, making the guidance effectively closed loop, or re-planned whenever the deviation of the actual condition from the reference trajectory exceeds a threshold. The proposed planning and guidance algorithm applies to a large class of parafoil canopies and payloads which encompasses wide variations in the lift-to-drag ratio, wing loading, and maximum turn rate. The guidance logic requires no tuning to accommodate variations in canopy performance. Monte Carlo simulations are conducted to evaluate the effectiveness of the algorithm with dispersions in canopy performance, loading, wind profile errors, navigation uncertainty, using lateral control only and with both longitudinal and lateral control.
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