Real time trajectory optimization of the powered descent phase in Mars pinpoint landing is presented in this paper. The optimal soft landing trajectory in the presence of large initial condition dispersion and inequality constraints on thrust, orientation and path is generated in real time. The guidance algorithm proposed in this paper can converge from a wide range of initial conditions within the capability of the propulsion system (propellant available and thrust). A two level optimization scheme is adopted for solving the trajectory parameters and optimizing fuel consumption. In the outer loop, the required burn duration is iterated using a simple optimization procedure for minimizing fuel consumption. In the inner loop, the trajectory for a given burn time obtained from the outer loop, is solved using a modified Newton-Raphson scheme in the pseudo-spectral global collocation framework. An important advantage of this scheme is that, it does not use any general gradient-based optimization procedures which are time consuming. This makes it attractive for on-board implementation. The algorithm is least sensitive to initial guess values while exhibiting very good convergence properties. The computation cost for solving the trajectory equations is also minimal in terms of optimization time. It is tested for robustness with respect to both initial guess and initial condition dispersion in the Monte Carlo sense.
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