Onboard computation of a fuel-optimal steering law is a pivotal technology for future lunar and planetary campaigns with pinpoint landings. This paper proposes a method of downrange estimation based on improved powered explicit guidance (PEG) focusing on the existing throttleable engine. The main contribution includes two aspects. First, the conventional PEG algorithm is improved to enforce prediction accuracy. The prediction of lander cutoff state can be used to estimate downrange position, and thus, the thrust switching time is corrected to recalculate optimal steering law. Second, in consideration of the thrust identification error and navigation uncertainty, the thrust switching condition triggered by the state of the Lander instead of time is established. The thrust is switched depending on estimated downrange feedback, and thus, a pinpoint landing with near-optimal fuel is achieved. The effectiveness of the proposed algorithm are demonstrated through Monte Carlo simulations in the presence of thrust deviation and navigation error. (C) 2021 American Society of Civil Engineers.
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