Terminal guidance for soft and accurate lunar landing for unmanned spacecraft.

Soft, precise landings of unmanned lunar spacecraft may be needed for logistic support of future manned missions. The terminal descent guidance concept proposed in this paper utilizes proportional navigation plus logarithmic deceleration in the initial guided phase of flight and switches at a predetermined range to an unguided gravity turn, thereby achieving a soft landing with vertical attitude at touchdown. Guidance simulation studies have shown this concept to be effective in reducing the target miss distance resulting from dispersions in initial flight conditions caused by uncertainties in retro burnout and errors in instrument measurement and command execution. With the present technology and available spaceflight-proven instruments, this guidance concept can be easily implemented without exceeding practical design limitations. Moreover, it requires no knowledge of the local vertical throughout the terminal phase of flight. A landing accuracy of 500-ft circular error probability is achievable.