Earth-based tracking of planetary encounter missions contains state vector uncertainties which may compromise the ability of the vehicle to control perifocal altitude to sufficient accuracies to effect the desired and/or safe orbital parameters. Using the Japanese PLANET B mission as an example of a planetary encounter requiring precise altitude control, it is the premise of this work to examine the use of a radar altimeter and a powerful, non-linear Kalman filter derived by the author to minimize the vertical estimation errors. It is shown that, not only is the altitude estimation error driven to acceptable values, but that the filter tightly couples the altitude estimation errors to the downtrack velocity errors thereby controlling orbital energy estimation as well. The result is precision control of the perifocal altitude estimation and the orbital energy estimation to the extent that aerodynamic braking is feasible as a replacement for propulsive deceleration resulting in a significantly increased payload capacity.
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