Riccati Controlled Terminal Landing of Unpowered Lifting Vehicles

Abstract

A Riccati controlled guidance scheme is developed for guiding a spacecraft during an unpowered terminal phase of flight. This method differs from those published previously /1,2/ in that: (a) An onboard backward integration method is used to generate a vehicle nominal trajectory. This technique differs from others which integrate in the forward direction (Hoffman and Nadkarni) and as a result reduces the vehicle state errors inherent in these models. (b) A three segment nominal trajectory technique is employed which decreases the complexity present in the eight segment model of Nadkarni /1/, without reducing the capability of the model, (c) Additionally, time has been chosen as the independent variable rather than altitude. This reduces errors which occur in other schemes /1,2/ during periods of slowly changing altitude. Digital simulation has indicated that the terminal guidance scheme presented in this paper is capable of handling a variety of off-nominal conditions.