Nonlinear trajectory control using drag-to-altitude transformation for entry guidance

Abstract

Existing entry guidance systems are based on the common concept of tracking a reference drag profile. For that purpose, trajectory control laws are incorporated into the guidance systems. Nonlinear trajectory control laws have been developed recently, while a linear control law has been employed for the Space Shuttles successfully. This paper proposes a new nonlinear control law using a device called the altitude-to-drag transformation. In the control law, the altitude is considered as the controlled variable instead of the drag force. The control law is easier to understand than other nonlinear approaches, since it has a natural structure based on the equation of motion with respect to the altitude. A numerical comparison through flight simulations shows that the proposed control law has satisfactory performance and robustness and it can be used as a superior substitute of such a linear control law as used by the Space Shuttle.