Drag-tracking guidance for entry vehicles without drag rate measurement

A robust entry guidance law without drag rate measurement is designed for drag-tracking in this paper. The bank angle is regarded as the control variable. First, a state feedback guidance law (bank angle magnitude) that requires the drag and its rate as feedback information is designed to make the system with the drag-tracking error and its rate as states be input-to-state stable (ISS) with respect to uncertainties. Then a high-gain observer is utilized to estimate the drag rate which is difficult for a vehicle to measure accurately in practice. Stability analysis as well as simulation results shows the effectiveness of the presented approach.