  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
drag-tracking error 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 show the efficiency of the presented
approach.
