# Observer-Based Drag-Tracking Guidance for Entry Vehicles Considering Input Saturation Constraint

## TL;DR

This paper introduces a novel observer-based guidance law for entry vehicles that effectively manages input saturation and uncertainties, ensuring accurate drag-tracking through a high-gain observer and Nussbaum function.

## Contribution

It proposes a new guidance law using a Nussbaum function and high-gain observer to handle input saturation and complex uncertainties in entry vehicle guidance.

## Key findings

- The guidance law achieves near-zero drag-tracking error despite uncertainties.
- The method effectively handles uncertainties depending on drag error and its integral.
- Monte Carlo simulations validate the approach's robustness and effectiveness.

## Abstract

This paper studies the drag-tracking guidance design problem of uncertain entry vehicles. With employing a Nussbaum type function to deal with input saturation constraint, an output feedback guidance law (bank angle magnitude) with a high-gain observer is constructed that makes the drag-tracking error converge near zero in the presence of uncertainties. It is also worthy to claim that, in contrast to the existing results whose envelope of uncertainty merely depends on the drag error, the considered uncertainty is allowed to be not bigger than a function of drag error and integral term of drag error, which inevitably occurs in practice. The Monte Carlo simulation is done to illustrate the advantage of the developed method.