## 1 Introduction
The control inputs of UAVs, as in all physical systems, are subjected to several constraints rising from the external environment, mechanical limitations, and energy restrictions. It is crucial to take these control constraints into account during the design and implementation of control systems. In practical scenarios, physical actuation limitations inevitably arise, introducing constraints in both the amplitude and the slew rate of the control input. These constraints can significantly impact the overall performance of the closed-loop system, potentially leading to undesired inaccuracies or even instability. A constrained nonlinear tracking controller for small fixed-wing kinematic UAVs was designed in [22]. The proposed algorithm involves the construction of control input that adhere to a predefined control Lyapunov function (CLF). From this set, a guidance law is selected to facilitate trajectory tracking. Moreover, the nested saturation technique to convert control input constraints into command angle constraints was presented in [23]. Recently, a backstepping control scheme providing trajectory tracking for an input constrained spacecraft was proposed in [24], exploiting a second-order anti-windup model. Owing to its inherent light-weight structure, the fixed-wing UAV is susceptible to the influence of external wind disturbances, which induce significant undesirable effects on trajectory tracking performance and may lead to instability. Thus, the disturbance rejection capability of fixed-wing UAV system is one of the most crucial factors to be considered when designing the controller. Methods to estimate the unknown external disturbances involve the utilization of \(L_{1}\) adaptive augmentation [25] and the disturbance observer based control (DOBC) approach [26; 27]. Thus, it is critical for the flight control scheme to be robust in order to achieve stable flights in realistic conditions. A robust nonlinear control technique introduced in [28] and further extended in approximation-free approach in [29], known as Prescribed Performance Control (PPC), guarantees the evolution of the tracking error within a user-prespecified performance envelope for bounded control input. In this vein, PPC is a robust control technique that imposes predefined transient and steady-state tracking characteristics to nonlinear systems. A PPC based control scheme for the trajectory tracking of small fixed-wing UAVs was recently presented in [30]. However, this algorithm does not take into account input constraints, which renders the practical implementation rather difficult, as any steady state perturbation may lead the system to singularity (the closed-loop signals become unbounded) as the saturated control effort fails to maintain the tracking error within the conventional prescribed envelope. Recently, input constrained issues have been addressed in [31; 32] within the approximation-free PPC framework, where an adaptive PPC (APPC) scheme counteracts input limitations by properly modifying the output constraints (i.e., the tracking error specifications), depending on the feasible control effort that can be applied to the plant. In this work, motivated by the above discussion, we aim at designing a novel robust scheme for the trajectory tracking control of the longitudinal motion of small fixed-wing UAVs subject to input and state constraints with guaranteed adaptive prescribed performance specifications.