Robust Predefined-Time Stabilization of Quadrotor Altitude and Attitude by Bounded Periodic Delayed Feedback under Actuator Saturation
Ali Hosseini Lagha
Abstract Introduction: In many unmanned aerial missions, driving the tracking error to zero by an instant fixed in advance is an operational requirement rather than a convenience: the vehicle must be stabilised before it reaches a moving landing platform or leaves a hazard zone. Asymptotic control offers no such guarantee, and conventional predefined-time designs rely on time-varying gains that diverge as the settling instant is approached, which causes actuator saturation, amplification of measurement noise near the end of the interval, and the need to truncate or replace the control law at that instant.
Methods: This work presents a robust control scheme based on bounded periodic delayed feedback that stabilises quadrotor altitude and attitude simultaneously. Because all four controlled subsystems are second-order strict-feedback chains, the law is designed in two backstepping steps with a delay equal to one quarter of the predefined time, and four explicit control laws are obtained. The gain remains bounded throughout, switches itself off at the predefined instant, and requires no external switching. Beyond convergence, boundedness of all closed-loop signals, a pre-flight verifiable attitude-cone condition, an extension to unknown mass and unknown moments of inertia, and a bound on the residual band under sampling and input delay are proved
Design and Simulation of a Wideband Double-Ridged Horn Antenna for Use in Air Defense Surveillance and Electronic Warfare Systems
Ahmadreza Talebian, milad nosrati, Javad Ranjbar
Abstract Introduction
Wideband double-ridged horn antennas are key candidates for air defense and electronic warfare systems owing to their wide bandwidth and linear polarization. Achieving a desirable beamwidth, compact dimensions, and stable performance across a wide band simultaneously remains a fundamental challenge. This study addresses the design and simulation of a wideband double-ridged horn antenna with ellipsoidal elements near the ridges, aimed at broadening the beamwidth over the 2–18 GHz band.
Method
Adouble-ridged horn antenna with horizontal and vertical linear polarizations was designed and simulated. To achieve wideband performance, ellipsoidal elements were employed near the ridges. The fabricated antenna has dimensions of 80 × 70 × 63 mm³. The antenna performance was evaluated through the return loss diagram (S11), radiation pattern, gain, and half-power beamwidth (HPBW), and a measurement uncertainty analysis was also performed.
Results
As the frequency increased, the antenna gain improved despite the fixed aperture dimensions. The HPBW in the azimuth and elevation planes was 90° and 60°, respectively, with a gain of approximately 3.5 dB at the edges. The S11 value remained below −10 dB across the entire band. With a 16 GHz bandwidth, the antenna is considered compact, and an expanded uncertainty of ±0.62 dB confirmed the acceptable accuracy of the results. The XPD was measured at 30 dB.
Conclusion
Given its wideband characteristics, appropriate gain, compact dimensions, high XPD, and high reliability, this antenna is suitable for tactical links, surveillance radars, electronic warfare, and C4ISR networks, and can serve as a key candidate for air defense communication infrastructure. The compliance of its characteristics with operational requirements confirms the applicability of the proposed design.