A Review of Over-the-Horizon Radar Systems: Global Perspectives on Design, Operational Principles, and Performance

This paper presents a comprehensive global review of over-the-horizon radar (OTHR) systems, spanning both skywave and surface-wave designs across operational, research, and historical contexts. The review integrates technical descriptions of system architectures, operating frequency ranges, waveform designs, signal processing methods, and performance characteristics, supported by corrected historical attributions and consolidated references. Major operational systems-including JORN, ROTHR, Nostradamus, the Russian 29B6 ”Container,” and Iran’s Sepehr-are examined alongside research networks such as SuperDARN and large-scale high-frequency surface-wave radar arrays used for oceanographic monitoring. Historical and retired systems, including the Soviet Duga, Cobra Mist, and the U.S. OTH-B network, are analyzed for the technical lessons they provide regarding antenna siting and noise suppression. A comparative table summarizes key parameters across all systems, offering a unified reference framework. Applications in defense surveillance, maritime domain awareness, environmental monitoring, and ionospheric research are outlined. Furthermore, this survey highlights emerging trends critical to the next generation of OTHR, such as multi-frequency agility for ionospheric resilience, machine learning-based clutter suppression, and the integration of spaceborne or UAV-borne sensors for enhanced data fusion. By linking historical technical challenges with modern adaptive signal processing and compact deployable architectures, this survey serves as both a strategic technical reference and a forward-looking roadmap for the continued evolution of high-frequency radar technology.