High frequency surface wave radar (HFSWR) is a type of radar system that operates in the high frequency (HF) band of the electromagnetic spectrum, typically ranging from 3 MHz to 30 MHz. Unlike traditional radar systems that rely on backscatter from objects in the atmosphere or space, HFSWR primarily uses surface waves that propagate along the boundary between the Earth’s surface and the atmosphere ( called the ionosphere). These surface waves can travel long distances (hundreds of kilometers) and are capable of detecting and tracking targets such as ships and aircraft at low altitudes over large sea areas. HFSWR systems are advantageous for maritime surveillance and coastal surveillance due to their ability to provide wide-area coverage and detect targets beyond the horizon, where line-of-sight radar systems would be limited.
Low frequency radar operates at frequencies typically below 1 GHz, including the very low frequency (VLF) and ultra low frequency (ULF) bands. Unlike higher frequency radar systems that rely on the reflection and scattering of electromagnetic waves from objects in the atmosphere or space, low frequency radar exploits the propagation characteristics of long wavelengths. These radar systems are capable of penetrating various atmospheric conditions and solid obstacles such as foliage or buildings, making them suitable for ground penetrating radar applications, subsurface imaging and environmental sensing. Low-frequency radar systems generally require larger antennas and are less sensitive to atmospheric attenuation, making them effective for long-range detection and surveillance in harsh environments where higher frequency radar may be limited .