White paper: Over the horizon - principles and challenges of operating in the HF band

Overview of OTH radar

Interest in OTH radar has recently surged due to the rise of atmospheric hypersonic weapons. These weapons fly at low altitudes and high speeds, making them difficult for conventional LoS radar to detect. OTH radar is also effective against aircraft and vessels that use stealth technology. Advances in digital signal processing, direct sampling and fully digital beamforming have also enhanced OTH radar capabilities, further driving this renewed interest.

OTH radar gets its name from its capability to detect targets beyond LoS. It does this using HF bands, specifically in the range of 3 MHz to 30 MHz. Traditional LoS radar operates within shorter distances limited by the Earth’s curvature. OTH radar, on the other hand, can reach over 4000 km. Its long effective range makes OTH radar useful for strategic early warning, missile tracking and detecting targets that evade microwave frequency radar.

Operating in the HF spectrum

OTH radar systems rely on the unique properties of the HF spectrum to detect targets beyond LoS. There are various modes of HF propagation, such as ground wave and sky wave. Sky wave propagation, in particular, involves reflecting HF signals off the ionosphere, allowing the signals to travel vast distances. The effectiveness of this process depends on ionosphere conditions, which are influenced by solar activity, time of day and other atmospheric factors.

The ionosphere’s different layers, especially D, E and F, play important roles in determining how HF signals travel. During the day, the D layer can absorb HF signals, reducing their range, while the E and F layers are more conducive to signal refraction. Understanding these layers and their behaviors is key to optimizing OTH radar performance.

Challenges of OTH radar operation

A primary challenge of OTH radar operation is ionosphere variability. The composition and density of the ionosphere change with solar activity, time of day and geographical location, affecting the propagation of HF signals crucial for OTH radar.

Other challenges are:

• Signal attenuation: HF signals used in OTH radar are prone to attenuation over long distances. This is due to factors such as absorption in the D layer during daylight hours.
• Noise: The HF spectrum is noisy, with interference from natural sources like lightning and cosmic noise, as well as man made sources.
• Resolution and accuracy: The long wavelength of HF signals necessitates physically large antenna arrays to achieve the narrow bandwidths for accurate target localization.
• Complex signal processing: OTH radar systems require sophisticated digital signal processing (DSP) techniques to handle weak signal returns and mitigate the effects of ionospheric scattering.
• Environmental factors: Environmental conditions such as weather, terrain and surface conductivity can affect OTH radar performance.

Ongoing advancements in DSP technology, antenna design and ionospheric modeling are improving the effectiveness of OTH radar systems. These advancements aim to enhance target detection capabilities and reduce the impact of environmental and ionospheric variability.