HF radar in marine hydrography

This article explains how ground-based High Frequency (HF) radar is used in hydrography as a remote sensing technique for mapping surface currents, ocean waves and wind direction over large sea areas. Radar techniques operating at microwave frequencies are described in the companion article X-band radar in marine hydrography.

Introduction

Radar remote sensing in Earth observation does not only cover the global and regional survey of geophysical parameters by satellite- or airborne radars, it also includes local observations by ground-based radar techniques. Due to their continuous measurement capabilities, the information provided by ground-based instruments installed at the coast or on offshore platforms does not suffer from the episodic character of the satellite- or airborne radar observations. In contrast to the satellite systems, the incident angle of ground-based radars is only a few degrees and near to zero (grazing) at far ranges. In this case, the working range of the radar strongly depends on the operating frequency:

• Decameter wave radars, also known as High Frequency (HF) radars, make use of ground wave propagation far beyond the horizon. HF radars provide a spatial resolution of one to several kilometers over working ranges of tens to more than 200 km working range.

• Microwave radars, also called X-band radars, are limited by line-of-sight propagation to the horizon. Their spatial radial resolution can be as fine as 5 m at 5 km working range.

The development of HF radars in hydrography started in 1955, when Crombie discovered that electromagnetic waves in the HF band were interacting with the ocean surface due to Bragg resonant scattering. The first radar for mapping of ocean surface currents was described by Barrick et al. (1977). Stewart and Joy (1974) observed that there are often small frequency differences between the observed Doppler shift of the backscattered radio waves and the Doppler shift predicted from surface gravity waves moving over still water. They showed that the observed differences resulted from near-surface ocean currents and that current speeds toward or away from the radars could be estimated.

A comprehensive overview on the use of radar in general and its use in geophysics is given by Paduan and Washburn (2013). In Europe, over 50 HF-radar systems were operational in 2017 (94 in 2024) to support coastal ocean forecasting, maritime safety, pollution-drift prediction and environmental monitoring.

Principles of HF radar

Figure 1. Successive reflections of an electromagnetic signal by a surface wave are in phase if the wavelength of the emitted signal is twice the wavelength of the surface wave. This positive interference is called 'Bragg scattering'. Image from Middleditch (2006).

Coastal HF radar systems operate in the frequency range 3–30 MHz. They exploit ground-wave propagation along the electrically conducting sea surface, allowing observations beyond the geometric horizon. Other types of HF radar use ionospheric reflection (sky-wave propagation) for much longer-range observations, but these are not considered here. The backscattered signal results from constructive interference of the electromagnetic radar wave with ocean surface waves of half the electromagnetic wavelength, a mechanism known as Bragg scattering (see Fig. 1). HF radars thus probe surface waves of relatively short length (a few tens of meters) which are travelling in the ocean as deep-water gravity waves. Coastal HF radar systems typically provide current measurements over distances of tens to more than 200 km, with spatial resolutions of approximately 1 to several kilometers, depending on radar frequency, configuration and operating conditions. The University of Hamburg developed a ground wave HF radar for coastal applications called WEllen RAdar (WERA), Gurgel et al. (1999). Fig. 2 shows a typical example of an antenna installation set up for a WERA radar. A typical backscatter spectrum from a selected patch of the ocean at a given range and angle is shown in Fig. 3: Two strong first-order Bragg peaks can be identified, which are generated by a resonant backscattering of the electromagnetic wave due to ocean waves of half the electromagnetic wavelength, e.g. 20 m long electromagnetic waves are scattered back by 10 m long ocean waves.

Figure 2: Typical example of an antenna installation for WERA radar.

Figure 3: Typical backscatter spectrum of sea echoes, normalized to the strongest signal measured by WERA. Two first-order Bragg peaks and the second-order sidebands (grey areas around the first-order peaks) can be seen. The two vertical grey lines mark the expected location of the first-order Bragg peaks in the absence of surface currents.

The angular frequency [math]\displaystyle{ \omega }[/math] and the wave vector [math]\displaystyle{ \vec{k} }[/math] (with components [math]\displaystyle{ k_x, k_y }[/math]) of the resonant ocean gravity waves obey the dispersion relationship

[math]\displaystyle{ \omega = \vec{k}.\vec{u}+\sqrt{gk \tanh(kd)}  ,  \qquad (1) }[/math]

where [math]\displaystyle{ g }[/math]  is the acceleration due to gravity, [math]\displaystyle{ k=|\vec{k}|=2 \pi/ \lambda }[/math] is the wave number, [math]\displaystyle{ \lambda }[/math] is the wavelength, [math]\displaystyle{ d }[/math] is the water depth and [math]\displaystyle{ \vec{u} }[/math] is the surface current velocity. The term [math]\displaystyle{ \tanh(kd) }[/math] which accounts for finite-depth effects, approximates 1 in sufficiently deep water, but must be considered when the Bragg-resonant wavelength is large relative to the local water depth.

As there are ocean waves travelling towards and away from the radar, two strong first-order Bragg peaks can be observed. The first-order Bragg peaks occur at Doppler frequencies corresponding to the propagation velocities of resonant ocean waves travelling towards and away from the radar. An underlying surface current causes an additional Doppler shift, which can be used to determine the radial current velocity. By changing the radar operating frequency, a specific ocean wavelength can be selected. The first-order Bragg peaks are accompanied by weaker second-order spectral contributions arising from nonlinear scattering interactions with the ocean wave field. These second-order sidebands contain information about the directional wave spectrum, see Fig. 3.

Applications and examples of HF radar

This section describes how HF radar is used for the measurement of maps of surface currents, ocean wave spectra and wind direction.

Measurement of surface current maps

Figure 4: Example of a surface velocity field measured by HF radar off the coast of Brest (France).

If an ocean surface current is transporting the Bragg resonant waves (cf. the two first-order Bragg peaks in Fig. 3) the first-order Bragg peaks are shifted in frequency, as indicated in the dispersion relationship (Eq. 1). This shift moves both peaks by the same amount and to the same direction and its amount can be used to calculate the radial component of the surface current. The current velocity derived from the Bragg peak displacement represents a depth-weighted average of the near-surface current. The contribution from deeper layers decreases approximately exponentially, with a characteristic depth depending on the wavelength of the Bragg-resonant ocean waves. For a radar frequency of 15 MHz, the current measurement is mainly sensitive to the upper meter of the water column.

A single HF radar measures only the radial component of the current. To reconstruct the horizontal current vector, radial measurements from at least two different viewing directions are generally combined. This is usually achieved with two or more radar stations sufficiently far apart whose observation areas overlap. For accurate vector reconstruction, the angle between radial viewing directions should preferably lie between approximately 30° and 150°. Fig. 4 shows an example of a surface velocity field measured off Brest, France.

As the surface current vector is calculated from two radial components, the resulting accuracy depends on the errors of the radial components themselves and on the angle between them. An angle of 90° between the radial components leads to the lowest errors, while angles around 0° or 180° dramatically increase the error orthogonal to the radial components. This degradation of accuracy is also known from the Global Positioning System (GPS) and is called Geometrical Dilution Of Precision (GDOP). To measure the accuracy, several comparisons between currents measured by current meters or ADCPs and HF radar have been made. The total RMS difference between these instruments is in the range 5-15 cm/s (Essen et al. 2000, Graber et al. 1997). Note that the two systems measure different things: HF radar currents represent averages over spatial resolution cells whose dimensions depend on the radar configuration and processing, whereas current meters and ADCPs provide more localized measurements. Differences between these observation scales, as well as differences in measurement depth, contribute to discrepancies between radar-derived and in situ currents. Large differences can occur in areas with strong spatial current gradients, particularly near coastal boundaries, tidal inlets and complex bathymetry.

It is important to keep in mind the following points about HF radar-derived currents: (1) the effective depth of the measurement depends on the depth of influence of the Bragg waves and is quite shallow (order of 1 m), (2) stable estimates require scattering from hundreds of wave crests plus ensemble averaging of the spectral returns, which sets the space-time resolution of the instruments, (3) the precision is limited by the frequency resolution of the Doppler spectrum and is typically 2-5 cm s^–1, and (4) the accuracy is controlled by numerous factors, such as signal-to-noise ratios, pointing errors, and geometry.

Measurement of ocean wave spectra maps

Figure 5: Wave field measured by HF radar off the coast of Brest, using the empirical algorithm. The colour scale represents the significant wave height, the arrows show the mean wave direction. The patches of irregular high sea state around Ushant island are due to strong local current variability resulting in multiple Bragg peaks, which makes it difficult to identify the second-order sidebands.

The second-order sidebands seen in Fig. 3 contain information about the ocean wave directional spectrum. Note that these sidebands may be hidden by stronger first-order Bragg peaks shifted by a different current velocity from other directions in case of non-directive receiving antennas. Unlike current measurements, which rely mainly on the displacement of the first-order Bragg peaks, retrieval of the directional wave spectrum requires inversion of the second-order backscatter spectrum. This inversion is more sensitive to measurement noise and assumptions about the wave field.

There are two kinds of algorithms to derive the ocean wave spectra from the second-order sidebands: Most are based on the inversion of the Barrick-Weber equations (Barrick et al. 1977), for example the method developed by Wyatt (2000) which allows resolving bi-modal sea states. Inversion methods based on second-order scattering theory become less reliable in energetic sea states, particularly when nonlinear scattering effects and overlap between spectral contributions make identification of the second-order sidebands difficult. The other kind of algorithms is based on an empirical approach, i.e. finding calibration factors by linear regression. The algorithm described by Gurgel et al. (2006) falls into this category. Fig. 5 shows an example of a wave field off the coast of Brest, France, obtained with this method.

At high sea states, increasing overlap between first-order Bragg peaks and second-order spectral contributions makes wave-spectrum inversion unreliable. Due to this problem, both algorithms show saturation effects: Illustrative limits reported for particular retrieval methods are [math]\displaystyle{ H_s \sim }[/math]7.5 m at 27 MHz radar frequency and [math]\displaystyle{ H_s \sim }[/math]16 m at 12.5 MHz. These values are not universal, as the limits also depend on radar characteristics, sea state and inversion method. Lower radar frequencies generally permit measurements under more energetic wave conditions, but may be less sensitive to small waves. Wave retrieval may also be unreliable at low sea states when the second-order backscatter signal becomes too weak relative to background noise.

There are several papers discussing the accuracy of HF radar wave measurements. An extensive comparison has been published by Wyatt et al. (2003) within the EuroROSE (European Radar Ocean Sensing, EU Mast-3, CT98-0168) project.

Measurement of wind direction maps

Figure 6: Example of wind direction measured by HF radar off the coast of Brest (France), derived from the ratio of the first-order Bragg peaks.

The relative strength of the two first-order Bragg peaks provides information on the directional distribution of the resonant ocean waves. When these waves are primarily generated by the local wind, their propagation direction can be used to estimate wind direction. The reliability of this estimate depends on radar frequency and the prevailing wave conditions. The wind-induced ocean waves follow changes in the wind direction with some delay (often order 10-15 min, depending on wind conditions and wave characteristics). Fig. 6 shows an example of the wind direction measured off the coast of Brest, France.

The algorithm to calculate the wind direction consists of fitting the ratio of the amplitudes of the two first-order Bragg peaks to a [math]\displaystyle{ \large\cos^s(0.5 \theta) }[/math] or [math]\displaystyle{ \large sech^s(\theta) }[/math] angular distribution model (Gurgel et al. 2006). The angle [math]\displaystyle{ \theta }[/math] is defined as the angle relative to the direction of maximum wave energy, presumed to coincide with the local wind direction. Because a single Bragg peak ratio does not uniquely determine wind direction, observations from different viewing directions should be combined, generally provided by two or more radar stations. The same multi-station configuration is commonly used to reconstruct horizontal current vectors.

Wind speed is more difficult to retrieve because the energy of the Bragg-resonant waves does not increase proportionally with wind speed, particularly at higher wind speeds. Nonlinear wave interactions and dissipation limit the growth of these short waves. The strength of the first-order Bragg peaks alone is therefore insufficient for reliable wind speed estimation.

Operational applications

Coastal HF radar networks provide near-real-time maps of near-surface currents, commonly at hourly intervals. These observations are used for monitoring coastal circulation, supporting search-and-rescue operations and predicting the drift of floating pollutants. They also provide information for validating and assimilating currents in operational ocean circulation models. In Europe, HF-radar observations from several regional networks are integrated and distributed through the European HF Radar Node and Copernicus Marine services.

Related articles

• X-band radar in marine hydrography

• Coastal meteorology

• Currents and turbulence by acoustic methods

• Statistical description of wave parameters

References

This article has been written by: Klaus-Werner Gurgel and Friedwart Ziemer

The main author of this article is Ziemer, Friedwart Please note that others may also have edited the contents of this article. Citation: Ziemer, Friedwart (2026): HF radar in marine hydrography. Available from http://www.coastalwiki.org/wiki/HF_radar_in_marine_hydrography [accessed on 14-11-2026] For other articles by this author see Category:Articles by Ziemer, Friedwart For an overview of contributions by this author see Special:Contributions/Ziemer, Friedwart

Article reviewed by

Job DronkersSee the discussion page