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Observing the oceans in coastal areas is essential for a number of environmental challenges such as climate and marine ecosystem monitoring or coastal hazard mitigation. It involves the measurement of key geophysical parameters that describe the dynamics of the sea surface, namely winds, waves and currents. Among the many remote sensing instruments that have been developed for this purpose, High-Frequency Radars (HFR) have the unique ability to provide synoptic and quasi-instantaneous maps of surface currents and, to a lesser extent, ocean wave parameters. They are deployed along the coasts and observe the sea at grazing incidence in the HF radio frequency band (3–30 MHz carrier frequency, 10–100 m radar wavelength). In this range of frequency and below, transmitted electromagnetic (EM) waves can be guided along the ocean surface well beyond the horizon following the Earth curvature, a phenomenon known as “ground wave” propagation. This remarkable property of radio waves at certain frequencies has been known since Marconi’s first experiments more than a century ago and was used to establish the first long-distance wireless communications. HFR take advantage of the ground wave mechanism to observe the sea surface over long distances, typically between 30 and 200 km offshore, depending on the radar frequency, with a spatial resolution of a few square kilometers. Due to these very efficient propagation characteristics and an adapted electromagnetic transmitted waveform, they consume little power and operate around the clock. They have a typical observation time of one hour, allowing processing of the data in near-real-time. They are “coherent on receive”, which means they can measure the phase shift of the backscattered echo and are therefore able to produce the ocean Doppler spectra which are used to characterize motion at the sea surface. Last but not least, they allow an unambiguous and accurate interpretation of the oceanic parameters since the physical mechanisms at play are fully described by an analytical electromagnetic and hydrodynamic theory.

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Introduction

  • 章节

  • 发表日期：

    2025年

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  • Charles-Antoine Guérin

摘要

Observing the oceans in coastal areas is essential for a number of environmental challenges such as climate and marine ecosystem monitoring or coastal hazard mitigation. It involves the measurement of key geophysical parameters that describe the dynamics of the sea surface, namely winds, waves and currents. Among the many remote sensing instruments that have been developed for this purpose, High-Frequency Radars (HFR) have the unique ability to provide synoptic and quasi-instantaneous maps of surface currents and, to a lesser extent, ocean wave parameters. They are deployed along the coasts and observe the sea at grazing incidence in the HF radio frequency band (3–30 MHz carrier frequency, 10–100 m radar wavelength). In this range of frequency and below, transmitted electromagnetic (EM) waves can be guided along the ocean surface well beyond the horizon following the Earth curvature, a phenomenon known as “ground wave” propagation. This remarkable property of radio waves at certain frequencies has been known since Marconi’s first experiments more than a century ago and was used to establish the first long-distance wireless communications. HFR take advantage of the ground wave mechanism to observe the sea surface over long distances, typically between 30 and 200 km offshore, depending on the radar frequency, with a spatial resolution of a few square kilometers. Due to these very efficient propagation characteristics and an adapted electromagnetic transmitted waveform, they consume little power and operate around the clock. They have a typical observation time of one hour, allowing processing of the data in near-real-time. They are “coherent on receive”, which means they can measure the phase shift of the backscattered echo and are therefore able to produce the ocean Doppler spectra which are used to characterize motion at the sea surface. Last but not least, they allow an unambiguous and accurate interpretation of the oceanic parameters since the physical mechanisms at play are fully described by an analytical electromagnetic and hydrodynamic theory.