Florent Jangal

Wavelet Contribution to Remote Sensing of the Sea and Target Detection for a High-Frequency Surface Wave Radar

High-frequency waves (3-30 MHz) interact with the sea surface. Thus, a high-frequency surface wave radar (HFSWR) is well suited to perform remote sensing of the sea. The HFSWR coverage range is not limited by the radio horizon: it is possible to keep watch over the sea up to a few hundred kilometers from the coastline. Oceanographic parameters (i.e., wave height, surface current velocity, wind direction, and wind velocity) are derived from the so-called sea spectrum. Moreover, the HFSWR can be used for maritime surveillance of the Exclusive Economic Zone. In that case, the sea spectrum is an unwanted signal because it can mask targets. Sea spectrum extraction is an important issue for HFSWR signal processing since it is a key point for the remote sensing accuracy and the target detection features. In this letter, we show how wavelets may be applied to improve the remote sensing of oceanographic parameters and the target detection using wavelet-based sea clutter extraction. The results obtained using real data with opportune targets validate our approach.

Negative Permittivity Media Able to Propagate a Surface Wave

In the fleld of High Frequency Surface Wave Radar (HFSWR), this paper deals with a study which determines the electric permittivity and conductivity values that a medium must hold to propagate a sole surface wave at its interface with air. Firstly, we demonstrate clearly the reason why the Zenneck Wave cannot be excited on sea surface. Kistovich decomposition is used for this purpose. Secondly, the reasoning is extended to identify electric permittivity and conductivity values that permit to excite a surface wave on an homogeneous medium. Finally, numerical validation is obtained by comparison with the analytic formulation of the fleld radiated by a vertical Hertzian dipole as it has been established by Norton.

Towards a better excitation of the surface wave

HF radar and communication systems take advantage of surface wave propagation at the interface of the sea to overcome the radioelectric horizon limitation. The paper reviews electromagnetic fields excited by a vertical electric dipole over both a flat and a spherical Earth in order to focus on the surface wave. In a first time, classical Norton and Zenneck results are discussed and compared with a modal decomposition technique for a plane interface. Secondly, the field on a spherical Earth is considered. Fock formulation is applied in the last part of this communication. Even if the expressions of these fields are not similar, the results are in good agreement. This make the way for analytic isolation of surface wave contribution whatever the antenna design.

Wavelets: a Versatile Tool for the High Frequency Surface Wave Radar

Widely used in image processing, wavelets seem to be a promising versatile tool for the high frequency surface wave radar (HFSWR). HFSWR is based on the ability of HF waves (3 MHz to 30 MHz) to propagate along the earth curvature. HFSWR can detect targets up to few hundred kilometers beyond the horizon. The two main applications of the HFSWR are the maritime surveillance of the exclusive economic zone (EEZ) and the remote sensing of the sea. Beside the classical noise which affects all radar signals, the detection capabilities of HFSWR suffer from two limitations: the sea clutter and the ionospheric clutter which are target masks. However sea clutter can be used to perform remote sensing of the sea. In this paper we are studying how wavelets may contribute to the improvement of radar signal processing. We consider general de-noising feature for radar signals, we carry out a wavelet-based improvement of remote sensing of oceanographic parameters and we show the first results of a wavelet-based processing for ionospheric clutter mitigation.

Ionospheric clutter cancellation and wavelet analysis

A new approach based on wavelet analysis to mitigate ionospheric clutter from the high frequency surface wave radar Doppler/range image is presented in this paper.

Near field to far field transformation by using equivalent sources in HF band

Initially designed for long distance communications, HF band (3-30 MHz) antennas are nowadays mainly used in over-the-horizon radar applications. The performances of such devices depend on the knowledge of the electromagnetic field radiated by the antennas. The “sky wave” and “surface wave” modes radiated by these HF antennas are drastically influenced by their environment and particularly by the electromagnetic properties of the ground upon which they are located. The conventional methods of radiation pattern computation, like full-scale radiation pattern measurement and mode expansion of the near field measured in an anechoic chamber, are, in that case, neither convenient nor suitable, respectively. Thus, this paper describes a new way to perform the near field to far field transformation by replacing the antenna under test by a set of equivalent sources for which the ground is taking into account.

Towards a better excitation of the surface wave: Electromagnetic field on the sea surface

HF radar and communication systems take advantage of Surface Wave propagation at the interface of the sea to overcome the radioelectric horizon limitation. The paper reviews electromagnetic fields excited by a vertical electric dipole over both a flat and a spherical Earth in order to focus on the Surface Wave. In a first time, classical Norton and Zenneck results are discussed and compared with a modal decomposition technique for a plane interface. Secondly, the field on a spherical Earth is considered. Fock formulation is applied in the last part of this communication. Even if the expressions of these fields are not similar, the results are in good agreement. This make the way for analytic isolation of Surface Wave contribution whatever the antenna design.

Observation of Zenneck-Like Waves over a Metasurface Designed for Launching HF Radar Surface Wave,

Since the beginning of the 20th century a controversy has been continuously revived about the existence of the Zenneck Wave. This wave is a theoretical solution of Maxwell’s equations and might be propagated along the interface between the air and a dielectric medium. The expected weak attenuation at large distance explains the constant interest for this wave. Notably in the High Frequency band such a wave had been thought as a key point to reduce the high attenuation observed in High Frequency Surface Wave Radar. Despite many works on that topic and various experiments attempted during one century, there is still an alternation of statements between its existence and its nonexistence. We report here an experiment done during the optimisation of the transmitting antennas for Surface Wave Radars. Using an infrared method, we visualize a wave having the structure described by Zenneck above a metasurface located on a dielectric slab.

Behaviour of antenna loaded with periodic structures in the High Frequency band: HF metamaterials

In the High Frequency band, antennas for Over-The-Horizon (OTH) applications suffer from the uncontrolled radiation of surface waves. We propose here to load antennas with metamaterial in order to control surface waves. As a consequence, the energy wasted in unwanted propagation mode is reduced and the coverage of radars or communication systems using those antennas is improved.

Remote Sensing of the Sea and Target Detection Improvement Using a Wavelet-based Extraction of Sea Echoes from High Frequency Radars

High frequency radars (HFR) use HF waves (3 MHz to 30 MHz). They interact with the sea surface and are well-suited radars to perform remote sensing of the sea. Moreover, HFR coverage range is not limited by the radio horizon: it is possible to keep watch over the sea up to few hundred kilometers from the coast line (surface wave mode) or up to few thousands kilometers (space wave mode). Oceanographic parameters (i.e. wave height, surface current velocity, wind direction and wind velocity) are derived from the so-called sea spectrum. Moreover, HFR can be used for maritime surveillance of the Exclusive Economic Zone (EEZ). In that case, the sea spectrum is an unwanted signal because it can mask targets. So, sea spectrum extraction is an important topic of HFR signal processing since it is a key point for remote sensing accuracy and target detection features. Wavelet-based processing allows an efficient extraction.