Wave radar is a type of radar for measuring wind waves. Several instruments based on a variety of different concepts and techniques are available, and these are all often called. This article (see also Grønlie 2004), gives a brief description of the most common ground-based radar remote sensing techniques. Instruments based on radar remote sensing techniques have become of particular interest in applications where it is important to avoid direct contact with the water surface and avoid structural interference. A typical case is wave measurements from an offshore platform in deep water, where swift currents could make mooring a wave buoy enormously difficult. Another interesting case is a ship under way, where having instruments in the sea is highly impractical and interference from the ship's hull must be avoided.
Radar remote sensing
Terms and definitions Basically there are two different classes of radar remote sensors for ocean waves.
Direct sensor measures directly some relevant parameter of the wave system (like surface elevation or water particle velocity). Indirect sensors observe the surface waves via the interaction with some other physical process as for example the radar cross section of the sea surface. Microwave radars may be used in two different modes;
The near vertical mode. The radar echo is generated by specular reflections from the sea surface. The low grazing angle mode. The radar echo is generated by Bragg scattering, hence wind generated surface ripple (capillary waves) must be present. The backscattered signal will be modulated by the large surface gravity waves and the gravity wave information is derived from the modulation of the backscattered signal. An excellent presentation of the theories of microwave remote sensing of the sea surface is given by Plant and Shuler (1980). The radar footprint (the size of the surface area which is illuminated by the radar) must be small in comparison with all ocean wavelengths of interest. The radar spatial resolution is determined by the bandwidth of the radar signal (see radar signal characteristics) and the beamwidth of the radar antenna. The beam of a microwave antenna diverges. Consequently, the resolution decreases with increasing range. For all practical purposes, the beam of an IR radar (laser) does not diverge. Therefore, its resolution is independent of range. HF radars utilize the Bragg scattering mechanism and do always operate at very low grazing angles. Due to the low frequency of operation the radar waves are backscattered directly from the gravity waves and surface ripple need not be present. Radar transceivers may be coherent or non-coherent. Coherent radars measure Doppler-modulation as well as amplitude modulation, while non-coherent radars only measure amplitude modulation. Consequently, a non-coherent radar echo contains less information about the sea surface properties. Examples of non-coherent radars are conventional marine navigation radars.
The radar transmitter waveform may be either unmodulated continuous wave, modulated or pulsed. An unmodulated continuous wave radar has no range resolution, but can resolve targets on the basis of different velocity, while a modulated or pulsed radar can resolve echoes from different ranges. The radar waveform plays a very important role in radar theory (Plant and Shuler, 1980).
Factors influencing performance
Mode of operation or measurement geometry (vertical or grazing) Class of system (direct or indirect) Frequency of operation Radar waveform (unmodulated CW or modulated/pulsed) Type of transceiver (coherent or non-coherent) Radar antenna properties
Remote sensing techniques
An excellent survey of different radar techniques for remote sensing of waves is given by Tucker (1991).
Microwave rangefinders Microwave rangefinders also operate in vertical mode at GHz frequencies and are not as affected by fog and water spray as the laser rangefinder. A continuous wave frequency modulated (CWFM) or pulsed radar waveform is normally used to provide range resolution. Since the beam diverges, the linear size of the footprint is directly proportional to range, while the area of the footprint is proportional to the square of range. One example of a microwave range finder is the Miros SM-094, which is designed to measure waves and water level, including tides. This sensor is used as an air gap (bridge clearance) sensor in NOAA's PORTS system. Another example is the WaveRadar REX, which is a derivative of a Rosemount tank radar.
From data on the elevation of the surface of the water at three or more locations, a directional spectrum of wave height can be computed. The algorithm is similar to the one which generates a directional spectrum from data on heave (vertical motion), pitch and roll at a single location, as provided by a disc-shaped wave buoy. An array of three vertical radars, having footprints at the vertices of a horizontal, equilateral triangle, can provide the necessary data on water surface elevation. “Directional WaveGuide” is a commercial radar system based on this technique. It is available from the Dutch companies Enraf and Radac.
Marine navigation radars Marine navigation radars (X band) provide sea clutter images which contain a pattern resembling a sea wave pattern. By digitizing the radar video signal it can be processed by a digital computer. Sea surface parameters may be calculated on the basis of these digitized images. The marine navigation radar operates in low grazing angle mode and wind generated surface ripple must be present. The marine navigation radar is non-coherent and is a typical example of an indirect wave sensor, because there is no direct relation between wave height and radar back-scatter modulation amplitude. An empirical method of wave spectrum scaling is normally employed. Marine navigation radar based wave sensors are excellent tools for wave direction measurements. A marine navigation radar may also be a tool for surface current measurements. Point measurements of the current vector as well as current maps up to a distance of a few km can be provided (Gangeskar, 2002). Miros WAVEX has its main area of application as directional wave measurements from moving ships. Another example of a marine radar based system is OceanWaves WaMoS II.
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