Wave radar
Encyclopedia
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
techniques. Several instruments based on a variety of different concepts and techniques are available to the user and these are all often called wave radars. This article (see also Grønlie 2004), gives a brief description of the most common ground-based radar remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals 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 waters with the presence of high currents making the mooring of a wave buoy enormously difficult. Another interesting case is a ship in transit where having instruments in the sea is highly impractical and interference from the ships hull must be avoided.
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 radial and azimuthal extent of the surface area to be illuminated by the radar) must be small in comparison with all ocean wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of interest. The radar spatial resolution is determined by the bandwidth of the radar signal (see radar signal characteristics
Radar signal characteristics
A radar system uses a radio frequency electromagnetic signal reflected from a target to determine information about that target. In any radar system, the signal transmitted and received will exhibit many of the characteristics described below....
)and the beamwidth of the radar antenna.
The beam of a microwave antenna is dispersive
Dispersive
*Dispersive partial differential equation*Dispersive phase from Biological dispersal*Dispersive medium*dispersive line*dispersive mass transfer*dispersive power*dispersive fading...
, consequently the resolution becomes a function of range. The beam of an IR
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
radar (laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
) is non dispersive, the radar footprint is therefore independent of range.
HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
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 wave
Gravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
s and surface ripple need not be present.
Radar transceivers may be coherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
or non-coherent. Coherent radars measure Doppler
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
-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 wave radar performance is highly dependent on
- 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).Laser altimeters
Laser altimeters are small and light weight and operate in the infra-red (IR) frequency band. They operate in vertical mode and normally use pulsed waveforms to perform direct measurements of sea surface elevation which easily can be converted to wave amplitude.Microwave range finders
Microwave range finders also operate in vertical mode at GHz frequencies and is not as affected by fog and water spray as the laser altimeter. A continuous wave frequency modulated (CWFM) or pulsed radar waveform is normally used to provide range resolution. The beam is dispersiveDispersive
*Dispersive partial differential equation*Dispersive phase from Biological dispersal*Dispersive medium*dispersive line*dispersive mass transfer*dispersive power*dispersive fading...
, hence the size of the footprint increases linearly with range.
One example of a microwave range finder is the Miros SM-094 which is designed for wave and water level (and tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
) measurements. This sensor is applied as air gap (bridge clearance) sensor in NOAAs PORTS system. Another example is the WaveRadar REX which is a derivative of a Rosemount tank radar.
Marine navigation radars
Marine navigation radars (X bandX band
The X band is a segment of the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of X band is rather indefinitely set at approximately 7.0 to 11.2 gigahertz . In radar engineering, the frequency range is specified...
) 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.
The range gated pulsed Doppler microwave radar
The range gated pulsed Doppler microwave radar operates the in low grazing angle mode. By using several antennas it may be used as a directional wave sensor, basically measuring the directional spectrum of the horizontal water particle velocity. The velocity spectrum is directly related to the wave height spectrum by a mathematical model based on linear wave theory and accurate measurements of the wave spectrum can be provided under most conditions. As measurements are taken at a distance from the platform on which it is mounted, the wave field is to a small degree disturbed by interference from the platform structure.Miros Wave and current radar is the only available wave sensor based on the range gated pulsed Doppler radar technique. This radar also uses the dual frequency technique (see below) to perform point measurements of the surface current vector
The dual frequency microwave radar
The dual frequency microwave radar transmits two microwave frequencies simultaneously. The frequency separation is chosen to give a “spatial beat” length which is in the range of the water waves of interest. The dual frequency radar may be considered a microwave equivalent of the high frequency (HF) radar (see below). The dual frequency radar is suitable for the measurement of surface current. As far as wave measurements are concerned, the back-scatter processes are too complicated (and not well understood) to allow useful measurement accuracy to be attained.The HF radar
The HF radar is well established as a powerful tool for sea current measurements up to a range of about 30 km. It operates in the MHz frequency band corresponding to a radar wavelength in the range of 10 to 300m. The Doppler shift of the first order Bragg lines of the radar echo is used to derive sea current estimates in very much the same way as for the dual frequency microwave radar. Two radar installations are normally required, looking at the same patch of the sea surface from different angles. CODAR Ocean Sensors (COS). The latest generation of shore-based ocean radar can reach more than 200 km for ocean current mapping and more than 100 km for wave measurements Helzel WERA. For all ocean radars, the accuracy in range is excellent. With shorter ranges, the range resolution gets finer. The angular resolution and accuracy depends on the used antenna array configuration and applied algorithms (direction finding or beam forming). The WERA system provides the option to use both techniques; the compact version with direction finding or the array type antenna system with beam forming methods.External links
Microwave range finders:- Physical Oceanographic Real-Time System (PORTS)
- NOAA Technical Report NOS CO-OPS 042; Microwave Air Gap-Bridge Clearance Sensor; Test, Evaluation, and Implementation Report
- ESEAS RI, Assessment of accuracy and operational properties of different tide gauge sensors.
- The Global Sea Level Observing System
- Volume IV of the IOC Manual on Sea Level Measurement and Interpretation
The range gated pulsed Doppler microwave radar:
X-band based wave sensors:
HF-Radar: