Wind waves can be measured by several radar remote sensing techniques. Several instruments based on a variety of different concepts and techniques are available, 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 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.
Basically there are two different classes of radar remote sensors for ocean waves.
Microwave radars may be used in two different modes;
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).
An excellent survey of different radar techniques for remote sensing of waves is given by Tucker (1991).
Microwave range finders also operate in vertical mode at GHz frequencies and are 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. 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 (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.
The range gated pulsed Doppler microwave radar operates 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 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 CODAR SeaSonde and Helzel WERA are well established as a powerful tool for sea current measurements up to a range of 300 km. It operates in the HF and low VHF frequencies 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. 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.
The FutureWaves technology was originally developed as an Environmental Ship and Motion Forecasting (ESMF) system for the Navy's ONR (Office of Naval Research) by General Dynamics' Applied Physical Sciences Corporation. The technology was adapted to be released in the commercial market and made its first public appearance at the 2017 Offshore Technology Conference in Houston Texas.
This technology differs from existing wave forecasting systems by using a customized wave sensing radar capable of measuring backscatter Doppler out to ranges of approximately 5 km. The radar antenna is vertically polarized to enhance the sea-surface backscatter signal. It also uses an innovative radar signal processing scheme that addresses the inherently noisy backscatter signals through a mathematical process termed least squares inversion. This approach applies a highly over-determined filter to the radar data, and rejects radar scans that do not observe incoming waves. The result is an accurate representation of the propagating incident wave field that will force ship motions over a 2-3 minute window. The wave processing algorithms also enable real-time calculation of wave field two-dimensional power spectra and significant wave height similar to that provided by a wave buoy.
It also uses a vessel motion prediction process that relies on a pre-calculated force/response database. Dynamic motional degrees of freedom are then represented as a lumped mechanical system whose future motions are predicted by numerically solving a multi-degree-of-freedom, forced, coupled differential equation with initial inertial state provided by vessel motion sensor outputs. The time-domain solution allows for nonlinear forcing mechanisms, such as quadratic roll damping and roll control systems, to be captured in the forecasting.
Finally, it uses the Gravity open architecture middleware solution to integrate the sensor feeds, processing subroutines and user displays. This open architecture approach will allow users to implement customized operator displays along with physics based models of specific vessels and machinery (e.g. cranes) into the system.
Microwave range finders:
The range gated pulsed Doppler microwave radar:
X-band based wave sensors:
The AN/AWG-9 and AN/APG-71 radars are all-weather, multi-mode X band pulse-Doppler radar systems used in the F-14 Tomcat, and also tested on TA-3B. It is a very long-range air-to-air system with the capability of guiding several AIM-54 Phoenix or AIM-120 AMRAAM missiles at the same time using its track while scan mode. The primary difference between the AWG-9 and APG-71 is the replacement of the former's analog computer with all-digital computer. Both the AWG-9 and APG-71 were designed and manufactured by Hughes Aircraft; contractor support is now being provided by Raytheon. The AWG-9 was originally developed for the failed naval F-111B program.The AN/AWG-9 offers a variety of air-to-air modes including long-range continuous-wave radar velocity search, range-while-search at shorter ranges, and the first use of an airborne track-while-scan mode with the ability to track up to 24 airborne targets, display 18 of them on the cockpit displays, and launch against 6 of them at the same time. This function was originally designed to allow the Tomcat to shoot down formations of bombers at long range.AN/SPG-51
The AN/SPG-51 is an American tracking / illumination fire-control radar for RIM-24 Tartar and RIM-66 Standard missiles. It is used for target tracking and Surface-to-air missile guidance on Virginia-class cruisers, California-class cruisers, and Kidd-class destroyers.
The Italian Navy used it aboard their Audace class, Durand de la Penne class and Impavido-class destroyers. The French Cassard class, Royal Netherlands Navy Tromp class and Spanish Baleares class frigates also use this system.
Older variants were used on Charles F. Adams-class destroyers, as well as the related German Lütjens class and Perth class used by the Royal Australian Navy.
Aircraft tracking is based on monopulse radar utilizing Pulse-Doppler radar signal processing in MK 74 MOD 14 and MK 74 MOD 15. The MK 74 MOD 15 configuration includes continuous-wave radar tracking in addition to pulse-Doppler tracking. It provides illumination for bistatic radar operation associated with missile guidance in all configurations. Older systems rely on conical scanning rather than monopulse.Continuous-wave radar
Continuous-wave radar is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. Continuous-wave (CW) radar uses Doppler, which renders the radar immune to interference from large stationary objects and slow moving clutter.
CW radar systems are used at both ends of the range spectrum.
Inexpensive radio-altimeters, proximity sensors and sport accessories that operate from a few dozen feet to several kilometers
Costly early-warning CW angle track (CWAT) radar operating beyond 100 km for use with surface-to-air missile systemsDielectric resonator antenna
A dielectric resonator antenna (DRA) is a radio antenna mostly used at microwave frequencies and higher, that consists of a block of ceramic material of various shapes, the dielectric resonator, mounted on a metal surface, a ground plane. Radio waves are introduced into the inside of the resonator material from the transmitter circuit and bounce back and forth between the resonator walls, forming standing waves. The walls of the resonator are partially transparent to radio waves, allowing the radio power to radiate into space.An advantage of dielectric resonator antennas is they lack metal parts, which become lossy at high frequencies, dissipating energy. So these antennas can have lower losses and be more efficient than metal antennas at high microwave and millimeter wave frequencies. Dielectric waveguide antennas are used in some compact portable wireless devices, and military millimeter-wave radar equipment. The antenna was first proposed by Robert Richtmyer in 1939. In 1982, Long et al. did the first design and test of dielectric resonator antennas considering a leaky waveguide model assuming magnetic conductor model of the dielectric surface .An antenna like effect is achieved by periodic swing of electrons from its capacitive element to the ground plane which behaves like an inductor. The authors further argued that the operation of a dielectric antenna resembles the antenna conceived by Marconi, the only difference is that inductive element is replaced by the dielectric material.Feed-through null
Feed-through null follows the duplexer and is commonly used with continuous-wave radar to improve performance.HJ-10
HJ-10 (Chinese: 红箭-10; pinyin: Hóng Jiàn-10; literally: 'Red Arrow-10') is a series of non-line-of-sight anti-helicopter / anti-tank missiles, indigenously developed by Norinco for People's Liberation Army. The primary version of HJ-10 utilizes fiber-optic wire-guided technology similar to European Polyphem while the lighter version equips laser homing and millimeter wave radar seeker as the primary weapon of the attack helicopters. HJ-10 can work both as land based anti-tank weapon as well as a Chinese equivalent of the AGM-114 Hellfire.HJ-9
The Hong Jian-9 (Chinese: 红箭-9; pinyin: Hóng Jiàn-9; literally: 'Red Arrow-9') is an advanced, third-generation anti-tank missile system deployed by the People's Liberation Army.
The missile was developed by China North Industries Corporation (Norinco), and one of the chief designers was Yang Chunming (杨春铭). It is similar in appearance to the Israeli MAPATS (man portable anti-tank system), causing speculation about the link between the two missiles. It is also similar to the South African ZT3 Ingwe anti-tank missile. Like the MAPATS and ZT3, the HJ-9 is guided by laser beam riding.
The HJ-9 has a maximum range of 5.5 km (3.4 mi), and a minimum range of 100 m (110 yd). Claimed armor penetration is 1,200mm, which is greater than that of the HJ-8. The missile may be fitted with high-explosive or thermal effect warheads for use against non-armored point targets, bunkers and fortifications. Like the HJ-8, the HJ-9 utilizes a disposable container/launching tube, but the one for HJ-9 is heavier, weighing 37 kg (82 lb) because HJ-9 is larger than HJ-8. The diameter of the HJ-9 is 152 mm (6.0 in) and the missile is compatible with a variety of thermal imaging sights.
In 2005, Norinco revealed in various public events that another version of HJ-9, the HJ-9A was already in service with Chinese armed forces, and this version used semi-active millimetre wave radar guidance. However, only photos of HJ-9A in service with Chinese paratroops were shown to the public. In these photos, the HJ-9A launcher is mounted on a jeep, and Norinco claimed the launcher on vehicles could be rapidly dismounted for foot soldiers.
Norinco also revealed a further-developed advanced version of the HJ-9A, designated HJ-9B.Mokopa
The ZT-6 Mokopa is a South African air-to-ground anti-tank guided missile. As of 2005 it is in its final stages of development, and is being integrated onto the South African Air Force's Rooivalk attack helicopters. The missile is produced by Denel Dynamics, formerly Kentron. The current version uses semi-active laser (SAL) guidance, requiring the target to be illuminated by a laser designator either on the launch platform or elsewhere; though there are alternative guidance packages available including a millimetre-wave radar (MMW) seeker and a two-colour imaging infrared (IIR) seeker.All variants of the Mokopa feature two launch modes, Lock-On Before Launch (LOBL) and Lock-On After Launch (LOAL). LOBL is the older, more conventional mode of missile launching, where the target has to be illuminated by the launch platform before launch. LOAL on the other hand allows the launch platform to launch the missile even though it may not be in sight of the target. In terms of the SAL version, this would then allow either the launch platform to move into place and only illuminate the target immediately prior to the missile striking the target, or it would allow an observer on the ground equipped with a laser designator to guide the missile in. This method of launching greatly reduces the exposure time of the launch platform to enemy fire.Multi-spectral camouflage
Multi-spectral camouflage is the use of counter-surveillance techniques to conceal objects from detection across several parts of the electromagnetic spectrum at the same time. While traditional military camouflage attempts to hide an object in the visible spectrum, multi-spectral camouflage also tries to simultaneously hide objects from detection methods such as infrared, radar, and millimetre-wave radar imaging.Among animals, both insects such as the eyed hawk-moth, and vertebrates such as tree frogs possess camouflage that works in the infra-red as well as in the visible spectrum.Radar gun
A radar speed gun (also radar gun and speed gun) is a device used to measure the speed of moving objects. It is used in law-enforcement to measure the speed of moving vehicles and is often used in professional spectator sport, for things such as the measurement of bowling speeds in cricket, speed of pitched baseballs, athletes and tennis serves.
A radar speed gun is a Doppler radar unit that may be hand-held, vehicle-mounted or static. It measures the speed of the objects at which it is pointed by detecting a change in frequency of the returned radar signal caused by the Doppler effect, whereby the frequency of the returned signal is increased in proportion to the object's speed of approach if the object is approaching, and lowered if the object is receding. Such devices are frequently used for speed limit enforcement, although more modern LIDAR speed gun instruments, which use pulsed laser light instead of radar, began to replace radar guns during the first decade of the twenty-first century, because of limitations associated with small radar systems.Rheinmetall KZO
KZO (Kleinflugzeug für Zielortung, German for small aircraft for target acquisition) is an unmanned aerial vehicle (UAV) with stealth characteristics manufactured by Airbus Defence and Space Airborne Solutions GmbH of Germany. Airbus Defence and Space Airborne Solutions GmbH is a joint venture of Airbus Defence and Space and Rheinmetall.
A KZO system consists of 10 UAVs and 2 ground units, consisting of one control station, one radio, one launch, one maintenance vehicle with a refuelling facility for the UAVs and one recovery vehicle.
The UAV is launched with a booster rocket directly out of its container. Landing is done with a parachute.
The KZO's main objective is to locate mobile threats and provide target locations for artillery. KZO replaced the German Army's other main UAV, Drohne CL 289.
The Taifun attack drone was a concept of an armed variant fitted with an intelligent millimeter-wave radar seeker that perform search and destroy attacks autonomously. It destroys its target with a hollow-charge warhead. However, the German Army decided against this concept in favor of the IAI Harop.
Two electronic warfare variants have also been developed as the Mücke ("mosquito") and the Fledermaus ("bat").
The initial system has been sold to China.SMArt 155
SMArt 155 is a German 155 mm artillery round, designed for a long range, indirect fire top attack role against armoured vehicles. The SMArt carrier shell contains two submunitions with infrared sensor and millimeter wave radar, which descend over the battlefield on ballutes and attack hardened targets with explosively formed penetrator warheads. Built with multiple redundant self-destruct mechanisms, these submunitions were specifically designed to fall outside the category of submunition weapons prohibited by the 2008 Convention on Cluster Munitions.
The name SMArt 155 is a contraction of its German name Suchzünder Munition für die Artillerie 155 (meaning "sensor-fuse munition for 155mm artillery"). SMArt is manufactured by GIWS mbh (Gesellschaft für Intelligente WirkSysteme mbH), a partnership between German armaments companies Rheinmetall and Diehl BGT Defence.
SMArt was first deployed by the Bundeswehr in 2000, and has been sold to the armies of Switzerland, Greece and Australia.Sea state
In oceanography, sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, period, and power spectrum. The sea state varies with time, as the wind conditions or swell conditions change. The sea state can either be assessed by an experienced observer, like a trained mariner, or through instruments like weather buoys, wave radar or remote sensing satellites.
In case of buoy measurements, the statistics are determined for a time interval in which the sea state can be considered to be constant. This duration has to be much longer than the individual wave period, but smaller than the period in which the wind and swell conditions vary significantly. Typically, records of one hundred to one thousand wave-periods are used to determine the wave statistics.
The large number of variables involved in creating the sea state cannot be quickly and easily summarized, so simpler scales are used to give an approximate but concise description of conditions for reporting in a ship's log or similar record.Semi-active radar homing
Semi-active radar homing (SARH) is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external (“offboard”) source — as it reflects off the target(in contrast to active radar homing, which uses an active radar: transceiver). Semi-active missile systems use bistatic continuous-wave radar.
The NATO brevity code for a semi-active radar homing missile launch is Fox One.Super high frequency
Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 and 30 gigahertz (GHz). This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres. These frequencies fall within the microwave band, so radio waves with these frequencies are called microwaves. The small wavelength of microwaves allows them to be directed in narrow beams by aperture antennas such as parabolic dishes and horn antennas, so they are used for point-to-point communication and data links and for radar. This frequency range is used for most radar transmitters, wireless LANs, satellite communication, microwave radio relay links, and numerous short range terrestrial data links. They are also used for heating in industrial microwave heating, medical diathermy, microwave hyperthermy to treat cancer, and to cook food in microwave ovens.
Frequencies in the SHF range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.Terahertz nondestructive evaluation
Terahertz nondestructive evaluation pertains to devices, and techniques of analysis occurring in the terahertz domain of electromagnetic radiation. These devices and techniques evaluate the properties of a material, component or system without causing damage.V band
The V band ("vee-band") is a standard designation by the Institute of Electrical and Electronics Engineers (IEEE) for a band of frequencies in the microwave portion of the electromagnetic spectrum ranging from 40 to 75 gigahertz (GHz). The V band is not heavily used, except for millimeter wave radar research and other kinds of scientific research. It should not be confused with the 600–1000 MHz range of Band V (Band Five) of the UHF frequency range.
The V band is also used for high capacity terrestrial millimeter wave communications systems. In the United States, the Federal Communications Commission has allocated the frequency band from 57 to 71 GHz for unlicensed wireless systems. These systems are primarily used for high capacity, short distance (less than 1 mile) communications. In addition, frequencies at 70, 80, and 90 GHz have been allocated as "lightly licensed" bands for multi-gigabit wireless communications. All communications links in the V band require unobstructed line of sight between the transmit and receive point, and rain fade must be taken into account when performing link budget analysis.W band
The W band of the microwave part of the electromagnetic spectrum ranges from 75 to 110 GHz, wavelength ≈2.7–4 mm. It sits above the U.S. IEEE-designated V band (40–75 GHz) in frequency, and overlaps the NATO designated M band (60–100 GHz). The W band is used for satellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications.YJ-7
The YJ-7 also known as C-701 is a Chinese missile that is roughly comparable to the American AGM-65 A/B/D/H Maverick air-to-surface missile. However, the C-701 is smaller and has less than half the weight of the AGM-65 A/B/D/H Maverick.