Abstract - the content of my term paper is based on the application of electromagnetic waves in RADAR. RADAR is one of the basic application of EMW. In my term paper the most important thing that i have added in my project is its application, principle of operation and also its basic area of application like in meteorology,Air -trafffic control etc. And i have tried my best to make it in a layman perspective.
Key words - EMW, carrier wave, transmission ,modulation
EMW- when the waveform account both electric and magnetic field for the transfer of energy or information propagation in one direction.
PRODUCTION OF EM-WAVE
Electromagnetic waves are produced by the motion of electrically charged particles. These waves are also called 'electromagnetic wave' because they radiate from the electrically charged particles. They travel through empty space as well as through air and other substances. Resarchers have observed that electromagnetic radiation has a dual 'property.' Besides acting like waves, it acts like a stream of particles that have no mass. The photons with the highest energy have the lowest wavelengths."
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Fig 1. Electromagnetic wave propogation
PROPERTIES OF EM- WAVE
1).Light travels in space in the form of electromagnetic waves.
2). Electromagnetic waves can travel through vacuum, means they do not require a medium for transmission.
3). Electromagnetic waves are two 2-D transverse waves, i.e., the transfer of energy is perpendicular to the oscillations.
4). Electromagnetic waves do not have no mass.
5). Polarization of electromagnetic waves is possible.
TheÂ electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.The electromagnetic spectrum extends from below frequencies used for modern radio to gamma radiation at the short-wavelength end, covering wavelengths from thousands of kilometers down to a fraction of the size of an atom..
Fig. 2 electromagnetic spectrum
APPLICATION OF EM-WAVE
1). Tansmission lines
2). High frequency and microwave circuits
4). Fibre optic communication
5). Mobile communication
6). Radio astronomy
RADAR- Radars are electromagnetic devices used for detection and location of objects. The term RADAR stands for Radio Detection And Ranging. the U.S. navy firstly used the word RADAR in 1940. In a typical radar system pulses of are transmitted to a distant object. the same antenna is used for transmitting and receiving ,so the time interval between the transmitted and reflected pulses is used to determine the range of the target object.
Fig3. image of simple radar
RADAR contains a high power Radio transmitter and an extremely sensitive receiver. when the transmitted radio signal strikes the object, some of the energy is reflected back. A highly directional antenna receives the reflected signal and The receiver is usually, but not always, in the same location as the transmitter. Although the signal returned is usually very weak, the signal can be amplified through use of electronic techniques in the receiver and in the antenna configuration. This enables RADAR to detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect.
The development of radar is not an individual work. Neither a single nation nor a single person is able to say, that he (or it) is the inventor of the RADAR method. Many researchers from different nation contributed parrallely in it There are nevertheless some milestones with the discovery of important basic knowledge and important inventions.Many scientist and engineers contributed to it. The first to use radio waves to detect "the presence of distant metallic objects" was Christian Hülsmeyer, who in 1904 proved the feasibility of detecting the presence of a ship in dense fog, but not its distance. He received Reichs patent for his nobel work in pre-radar device in April 1904. However, it was the Britishers who were the first to fully exploit it as a defence against aircraft attack.
Invention in RADAR
The English scientist James Clerk Maxwelldeveloped his electro-magnetic light theory (Description of the electro-magnetic waves and her propagation)
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The German scientist Heinrich Rudolf Hertzdiscovers the electro-magnetic waves and prove the theory of Maxwell with that.
The German high frequency engineer Christian Hülsmeyerinvents the â€žTelemobiloskop" to the traffic supervision on the water. He measures the running time of electro-magnetic waves to a metal object (ship) and back. A determination of the distance is thus possible. This is the first practical radar test. Hülsmeyer registers his invention to the patent in Germany and in the United Kingdom.
The invention of the Magnetron as an efficient transmitting tube by the US-american inventer Albert Wallace Hull
The American electrical engineers Albert H. Taylorand Leo C. Youngof the Naval Research Laboratory (USA) locate a wooden ship for the first time.
Lawrence A. Hyland(also of the Naval Research Laboratory), locates an aircraft for the first time.
A ship is equipped with radar. As antennae are used parabolic dishes with horn radiators.
The development of the Klystron by the technicians George F. Metcalfand William C. Hahn, both General Electric. This will be an important part in radar units as an amplifier or an oscillator tube.
Two engineers from the university in Birmingham, John Randalland Henry Bootbuilt a small but powerful radar using a Cavity-Magnetron. The B- 17airplanes were fitted with this radar.
Different radar equipments are developed in the USA, Russia, Germany, France.
Table.1 history of radar
III. CLASSIFICATION OF RADAR
I. on the basis of desired information, radar sets must have different qualities and technologies. One possible reason for these different qualities and techniques radar sets are classified in:
Fig 4. Types of RADAR on the basis of information
1). Primary RADAR: A Primary Radar transmits high-frequency signals which are reflected at targets object. The arised echoes are received and evaluated. This means, unlike secondary radar sets a primary radar unit receive it's own emitted signals as an echo.
2). Secondary RADAR: At these radar sets the airplane must have a transmitting responder on board and this transponder responds to interrogation by transmitting a coded reflected signal. This response may contain much more information, than a primary radar unit is able to acquire (E.g. an height, an identification code or also any technical problems on board such as a radio contact loss.
3). Pulse RADAR sets: pulse RADAR transmit a very high-frequency impulse signal of high power. After this impulse signal, a longer break follows in which the echoes can be received, before a new transmitted signal is sent out. Direction, distance and sometimes if necessary the height of the target can be determined from the measured antenna position and propagation time of the pulse-signal.
4). Continuous wave RADAR sets: CW radar sets transmit a high-frequency signal continuously. This echo signal is received and processed. The receiver need not to be mounted at the same place as the transmitter. In every firm civil radio transmitter can work as a radar transmitter at the same time, if a remote receiver compares the propagation times of the direct signal with the reflected one. Tests are known that the correct location of an airplane can be calculated from the evaluation of the signals from three different television stations.
5). Unmodulated CW RADAR- The transmitted signal of these equipments is constant in amplitude and frequency. These equipment is specialized in speed measurings. Distances cannot be measured. E.g. they are used as speed gauges for police. Newest equipments (LIDAR) work in the laser frequency range and measure not only the speed.
6). Modulated CW RADAR -The transmitted signal is constant in the amplitude but modulated in the frequency. This thing gets possible after the principle of the propagation time measurement with that again. It is an advantage of this equipment that an evaluation is carried out without reception break and the measurement result is therefore continuously available. These radar sets are used where the measuring distance is not too large and it's necessary a continuous measuring (e.g. an altitude measuring in airplanes or as weather radar).
7). Intrapulse modulated RADAR
8).. Pulse modulated RADAR
II. On the basis of use-
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Fig 5. Types of radar
Fig 6. classification On the basis of use
1). AIR DEFENCE : Air-Defense Radars can detect air targets and determine their position, course, and speed in a relatively large area. The maximum range of Air-Defense Radar can exceed 300 miles, and the bearing coverage is a complete 360-
2).MISSILE CONTROL: the patriot is an army-surface to air, mobile air defence missile system. The system had evolved to defence against cruise missile and and recently against ballisiic missile.
3).weapon control.: A Mortar Locating Radar provides quick identification to pinpoint enemy mortar positions in map co-ordinates, enabling artillery units to launch counter attacks. The system electronically, scans the horizon over a given sector several times a second, intercepting and
automatically tracking hostile projectiles, then computing back along the trajectory to the origin. The co-ordinates and altitude of the weapons site are then presented to the operator.
4). ASR RADAR: Airport Surveillance Radar (ASR) is an approach control radar used to detect and display an aircraft's position in the terminal area. These radar sets operate usually in E-Band, and have ability of reliably
Fig 7. Classification on the basis of use
5).PAR RADAR: The ground-controlled approach is a control mode in which an aircraft is able to land in bad weather. The pilot is guided by ground control using precision approach radar. The guidance information is obtained by the radar operator and passed to the aircraft by either voice radio or a computer link to the aircraft.
6).SMR: The Surface Movement Radar (SMR) scans the airport surface to locate the positions of aircraft and ground vehicles and displays them for air traffic controllers in bad weather. Surface movement radars operate in J to X- Band and use an extremely short pulse-width to provide an acceptable range-resolution.
RADAR FREQUENCY BAND
Fig8. Radio frequency band
IV. PRINCIPLE OF OPERATION
Radar, like sonar and seismology, uses a man-made pulse of radio energy to map distance based on the length of time it takes the pulse to return from the source. Radar (short for "Radio Detection And Ranging"), which can be airborne or space borne, has greatly changed the way wesee the land and ocean surfaces. Radar is based on the principle of sending very long wavelength radiation (called microwaves) from an antenna, and then detecting that energy after it bounces off a remote target. The wavelength of the microwave, its polarization (vertical or horizontal orientation) and strength can be controlled at the source and measured when it returns. Many common land-cover types and materials affect the polarity and strength of the radar return differently, which helps in their identification.
The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter. The time it takes for the reflected waves to return to the dish enables a computer to calculate how far away the object is, its radial velocity and other characteristics.
Fig.9 block diagram of primary radar
Fig.10 diagram to show hoow RADAR work
1).REFLECTION: The electromagnetic waves are reflected if they meet an electrically leading surface. If these reflected waves are received again at the place of their origin, then that means an obstacle is in the propagation direction. This means that a solid object in air or a vacuum, or other significant change in atomic density between the object and what is surrounding it, will usually scatter radar (radio) waves. Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of the target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light is reflected by a mirror. If the wavelength is much longer than the size of the target, the target is polarized (positive and negative charges are separated), like a dipole antenna. This is described by Rayleigh scattering, an effect that creates the Earth's blue sky and red sunsets. When the two length scales are comparable, there may be resonances. Early radars used very long wavelengths that were larger than the targets and received a vague signal, whereas some modern systems use shorter wavelengths (a few centimeters or shorter) that can image objects as small as a loaf of bread. Short radio waves reflect from curves and corners, in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90Â° angles between the reflective surfaces. A structure consisting of three flat surfaces meeting at a single corner, like the corner on a box, will always reflect waves entering its opening directly back at the source. These so-called corner reflectors are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect, and are often found on boats in order to improve their detection in a rescue situation and to reduce collisions.
2). Polarization: Radars use horizontal, vertical, linear and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces. Random polarization returns usually indicate a fractal surface, such as rocks or soil, and are used by navigation radars .
Fig11. Principle of operation
3). Interference: Radar systems must overcome unwanted signals in order to focus only on the actual targets of interest. These unwanted signals may originate from internal and external sources, both passive and active. The ability of the radar system to overcome these unwanted signals defines its signal-to-noise ratio (SNR). SNR is defined as the ratio of a signal power to the noise power within the desired signal.
4). Clutter: Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to the radar operators. Such targets include natural objects such as ground, sea, precipitation (such as rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as ionosphere reflections, meteor trails, and three body scatter spike. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff.
There are several methods of detecting and neutralizing clutter. Many of these methods rely on the fact that clutter tends to appear static between radar scans. Therefore, when comparing subsequent scans echoes, desirable targets will appear to move and all stationary echoes can be eliminated. Sea clutter can be reduced by using horizontal polarization, while rain is reduced with circular polarization (note that meteorological radars wish for the opposite effect, therefore using linear polarization the better to detect precipitation). Other methods attempt to increase the signal-to-clutter ratio.
Clutter may also originate from multipath echoes from valid targets due to ground reflection, atmospheric ducting or ionospheric reflection/refraction. This clutter type is especially bothersome, since it appears to move and behave like other normal (point) targets of interest, thereby creating a ghost. In a typical scenario, an aircraft echo is multipath-reflected from the ground below, appearing to the receiver as an identical target below the correct one. The radar may try to unify the targets, reporting the target at an incorrect height, or - worse - eliminating it on the basis of jitter or a physical impossibility. These problems can be overcome by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below ground or above a certain height. In newer Air Traffic Control (ATC) radar equipment, algorithms are used to identify the false targets by comparing the current pulse returns, to those adjacent, as well as calculating return improbabilities due to calculated height, distance, and radar timing.
5).Jamming: Radar jamming refers to radio frequency signals originating from sources outside the radar, transmitting in the radar's frequency and thereby masking targets of interest. Jamming is considered an active interference source, since it is initiated by elements outside the radar and in general unrelated to the radar signals. Jamming may be intentional, as with an electronic warfare (EW) tactic, or unintentional, as with friendly forces operating equipment that transmits using the same frequency range.
Figure 12: noise-modulated jamming, the jammer in 150Â° (VHF-Band radar)
RADAR BASIC PRINCIPLE
1). Transmitter: The radar transmitter produces the short duration high-power rf pulses of energy that are into space by the antenna.
2). Duplexer: The duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver.
3). Receive: The receivers amplify and demodulate the received RF-signals. The receiver provides video signals on the output.
4). Radar antenna: The Antenna transfers the transmitter energy to signals in space with the required distribution and efficiency. This process is applied in an identical way on reception.
5). Indicator: The indicator should present to the observer a continuous, easily understandable, graphic picture of the relative position of radar targets.
Fig13. Schematic diagram of basic operation
RADAR SIGNAL PROCESSING
1). Distance measurement: The distance is determined from the running time of the high-frequency transmitted signal and the propagation Â c0. The actual range of a target from the radar is known as slant range. Slant range is the line of sight distance between the radar and the object illuminated. While ground range is the horizontal distance between the emitter and its target and its calculation requires knowledge of the target's elevation. Since the waves travel to a target and back, the round trip time is dividing by two in order to obtain the time the wave took to reach the target. Therefore the following formula arises for the slant range. The distance resolution and the characteristics of the received signal as compared to noise depends heavily on the shape of the pulse. The pulse is often modulated to achieve better performance using a technique known as pulse compression.
Fig 14. Diagram showing radar principle
Expression of distance
where c = speed of light=3*108
t = measured running time
R = slant range antenna
The distances are expressed in kilometers or nautical miles (1 NM = 1.852 km)
2).Direction measurement: The angular determination of the target is determined by the directivity of the antenna. Directivity, sometimes known as the directive gain, is the ability of the antenna to concentrate the transmitted energy in a particular direction. An antenna with high directivity is also called a directive antenna. By measuring the direction in which the antenna is pointing when the echo is received, both the azimuth and elevation angles from the radar to the object or target can be determined. The accuracy of angular measurement is determined by the directivity, which is a function of the size of the antenna.
This angle is measured in the horizontal plane and in a clockwise direction from true north. In order to have an exact determination of the bearing angle, a survey of the north direction is necessary. Therefore, older radar sets must expensively be surveyed either with a compass or with help of known trigonometrically points. More modern radar sets take on this task and with help of the GPS satellites determine the northdirection independently.
Fig15. Direction determination
3).Speed measurement: Speed is the change in distance to an object with respect to time. Thus the existing system for measuring distance, combined with a memory capacity to see where the target last was, is enough to measure speed. At one time the memory consisted of a user making grease-pencil marks on the radar screen, and then calculating the speed using a slide rule. Modern radar systems perform the equivalent operation faster and more accurately using computers. The Doppler effect is only able to determine the relative speed of the target along the line of sight from the radar to the target. Any component of target velocity perpendicular to the line of sight cannot be determined by using the Doppler effect alone, but it can be determined by tracking the target's azimuth over time. Additional information of the nature of the Doppler returns may be found in the radar signal characteristics article.
4). Maximum unambiguous range: The maximum measuring distance Rmax of a radar unit isn't orientated only at the value determined in the radar equation but also on the duration of the receiving time.
The radar timing system must be reset to zero each time a pulse is radiated. This is to ensure that the range detected is measured from time zero each time. Echo signals arriving after the reception time are placed either into theThe maximum range at which a target can be located so as to guaratee that the leading edge of the recieved backscatter from that target is receivd before transmission begins for the next pulse. This range is called maximum unambiguous range or the first range ambiguity. The pulse-repetition frequency (PRF) determines this maximum unambiguous range of a given radar before ambiguities start to occur. This range can be determined by using the following equations:
Where c is the speed of light with 3Â·108 m/s. The pulse width (PW) in these equations indicates that the complete echo impulse must be received. If the transmitted pulse is very short, e.g. one microsecond can be ignored. But some radars uses very long pulses (up to 800Â microseconds) and the backscattered signal must be compressed in the receiver.
Fig 16. a second-sweep echo in a distance of 400Â km
5). Radar accuracy : Accuracy is the degree of conformance between the estimated or measured position and/or the velocity of a platform at a given time and its true position or velocity. Radio navigation performance accuracy is usually presented as a statistical measure of system error and is specified as:
Fig17. Graph showing RADAR accuracy
The power Pr returning to the receiving antenna is given by the radar equation:
Pt = transmitter power
Gt = gain of the transmitting antenna
Ar = effective aperture (area) of the receiving antenna
Ïƒ = radar cross section, or scattering coefficient, of the target
F = pattern propagation factor
Rt = distance from the transmitter to the target
Rr = distance from the target to the receiver
1).RADAR MODULATOR: Modulators act to provide the short pulses of power to the magnetron, a special type of vacuum tube that converts DC (usually pulsed) into microwaves. This technology is known as Pulsed power. In this way, the transmitted pulse of RF radiation is kept to a defined, and usually, very short duration. Modulators consist of a high voltage pulse generator formed from an HV supply, a pulse forming network, and a high voltage switch such as a thyratron.
2). RADAR COOLANT: Coolanol and PAO (poly-alpha olefin) are the two main coolants used to cool airborne radar equipment today. A synthetic coolant/lubricant composition, comprising an ester mixture of 50 to 80 weight percent of poly (neopentyl polyol) ester formed by reacting a poly (neopentyl polyol) partial ester and at least one linear monocarboxylic acid having from 6 to 12 carbon atoms, and 20 to 50 weight percent of a polyol ester formed by reacting a polyol having 5 to 8 carbon atoms and at least two hydroxyl groups with at least one linear monocarboxylic acid having from 7 to 12 carbon atoms, the weight percents based on the total weight of the composition
3).RADAR CROSS SECTION: The size and ability of a target to reflect radar energy can be summarized into a single term, Ïƒ, known as the radar cross-section, which has units of mÂ². If absolutely all of the incident radar energy on the target were reflected equally in all directions, then the radar cross section would be equal to the target's cross-sectional area as seen by the transmitter. In practice, some energy is absorbed and the reflected energy is not distributed equally in all directions. Therefore, the radar cross-section is quite difficult to estimate and is normally determined by measurement.
Fig18.reflected signal from spherical shape
Ïƒmax = Ï€Â Â·R2
Fig19. Reflected signal for cylindrical shape
ÏƒmaxÂ =Â 2Â·Ï€Â·rÂ·h2 / Î»
3). Flate plate
Fig20.reflected signal from flat plate
ÏƒmaxÂ =Â 4Â·Ï€Â·b2Â·h2 / Î»2
Table 2. RCS for point like target
APPLICATION OF RADAR
It is used in many different fields where the need for such positioning is crucial.
1). The first use of radar was for military purposes; to locate air, ground and sea targets. This has evolved in the civilian field into applications for aircraft, ships and roads.
2). Marine radar are used to measure the bearing and distance of ships to prevent collision with other ships, to navigate and to fix their position at sea when within range of shore or other fixed references such as islands, buoys, and lightships.
3). In port or in harbour, Vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters. Police forces use radar guns to monitor vehicle speeds on the roads.
4). In air traffic control-its main application is in the air traffic control. As a general example we can put as conversation between pilot and air traffic department. In aviation, aircraft are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings. They can land in fog at airports equipped with radar-assisted ground-controlled approach (GCA) systems, in which the plane's flight is observed on radar screens while operators radio landing directions to the pilot.
Fig 21. Showing frequency band of radio waves with the area of application
5). Meteorologists use radar to monitor precipitation. It has become the primary tool for short-term weather forecasting and to watch for severe weather such as thunderstorms, tornadoes, winter storms precipitation types, etc.
6) Geologists use specialised ground-penetrating radars to map the composition of the Earth crust.
7). RADAR can measure pressure: The strength of the echo received from the ionosphere measures the number of electrons able to scatter radio waves or what we call electron pressure
8).RADAR can measure temperature: Some electrons are moving due to heat- In this case the echo is scattered
The echo will contain a range of frequencies close to the transmitter frequencyAs the temperature increases, the electrons move faster So radar can act like a thermometer and measure the temperature of the ionosphere
9).measuring wind speed: When an electron is removed from an atom, the remaining charged atom is called an ion
The ion gas can have a different temperature from the electron gasThe electron/ion mixture is known as a plasma and is usually in motion (like our wind),So incoherent scatter radar can also measure wind speed
Fig 22. Incoherent scattering
APPLICATION OF RADAR IN AIR-TRAFFIC CONTROL
The air traffic control radar is a system used in air traffic control (ATC) to enhance surveillance radar monitoring and separation of air traffic. ATCRBS assists ATC surveillance radars by acquiring information about the aircraft being monitored, and providing this information to the radar controllers. The controllers can use the information to identify radar returns from aircraft (known as targets) and to distinguish those returns from ground clutter.
1).theory of operation: First, the ATCR interrogator periodically interrogates aircraft on a frequency of 1030Â MHz. This is done through a rotating or scanning antenna at the radar's assigned Pulse Repetition Frequency (PRF). Interrogations are typically performed at 450 - 500 interrogations/second. Once an interrogation has been transmitted, it travels through space in the direction the antenna is pointing at the speed of light until an aircraft is reached. When the aircraft receives the interrogation, the aircraft transponder will send a reply on 1090Â MHz after a 3.0Î¼s delay indicating the requested information. The interrogator's processor will then decode the reply and identify the aircraft. The range of the aircraft is determined from the delay between the reply and the interrogation. The azimuth of the aircraft is determined from the direction the antenna is pointing when the first reply was received, until the last reply is received. This window of azimuth values is then divided by two to give the calculated "centroid" azimuth. The errors in this algorithm cause the aircraft to jitter across the controllers scope, and is referred to as "track jitter." The jitter problem makes software tracking algorithms problematic, and is the reason why monopulse was implemented.
Fig23. Antenna system of a typical ground radar
2). Radar display: The beacon code and altitude were historically displayed verbatim on the radar scope next to the target, however modernization has extended the radar data processor with a flight data processor, or FDP. The FDP automatically assigns beacon codes to flight plans, and when that beacon code is received from an aircraft, the computer can associate it with flight plan information to display immediately useful data, such as aircraft callsign the aircraft's next navigational fix, assigned and current altitude, etc. near the target in a data
Fig24. Radar screen picture on ATCRS
On one side the RADAR research focuses on detecting objects with highest resolution and sensitivity, and on the other , active research is carried out in designing aircraft and ships which have extremely low RADAR cross sections. Special electromagnetic are used to develop targets which are practically invisible to radar. Of course, it is not possible to design an object which will be visible at all frequencies. However, the effort is made to make the target invisible over as large a frequency band is possible.
The invisible air craft is a spy over enemy territory.
It is not a easy task to conclude on a such a topic which has a vast area of application but as far as what i learnt from this project is that radar technology has a wide future aspects, since we found that it has application in wide areas like air traffic control, weather forecasting, defence activity and research is going on to make a target which will be invisible as large as frequency band as possible. The invisible air craft is used as spy over the enemy territory. So i can conclude that it is one of the important application as far as study of electromagnetic wave is concerned.