Characteristics of the Radar and Their Applications Sung Yoon-Pill(0341074) , Kim Jae-Woo(0341118)

Electric & Electronic Engineering, Yonsei Univ.

Abstract — No one say wh doesn't know what is radar. And every one say that, think of radar used in war, especially used to detect enemy in ship or aircraft. We are also that people before completing this project. We will examine in many direction of radar that is not well known although it seems friendly. After discussing how the word 'radar' was formed, and how it developed, we will see the radar's fundamental theory. And then we'll concentrate on where radar's been used and where it is heading.

I. Introduction. *Definition : Radar is defined as “a device for transmitting electromagnetic (EM) signals and receiving echoes from objects of interest (targets) within its volume of coverage”. Radar was originally an acronym for radio detection and ranging.

The radar, we commonly use, is monostatic radar. It is that EM energy is transmitted from the radar hardware to an antenna, radiated from the antenna, scattered from one or more external objects, collected back at the transmitting antenna, and received by the radar hardware. That is, the transmitting and receiving locations are the same. Occasionally, reference is made to bistatic radars, in which the transmitting and receiving locations are different, or to multistatic radars, which involve transmission from one or more locations and reception at one or more locations

*History : The history of radar, especially the early history, is a fascinating subject in itself. Excellent treatises have been prepared by Budieri, Burns, Swords, and others. Briefly, the history of radar is as follow. In 1886, Heicrich Hertz confirmed radio wave propagation. The first “radar” was patented in 1904 by Huelsmeyer, of Dusseldorf, Germany. Huelsmeyer called his device “Hertzian-wave projecting and receiving apparatus adapted to indicate or give warning of the presence of a metallic body, such as a ship or train, in the line of projection of such wave”. Taylor and Young accomplished systematic ship detection at the Naval Research Laboratory, Washington, D.C, in 1922, and Hyland performed the first aircraft detection in 1930. In 1941, a U.S Army radar detected Japanese planes approaching Pearl Harbor, but the supervisor in charge decided that the signals were spurious. The rapid development of the Chain Home radar system in Britain was essential to Britain’s successful defense against air attacks by Germany during World WarⅡ. Following the war, radar burgeoned to produce the many types in use today.

II. The principal of the operation of the radar

It operates by radiating energy into space and detecting the echo signal reflected from an object, or target. The reflected energy that is returned to the radar not only indicates the presence of a target, but by comparing the received echo signal with the signal that was transmitted, its location can be determined along with other target-related information. Radar can perform its function at long or short distances and under conditions impervious to optical and infrared sensors. It can operate in darkness, haze, fog, rain, and snow. Its ability to measure distance with high accuracy and in all weather is one of its most important attributes. The basic principle of radar is illustrated in Fig.1. A transmitter (in the upper left portion of the figure) generates an electromagnetic signal that is radiated into space by an antenna. A portion of the transmitted energy is intercepted by the target and reradiated in many directions. The reradiation directed back towards the radar is collected by the radar antenna, which delivers it to a receiver. There it is processed to detect the presence of the target and determine its location. A single antenna is usually used on a time-shared basis for both transmitting and receiving when the radar waveform is a repetitive series of pulse. The range, or distance, to a target is found by measuring the time it takes for the radar signal to travel to the target and return back to the radar. The target's location in angle can be found from the direction the narrow-bandwidth radar antenna points when the received echo signal is of maximum amplitude. If the target is in motion, there is a shift in the frequency of the echo signal due to the doppler effect. this frequency shift is proportional to the velocity of the target relative to the radar. The doppler frequency shift is widely used in radar as the basis for separating desired moving targets from fixed "

clutter" echoes reflected from the natural environment such as land, sea, or rain. Radar can also provide information about the nature of the target being observed.

*Range to a Target : The most common radar signal, or waveform, is a series of short-duration somewhat rectangular-shaped pulses modulating a sinwave carrier. (This is sometimes called a pulse train) The range to a target is determined by the time TR it takes the radar signal to travel to the target and back. Electromagnetic energy in free space travels with the speed of light, which is c=3*108 m/s. Thus the time for the signal to travel to a target located at a range R and return back to the radar is 2R/c. The range to a target is the

R = cT R2 [1.1]

With the rage in kilometers or in nautical miles, and T in microseconds, Eq(1.1) becomes

R(km) = 0.15TR(μs) or R(nmi)=0.081TR(μs)

Each microsecond of round-trip travel time corresponds to a distance of 150 meters. 164 yards, 492 feet, 0.081 nautical mile , or 0.093 statute mile. It takes 12.35 μs for a radar signal to travel a nautical mile and back.

*Maximum Unambiguous Range : Once a signal is radiated into space by a radar, sufficient time must elapse to allow all echo signals to return to the radar before the next pulse is transmitted. The rate at which pulses may be transmitted, therefore, is determined by the longest range at which targets are expected. If the time between pulses Tp is too short, an echo signal from a long-range target might arrive after the transmission of the next pulse and be mistakenly associated with that pulse rather than the actual pulse transmitted earlier. This can result in an incorrect or ambiguous measurement of the range. Echoes that arrive after the transmission of the next pulse are called second-time-around echoes. Such and echo would appear to be at a closer range than actual and its range measurement could be misleading if it were not known to be a second-time-around echo. The range beyond which targets appears as second-time-around echoes is the maximum unambiguous range, Run, and is given by

R un =cT p2 = c

2fp [1.2]Where Tp=pulse repetition period = 1/fp, and fp = pulse repetition frequency(prf), usually given in hertz or pulses per

second (pps). A plot of the maximum unambiguous range as a function of the pulse repetition frequency is shown in Fig. 2 The term pulse repetition rate is sometimes used interchangeably with pulse repetition frequency.

III. Em device used for radar

AntennaAn antenna is defined as "that part of a transmitting or receiving system which is designed to radiate or to receive EM

waves". The antenna produces an EM wave in either free space from an EM wave in the hardware (called transmission or transmitting) or the hardware from an EM wave in free space (called reception or receiving.)

Transmitter The low-power (milliwatts) modulated carrier enters the transmitter, which amplifies it to a relatively high power

(typically kilowatts). Many types of transmitters are used. The traveling-wave tube (TWT) is widely employed because it is both coherent and relatively wideband, that is, it amplifies the input power with high gain over a relatively wide range of frequencies, typically hundreds of megahertz at X band. Other types of transmitters include the magnetron (noncoherent), klystron, crossed-field amplifier (CFA), and solid state transmitters.

Waveguide After leaving the transmitter, the high-power microwaves must be "guided" to the antenna with low loss. For that

purpose, a waveguide usually is used. A waveguide is a hollow metal pipe with a cross-section that usually is rectangular but may be circular or elliptical. The interior may be vacuum, air, or some other dielectric. To find the propagation characteristics inside the wave guide, we solve Maxwell's equations with appropriate boundary conditions. The solution reveals a number of possible modes of increasing frequencies. In rectangular waveguide, modes are either transverse electric (TE) or transverse magnetic (TM), indicating that the electric (for TE) or the magnetic (for TM) field

is entirely tranverse to the propagation direction. Fig.3 illustrates the cross-section of a rectangular waveguide; the interior dimensions are denoted as a and b, with a>b.

For the most common propagation mode in a rectangular waveguide (the TE 10 mode), if cD is the speed of EM waves in the dielectric (cD<C), the minimum frequency that will propagate is the cutoff frequency :

fmin = fc =cD2a

Futhermore, the maximum frequency that will propagate is fmax=cD/a. The effective speed of propagation through the guide is the group velocity:

cG = cD 1 -

fcf

2 1/ 2

IV. The use of radar nowadays & application of radar in many fields

Radar has been employed to detect targets on the ground, on the sea, in the air, in space, and even below ground. The major areas of radar application are briefly described below.

The information secured by radar includes the position and velocity of the object with respect to the radar unit. In some advanced systems the shape of the object may also be determined. Commercial airliners are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings. Planes 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. A ground-based radar system for guiding and landing aircraft by remote control was developed in 1960.

Radar is also used to measure distances and map geographical areas (shoran) and to navigate and fix positions at sea. Meteorologists use radar to monitor precipitation; it has become the primary tool for short-term weather forecasting and is also used to watch for severe weather such as thunderstorms and tornados. Radar can be used to study the planets and the solar ionosphere and to trace solar flares and other moving particles in outer space.

Various radar tracking and surveillance systems are used for scientific study and for defense. For the defense of North America the U.S. government developed a radar network known as the Ballistic Missile Early Warning System (BMEWS), with radar installations in Thule, Greenland; Clear, Alaska; and Yorkshire, England. A radar system known as Space Detention and Tracking System (SPADATS), operated collaboratively by the Canada and the U.S., is used to track earth-orbiting artificial satellites.

V. The future direction of the radar

When the Radar was developed, it was believed that the techniques of high output amplifier and high sensitive devices were important. But nowadays and future, they are important that software technique that can process high speed and ultra capacity signals. As a respect of system, radar will become compact, lightweight, digitalize, semiconductize, high trust, automatize, and improvement of ability to detection. Aspect of technology, it is expected to be developed to adapt array antenna that has ECM(Electronic Counter Measures) ability by Null directed with interfere wave. And it is also expected that development of adaptation antenna and transmitter and receiver are expected that are expand the range of frequency even to laser range, it will be applied to compact lightweight, etc.(in systemic aspect) The signal processing device will be developed to multifunction, multidimension and mutirange and by this device, it will be able to real-time process. It would be able to recognize the shape of the ships or aircrafts, and expected to be developed that it can notice the kinds of those.

VI. Conclusion

We found lots of facts about radars during this project. Also We could find many connections with what we learned in ‘Electric & Electronic Fields’ class. As many people know, radar made the most acknowledged development during

World War 2. Although the fact that science techniques are used to kill people is very sad, as we can see in the end of the project, it is now developing to enhance human's life. As a student studying electric engineering, I hope many science techniques would be used to help human's life.

References

[1] Merrill l. Skolnik, Introduction to RADAR system [2] Roger j. Sullivan, Microwave radar : imaging and advanced concepts[3] Eli Brookner, Radar technology

Fig 1.Basic Principle of radar.

Fig. 2. Plot of Eq.1.2, the maximum unambiguous range Run as a function of the pulse repetition frequency fp.

Fig 3.Cross-section of rectangular waveguide.

Fig 4. ATCR-2T L-Band Enroute 2-D Air Surveillance Radar at Vienna Site Site.