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Radar Radio Waves

A Comprehensive Report on Radar

1. Abstract

The two main tasks of radar: detecting a target and determining its range. Radar have many types which perform different functions like “Search radars” scan a wide area with pulses of short radio waves. The radar measures the distance to the reflector by measuring the time from emission of a pulse to reception. The majority of the radars illuminates targets by transmitting short pulses and listen for echoes. These are known as pulse radar. Most radar function can also be accomplished by transmitting a continuous wave (cw) and listening for echoes while the transmitter is radiating.

2. Introduction

RADAR (derived from “Radio Detection And Ranging”) is a system that uses electromagnetic waves to remote-sense the range, position, velocity and identifying characteristics of the targets. [1]

The term RADAR stands for Radio Detection and Ranging. Radar was originally called RDF (Radio Direction Finder). Radar is an electromagnetic system which is used for detection and location of the reflected objects. It operates by radiating energy in the space and detecting the echo signal reflected from an object. The location of the object or target can be determined by comparing the received echo signal with the transmitted signal. Radar can work in rain, snow, fog, haze and darkness. The most important attributes of the radar is to measure distance with high accuracy and in all weather. [2]

3. History Of Radar

Radar was an accident similar as penciline. It is not an invention that was created overnight. It started from 18th century and is still being improved.

3.1. 1887

In 1887 the German physicist Heinrich Hertz began experimenting with radio waves in his laboratory. He found that radio waves could be transmitted through different types of materials, and were reflected by others, such as conductors and dielectrics. The existence of electromagnetic waves was predicted earlier by the British physicist James Clerk Maxwell, but it was Hertz who first succeeded in generating and detecting radar waves

3.2.1900

“The amazing technical feat which provoked the radar in electronics was naturally accompanied by an abundant flowering of memoires, among which legend and technical nationalism were regrettably not always absent. The fundamental principle of the radar belongs to the common patrimony of the physicists : after all, what is left to the real credit of the technicians is measured by the effective realisation of operational materials". — Maurice Ponte in France 1934 [3]

Several inventors, scientists, and engineers contributed to the development of radar. The first to use radio waves to detect "the presence of distant metallic objects" was Christian Hülsmeyer, who in 1904, some of the major names are as follows:

4. Early Radar Technology

Radar technology came of age in World War II, with the technology development during that period laying the groundwork for further elaboration in the postwar period.

One of the early improvements was to build a radar that could automatically sweep around the sky to search for intruders. The early floodlight systems could cover a wide sector of the sky, but as mentioned they were inefficient. A simple steerable radar with an A-scope display was more efficient, but it had to be manually steered to find a target. Building an improved radar that could be swept around 360 degrees was a bit tricky, since it implied that the electrical connection between the antenna system and its associated electronics had to freely rotate, and designing reliable "rotary couplers" was troublesome.

It also implied a different type of display, the "plan position indicator (PPI)", also known as the "polar plot indicator". The PPI is a circular display, with a sweep rotating around the center in sync with the transmitter antenna, and the return for a particular angle displayed along the display sweep.

5. Modern Radar Technology

Although radars went through a rapid evolution during World War II, the rate of change in the technology leveled out in the postwar period. The pace picked up again in the 1960s with the introduction of solid-state and digital technology, in particular leading to the development of the "pulse Doppler" radar, which finally managed to solve the problem of ground clutter.

Suppose a radar transmitter antenna has two feed horns, each slightly offset from the antenna bore sight. This allows two pulses to be sent at the same time, with the returns from both pulses being picked up by the receiver subsystem at the same time. The radar tracking system will be provided both the sum and the difference of these two returns, and it will then rotate the antenna to ensure that the sum remains at a maximum and the difference remains at a minimum. This is known as "monopulse" operation. [3]

6. Types of Radar

There are two main types of radar.

6.1. Pulse Wave Radar

Pulse Radar sends out signals in short (few millionths of a second) but powerful bursts or pulses. Pulse Radar determines distance (range) by measuring the time it takes a radar wave to get to the target object and to come back (time of flight) and then divides that time in two (distance to the target). Since all radio waves travel at the same speed of light, this known speed multiplied by the time of flight can be used to determine distance. By continuing to track an object with a pulse radar the speed of the object can also be determined. [4]

The Majority of radar illuminate target s by transmitting short bursts of illumination energy and listen for echoes with the transmitter silent. These are known as pulse radar.

6.1.1. Pulse repetition frequency (PRF)

a. Pulses per second

b. Relation to pulse repetition time (PRT)

c. Effects of varying PRF

(i) Maximum range

(ii) Accuracy

6.1.2. Peak power

a. Maximum signal power of any pulse

b. Affects maximum range of radar

6.1.3. Average power

a. Total power transmitted per unit of time

b. Relationship of average power to PW and PRT

6.1.4. Duty cycle

a. Ratio PW (time transmitting) to PRT (time of entire cycle, time transmitting plus rest time)

b. Also equal to ratio of average power to peak power.

6.1.5. Determining Range with Pulse Radar

Range=(c*t)/2

c = 3 x 108 m/sec

t is time to receive return

divide by 2 because pulse traveled to object and back

6.1.6. Pulse Width (PW)

Length or duration of a given pulse

6.1.7. Pulse Repetition Time (PRT=1/PRF)

PRT is time from beginning of one pulse to the beginning of the next

PRF is frequency at which consecutive pulses are transmitted.

Basic Block Diagram of Pulse Wave Radar

6.1.8. Modulator

You can see on the block diagram that the heart of the radar system is the modulator. It generates all the necessary timing pulses (triggers) for use in the radar and associated systems. Its function is to ensure that all subsystems making up the radar system operate in a definite time relationship with each other and that the intervals between pulses, as well as the pulses themselves, are of the proper length.[4]

6.1.9. Transmitter

The transmitter generates powerful pulses of electromagnetic energy at precise intervals. The

Required power is obtained by using a high-power microwave oscillator, such as a magnetron, or a microwave amplifier, such as a klystron, that is supplied by a low-power rf source. (You can review the construction and operation of microwave components in NEETS module 11, Microwave Principles.) [1]

6.1.10. Duplexer

The duplexer is essentially an electronic switch that permits a radar system to use a single antenna to both transmit and receive. The duplexer must connect the antenna to the transmitter and disconnect the antenna from the receiver for the duration of the transmitted pulse. As we mentioned previously, the switching time is called receiver recovery time, and must be very fast if close-in targets are to be detected. [4]

6.1.11. Antenna System

The antenna system routes the pulse from the transmitter, radiates it in a directional beam, picks up the returning echo and passes it to the receiver with a minimum of loss. The antenna system includes the antenna, transmission lines, and waveguide from the transmitter to the antenna, and transmission lines and waveguide from the antenna to the receiver. [4]

6.1.12. Receiver

The receiver accepts the weak RF echoes from the antenna system and routes them to the indicator as discernible video signals. Because the radar frequencies are very high and difficult to amplify, a super heterodyne receiver is used to convert the echoes to a lower frequency, called the intermediate frequency (IF), which is easier to amplify. [1]

6.1.13. Indicator

The indicator uses the video output of the receiver to produce a visual indication of target information including range and bearing (or in the case of height-finding indicators, range and height). [4]

6.2. Continuous Wave Radar

Continuous Wave Radar sends out a continuous signal instead of short bursts. There are two types of Continuous Wave Radar:

6.2(a). Doppler Radar

“A Radar that can determine the frequency shift through measurement of the phase change that occurs in electromagnetic waves during a series of pulses.” [6]

Doppler Radar is used mostly to make precise speed measurements and is most often utilized by police traffic radars. Doppler Radar transmits a continuous wave of a constant frequency. When this frequency strikes a moving object the frequency is changed and the new frequency returning to the radar is used to determine the speed of the moving target.

Doppler is a means to measure motion. Doppler radars not only detect and measure the power received from a target, they also measure the motion of the target toward or away from the radar.

The electric field of a transmitted wave

The returned electric field at some later

time back at the radar

The time it took to travel

Substituting:

The received frequency can be determined by taking the time derivative if the quantity in parentheses and dividing by 2 pi. [8]

6.2(a).1. Principles of Doppler Radar

Doppler radars can measure the velocity of targets relative to the radar. For example, at time T1 a pulse is sent towards a target and it returns a target distance "D". At time T2, another pulse is sent towards the same target and returns a target distance "D + D".

The distance to target has changed from times T1 to T2, resulting in a phase shift between the two return signals, which Doppler radars are capable of measuring. By knowing the phase shift, the wavelength and the time interval from T1 to T2, the distance D that the target has moved relative to the radar can be computed. These pieces of information are then used to compute the target velocity relative to the radar. If the target is moving sideways so that its distance relative to the radar does not change, the radar will record zero velocity for that target.

6.2(a).2. Sign Conventions

The Doppler frequency is negative (lower frequency, red shift) for objects receding from the radar.

The Doppler frequency is positive (higher frequency, blue shift) for objects approaching the radar.

These “color” shift conventions are typically also used on radar displays of Doppler velocity.

Red: Receding from radar

Blue: Toward radar

6.2(a).3. Doppler Shift

A frequency shift that occurs in electromagnetic waves due to the motion of scatterers toward or away from the observer.

In 1842, the Austrian physicist Johann Christian Doppler first related motion to frequency changes in light and sound. Doppler discovered that the shift in frequency  caused  by  moving  sources  of  sound  was directly  proportional  to  the  speed  of  the  source.  He then developed mathematical formulas to describe this effect called the Doppler Shift. While not given much thought, you experience Doppler shifts many times each day. The change in pitch of a passing train whistle and a speeding automobile horn demonstrate its effects. When you hear a train or automobile, you can determine its approximate location and movement. [7]

6.2(a).4. Magnitude of the Doppler Shift

Transmitted Frequency

X band C bandS band

9.37 GHz 5.62 GHz 3.0 GHz

Radial velocity

1 m/s 62.5 Hz 37.5 Hz 20.0 Hz

10 m/s 625 Hz 375 Hz 200 Hz

50 m/s 3125 Hz 1876 Hz 1000 Hz

These frequency shifts are very small, for this reason, Doppler radars must employ very stable transmitters and receivers. [8]

Typical period of Doppler frequency = 0.3 to 50 milliseconds

Typical pulse duration = 1 microsecond

Only a very small fraction of a complete Doppler frequency cycle is contained

Within a pulse.

We can understand how the phase shift can be related to the radial velocity by considering a single target moving radially.

Distance target moves radially in one pulse period Tr

The corresponding phase shift of a wave between two Consecutive pulses (twice (out and back) the fraction of a wavelength traversed between two consecutive pulses).

Solving for the radial velocity:

In practice, the pulse volume contains billions of targets moving at different radial speeds and an average phase shift must be determined from a train of pulses.

6.2(b). Frequency-Modulated Radar

Frequency-Modulated Radar also transmits a continuous signal, but it rapidly increases or decreases the frequency of the signal at regular intervals. As a result FM Radar, unlike Doppler Radar, can determine distance (range) as well as velocity (speed). [6]

7. Difference Between Pulse and Continuous Wave Radar

Pulse Echo

Continuous Wave

8. Important Rules/ Facts about Radar

8.1. Inverse Square Rule

The inverse square rule states that the decrease in strength of a radar signal is inversely proportional to the square of the change in distance from the antenna. 8.2. Contour Lines of Equal Sensitivity

The contour lines of equal sensitivity rule states that the strongest reflected signal is determined by the location of the target vehicle to the main power beam. To better understand contour lines of equal sensitivity it is helpful to review all beam reflection rules. The inverse square rule demonstrated that the distance from the radar determined the strength of the reflected signal. We also learned from lines of equal sensitivity that two vehicles of equal size, located at an equal distance from the axis of the main beam, will reflect a radar signal equally. However, if two identical vehicles are positioned so that one vehicle is located directly along the main power axis and one vehicle is located at the edge of the radar beam, the vehicle located on the main power axis will reflect the stronger signal.

8.3. Beam Range Sensitivity

As mentioned earlier, the radar beam will continue outward from the radar antenna for an indefinite distance. In reality the beam range as referred to in this manual is that distance where the radar signal may be reflected from an object and then accurately received by the radar antenna. All radar manufacturers specify the range of their radars within these specifications. Nevertheless, the radar range will vary considerably due to several conditions. Atmospheric conditions such as rain, snow, and fog will decrease the effective range of the beam of energy. Terrain such as hills, curves, fences, and buildings will obviously affect the radar signal. Large volumes of traffic or stronger reflective signals may also reduce the effective range of the radar. Most modern radars have a sensitivity adjustment to control the beam range.

9. Application of Radar

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

9.1. Military

Radar is used for detecting ground moving vehicles, such as tanks, for defense purpose. [1]

Radar is an important part of air-defense systems as well as the operation of offensive missiles and other weapons. In air defense it performs the function of surveillance and weapon control. Surveillance includes target detection, target recognition, target tracking and designation to a weapon system. [2]

9.2. Remote Sensing

All Radars are remote sensors; however, this term is used to imply the sensing of the environment.

9.3. Air Traffic Control (ATC)

Radar have been employed around the world to safely control air traffic in the vicinity of air port (air surveillance radar), and en-route from one airport to another (Air Route Surveillance Radar ) as well as ground-vehicular traffic and taxiing aircraft on the ground (air surface detection equipment).[6]

9.4. Ship Safety

Radar is found on ships and boats for detecting and locating ships and land features for ship collision avoidance. Radars are also used for navigating ships in bad weather or at night.[1]

9.5. Space

Radar used for measuring distance and velocity for spacecraft and docking. [1]Space vehicles have used radar for docking, and for landing on the moon. Large ground based radars are used for detection and tracking of the satellites and other space objects. The field of radar astronomy using earth based systems held in understanding the nature of meteors establishing an accurate measurement of the astronomical unit (the basic yard stick for measuring distances in the solar system). [2]

9.6. Aircraft Safety and navigation

Military aircraft employ ground-mapping Radars to image a sense the radio altimeter is also a radar used to indicate the height of an aircraft above the terrain and as a part of self-contain guidance systems overland.[2]

10. References

[1] Byron Edde, “Radar Fundamentals”, Pearson Education.Inc, 1995

[2] Merrill I.Skolnik, “Introduction to Radar Systems” , Irwin/McGraw-Hill.Co, 1980

[3] Mc Krane, “General History of Radar”, Jhon Wiley& Sons.Inc, 1975

[4] A.Farina et-all, “Radar Data Processing”, Research Study Press.Ltd, 1985

[5] Lt.Mazat, “Overview of Radar”, Presentation.

[6] Nadav Levanon et-all, “Radar Signals”, Jhon Wiley & Sons.inc, 2004

[7] www. Wikipedia.com

[8] JhonWurman, “Lecture on Radar”, Presentation

[9] University of Illinois, “Research Paper On Radar”, 1996

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