Topic:
Radar Systems
Subject: Principles of Communication Systems
Submitted By: Muhammad Imran FA09-BEE-145
Submitted To: Sir. Yousaf Murtaza Rind
Date:
December 28, 2011
Outline Ø Introduction to Radar Ø History Ø Working Principle Ø Types of Radar Ø Components of a Radar System Ø Mathematical Model of Radar Processing Ø Characteristic of Radar Waves Ø Applications of Radar Ø Stealth Technology
RADAR SYSTEMS Introduction 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, missiles, motor, weather formations, and terrain. “The term RADAR stands for Radio Detection and Ranging.”
History In 1886, Heinrich Hertz showed that radio waves could be reflected from solid objects. In1904, Huelsmeyer used radio waves to detect the presence of distant metallic objects.. He invented a device named telemobiloscope which was the start of the invention of radar systems. Telemobiloscope was used only to locate the objects only; its distance couldn’t be measured. However in 1922, US Navy broadcasted radio waves at 60MHz, and succeeded to determine the range and bearing of nearby objects, like ships. Before the Second Worl War, many Europeans countries developed technologies that led to the modern version of radar. In 1935, Soviet military engineer P.K.Oschepkov produced an experimental apparatus, RAPID, capable of detecting an aircraft within 3 km of a receiver. The British were the first to fully exploit radar as a defence against aircraft attack. After this, work on radar was raised exponentially leading a complex to more complex radar systems.
Principle “Radar operates by transmitting a particular kind of radio frequency waveform and detecting the nature of the reflected echo. When radio waves strike an object, some portion is reflected, and some of this reflected energy is returned to the radar set, where it is detected. The location and other information about these reflective objects, targets, can be determined by the reflected energy.” Radio wave is the main consideration in radar systems. A radar system has a transmitter that emits radio waves called radar signals in predetermined directions. When these come into contact with an object they are usually reflected and/or scattered in many directions. Radar signals are reflected especially well by materials of considerable electrical conductivity especially by most metals, by seawater and by wetlands. The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work. If the object is moving either closer or farther
away, there is a slight change in the frequency of the radio waves, caused by the Doppler effect. Radar relies on its own transmissions rather than light from the Sun or the Moon, or from electromagnetic waves emitted by the objects themselves, such as infrared wavelengths (heat). This process of directing artificial radio waves towards objects is called illumination, regardless of the fact that radio waves are invisible to the human eye or cameras.
Types of Radar There are two Types of Radar Detectors: 1. Pulse radar 2. Continuous-wave radar a) Doppler Radar b) Frequency-Modulated Radar
1. Pulse 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 pulse radar the speed of the object can also be determined.
Block Diagrm of Pulse Radar 2. Continuous-Wave Radar
Continuous-Wave Radar sends out a continuous signal instead of short bursts. There are two types of Continuous Wave Radar: a. Doppler Radar b. Frequency Modulated (FM radar) a) Doppler Radar 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. Examples of Doppler Radar - Decatur Police Radar Guns. A lot of sports radar guns, such as Bushnell Velocity Speed Gun, are Doppler Radars as well.
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).
Components of a Radar Systems A practical radar system requires seven basic components 1. Transmitter 2. Receiver. 3. Power Supply. 4. Synchronizer 5. Duplexer 6. Antenna. 7. Display 1. Transmitter. The transmitter creates the radio wave to be sent and modulates it to form the pulse train. The transmitter must also amplify the signal to a high power level to provide adequate range. The source of the carrier wave could be a Klystron, Traveling Wave Tube (TWT) or Magnetron. Each has its own characteristics and limitations. 2. Receiver. The receiver is sensitive to the range of frequencies being transmitted and provides amplification of the returned signal. In order to provide the greatest range, the receiver must be very sensitive without introducing excessive noise. The ability to discern a received signal from background noise depends on the signal-to-noise ratio (SNR). Some common features of receiver are: a) Pulse Integration. The receiver takes an average return strength over many pulses. Random events like noise will not occur in every pulse and therefore, when averaged, will have a reduced effect as compared to actual targets that will be in every pulse.
b) Sensitivity Time Control (STC). This feature reduces the impact of returns from sea state. It reduces the minimum SNR of the receiver for a short duration immediately after each pulse is transmitted. The effect of adjusting the STC is to reduce the clutter on the display in the region directly around the transmitter. The greater the value of STC, the greater the range from the transmitter in which clutter will be removed. However, an excessive STC will blank out potential returns close to the transmitter. c) Fast Time Constant (FTC). This feature is designed to reduce the effect of long duration returns that come from rain. This processing requires that strength of the return signal must change quickly over it duration. Since rain occurs over and extended area, it will produce a long, steady return. The FTC processing will filter these returns out of the display. Only pulses that rise and fall quickly will be displayed. In technical terms, FTC is a differentiator, meaning it determines the rate of change in the signal, which it then uses to discriminate pulses which are not changing rapidly. 3. Power Supply. The power supply provides the electrical power for all the components. The largest consumer of power is the transmitter which may require several kW of average power. The actually power transmitted in the pulse may be much greater than 1 kW. The power supply only needs to be able to provide the average amount of power consumed, not the high power level during the actual 4. Synchronizer. The synchronizer coordinates the timing for range determination. It regulates that rate at which pulses are sent and resets the timing clock for range determination for each pulse. Signals from the synchronizer are sent simultaneously to the transmitter, which sends a new pulse, and to the display, which resets the return sweep. 5. Duplexer. This is a switch which alternately connects the transmitter or receiver to the antenna. Its purpose is to protect the receiver from the high power output of the transmitter. During the transmission of an outgoing pulse, the duplexer will be aligned to the transmitter for the duration of the pulse. After the pulse has been sent, the duplexer will align the antenna to the receiver. When the next pulse is sent, the duplexer will shift back to the transmitter. A duplexer is not required if the transmitted power is low. 6. Antenna. v The antenna takes the radar pulse from the transmitter and puts it into the air.
v The antenna focuses the energy into a well-defined beam which increases the power and permits a determination of the direction of the target. v The antenna keeps track of its own orientation which can be accomplished by a synchronic-transmitter. v There are also antenna systems which do not physically move but are steered electronically. 7. Display. The display unit may take a variety of forms but in general is designed to present the received information to an operator. The most basic display type is called an A-scan. It shows the graph b/w amplitude and Time delay. The vertical axis is the strength of the return and the horizontal axis is the time delay or range. The A-scan provides no information about the direction of the target. The most common display is the PPI (plan position indicator). The A-scan information is converted into brightness and then displayed in the same relative direction as the antenna orientation. The result is a top-down view of the situation where range is the distance from the origin. The PPI is perhaps the most natural display for the operator and therefore the most widely used.
Mathematical Model of Radar Processing We build the mathematical model of radar processing, by and large following Borden’s treatment. Usually a transmitted signal comprises a slowly varying waveform superimposed on a rapidly oscillating sinusoid. Thus s(t)=w(t).cos(2π(f t+Ω(t)) where w(t) is the amplitude modulation waveform, Ω(t) represents the frequency or phase modulation and fc is the carrier frequency. It is important to make the rather obvious observation at this stage that all signals transmitted and received are real valued. Of course we can represent the signal as the result of amplitude modulating a two sinusoids with opposite phase s(t)=w(t).cos(2πf t)cos(2πΩ(t))- sin(2πf t)sin(2πΩ(t)) which in complex form is s(t)=real{w(t)e
π
e
πΩ
}
If w(t) is real then, s(t)=w(t)e
πΩ
The variability of the waveform of course depends on its form. Often phase coded pulses are used which, in theory, switch instantaneously, but limitations in the practical
implementation usually restrict the highest frequency components of the waveform to be no more than of the carrier frequency. In the complex domain we can write the signal as s(t)=real{e
π
}
The transmitted signal hits a target whose distance from the (collocated) transmitter and receiver is R. Let us assume for the moment that the target is stationary relative to the radar system and that it comprises a single scatterer. Then the return signal will, to a good approximation be a delayed version of the original signal which depends on the range R of the target. Specifically the signal voltage at the antenna of the receiver is
s
=As(t-- )
where c is the speed of light and A represents the attenuation of the signal by the reflection. Now we insert the possibility that the target is moving. This has the effect of modifying the transmitted waveform other than just by delay. It imposes the Doppler Effect on it. If this is done correctly it results in a “ time dilation” of the return signal, so that, If the target has a radial velocity v, the return signal s becomes
s
=As(αt-- )
Where
α=
⁄ ⁄
When v is much smaller than c this is approximated by
α=1-
Characteristics of Radio Waves Reflection Radio wave bounces off an object in direct relationship to the angle of approach.
Refraction Refraction refers to the breaking up of the radio signal when passing through a material which neither reflects nor absorbs the signal. An example of refraction is found when light passes through a broken windshield. The light is diffused into different directions.
Absorption
Absorption occurs when radio waves strike an object which neither reflects nor refracts the signal. Radio waves can be absorbed by the soil, leaves, rocks, etc. Most material will effect the radio wave with a combination of all three; reflection, refraction, and absorption.
Doppler Effect Ground-based radar systems used for detecting speeds rely on the Doppler Effect. The apparent frequency (f) of the wave changes with the relative position of the target. The Doppler equation is stated as follows for vobs (the radial speed of the observer) and vs (the radial speed of the target) and f0 frequency of wave :
However, the change in phase of the return signal is often used instead of the change in frequency. Only the radial component of the speed is available. Hence when a target is moving at right angle to the radar beam, it has no relative velocity, while one parallel to it has maximum recorded speed even if both might have the same real absolute motion.
Polarization In the transmitted radar signal, the electric field is perpendicular to the direction of propagation, and this direction of the electric field is the polarization of the wave. Radars use horizontal, vertical, linear and circular polarization to detect different types of reflections. For example, circular 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.
Cosine Effect If the target is in a direct line (collision course) with the police radar or sports radar gun the measured speed will be exact. As the angle of incidence increases, if you move either right or left of this direct line, the accuracy of radar decreases. The measured speed will decrease as you move off this centerline. This phenomenon is called the Cosine Effect. It is called this because the measured speed is directly related to the cosine of the angle between the radar and the target’s direction of travel. As a quick reference to radar accuracy, remember to keep your targets direction of travel in a direct line with you, and not perpendicular.
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.
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. 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. Beam Range Sensitivity 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.
Applications of Radar Military. Radar originally was developed to meet the needs of the military, and it continues to have significant application for military purposes. It is used to detect aircraft, missiles, artillery and mortar projectiles, ships, land vehicles, and satellites. In addition, radar controls, guides, and fuses weapons; allows one class of target to be distinguished from another; aids in the navigation of aircraft and ships; performs reconnaissance; and determines the damage caused by weapons to targets. The importance of radar in modern warfare is borne out by the many measures designed to negate its effectiveness (in addition to direct attack, which is an option for any military target of value). Attempts to degrade military radar capability include electronic warfare (jamming, deception, chaff, decoys, and interception of radar signals), antiradiation missiles that home on radar transmissions, reduced radar cross-section targets to make detection more difficult (stealth), and high-power microwave energy transmissions to
degrade or burn out sensitive receivers. A major objective of military radar development is to insure that a radar system can continue to perform its mission in spite of the various measures that attempt to degrade it.
Air traffic control Radar supports air traffic control by providing surveillance of aircraft and weather in the vicinity of airports as well as en route between airports. Many major airports also employ airport surface detection equipment (ASDE), which is high-resolution radar that provides the airport controller with the location and movement of ground targets within the airport, including service vehicles and taxiing aircraft. The location of dangerous weather phenomena such as "downbursts" (downward blasts of air associated with storms that have been identified as a major cause of fatal weather-related aircraft accidents) can be pinpointed with a specially configured terminal Doppler weather radar (TDWR) located near airports. Radar also has been used to "talk down" pilots to safe landings in adverse weather conditions. This is called ground-controlled approach (GCA) by the military.
Remote sensing One of the early applications of remote sensing involved the observation of rainfall. The radar measurement of the radial velocity of precipitation (from the Doppler frequency shift) in conjunction with the strength of the reflected signals (reflectivity) can indicate the severity of storms, as well as provide other important information for reliable weather forecasting. Astronomers have made radar observations of meteors, auroras, and certain planets. Synthetic aperture radars on orbiting spacecraft have mapped the surface of Venus beneath the ever-present cloud cover that blocks observation at optical wavelengths. Space-based radar systems have measured the Earth's geode and ocean roughness. An important application of imaging radar from either aircraft or spacecraft is the surveillance of sea ice; information about pack ice distribution and concentration is used to route shipping in cold-weather regions. Radar has even been used to study the movement of birds and insects at distances and under conditions where visual observation would not be possible.
Aircraft navigation The radar altimeter measures the height of an airplane above the local terrain, Doppler navigation radar determines the plane's own speed and direction, and high-resolution radar mapping of the ground contributes to its navigation. Radars carried aboard aircraft also provide information about the location of dangerous weather so that it can be avoided. Military aircraft can fly at low altitudes with the aid of terrain-avoidance and terrain-following radars that warn of obstacles.
Ship safety Small, relatively simple radar systems on board ships aid in piloting and collision avoidance. Similar radars on land provide harbor surveillance.
Space Applications Radars have been used in space for rendezvous, docking, and landing of spacecraft. Since size and weight are important in space, the same equipment is used on a timeshared basis aboard the U.S. Space Shuttle both as radar to allow rendezvous with (and sometimes retrieve) other spacecraft and as a two-way data link to relay satellites that communicate with ground stations. Besides providing remote sensing of the Earth's surface (see above), radar carried by orbiting spacecraft is able to monitor rainfall over the oceans. Large land-based radar systems permit the detection and tracking of satellites and space debris.
Law Enforcement The familiar police radar is a relatively simple, low-power continuous-wave system that measures the speed of vehicles by detecting the Doppler frequency shift introduced in the echo signal by a moving vehicle. The Doppler shift is directly proportional to the radial speed of the vehicle. A similar kind of CW radar is used to measure the speed of a baseball to determine how fast a pitcher can throw. Radar also has been used in security systems for intrusion detection; it can "sense" the movement of people attempting to penetrate a protected area.
Instrumentation Surveyors may make use of special radars to measure distances. CW radars are used to measure speed in certain industrial applications; the sensor does not make contact with the object whose speed is to be determined. Instrumentation radars are employed at missile test ranges for precision tracking of targets.
Latest Radar-Laser Radar It is also known as LIDAR or LADAR. “It is named from Light Detection And Ranging”. According to scientific research, Lidar does not emit any harmful radiation. The Lidar laser beam shoots 280 pulses per second and at 1,000 feet the beam is 3 feet square. The Laser Radar Beam travels 186,282 miles per second and that is 1 foot per billionth of a second. You can see it is important to keep the beam on one spot on the target. Cosine effect applies to Lidar in the same way it applies to Radar. It is most commonly used bad wheather.
Stealth Technology Stealth technology is a use of advanced design and specialized materials to make an aircraft difficult or even impossible to detect by radar. It is also termed as LO technology i.e. low observable technology. It is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, to make them less visible (ideally invisible) to radar, infrared, sonar and other detection methods. There are two different ways to create invisibility: v The airplane can be shaped so that any radar signals it reflects are reflected
away from the radar equipment. v The airplane can be covered in materials that absorb radar signals.