Examination Of Radio Detection And Ranging English Language Essay

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This study examines the term radar which stands for radio detection and ranging. Imagine a pilot travelling through thick fogs at night, he can't see where he is going, basically he uses radar to help him.

This study examines how by using electromagnetic wave (radio wave), the range, location, altitude, direction, or speed of an object whether fixed or moving examples which are aircrafts, ships, can be determined. This is done by transmitting a radio wave into the air with the use of an antenna and listens for anything for several seconds before it transmit again. The reflected radio wave received by the antenna is forwarded to a computer in the receiver unit which processes it that is filters out the noise and then display only important reflections on a radar screen usually television-like, and is constantly monitored by a human operator

This is practically done using the computer software called mat-lab where the concept of cross-correlation is applied. "Cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them"

A signal is transmitted; the signal is reflected off the target and returns. There is obviously a time difference (delay) between the time the signal is transmitted and the time it returns. The time difference between the transmitted signal and returned signal is then estimated or measured. "A common method of estimating the time delay is to compute the cross-correlation function of the received signal with the transmitted signal" [1].

Attestation

The work presented in this dissertation is entirely from the studies of the individual stu dent, except where otherwise stated. Where derivations are presented and the origin of the work is either wholly or in part from other sources, then full reference is given to the original author. This work has not been presented previously for any degree, nor is it at present under consideration by any other degree awarding body.

Signature (you must sign and date this page) Date: March 22nd 2011

Statement of Availability

I hereby acknowledge the availability of any part of this dissertation for viewing, photocopying or incorporation into future studies, providing that full reference is given to the origins of any information contained herein.

Acknowledgements

I would like to thank the Almighty God for life and his guidance, because without him nothing would have been possible.

My appreciation also extends to my supervisor, Prof. Paul Spencer for his thorough guidance, invaluable feedback, support and encouragement during the course of the project.

I would also like to thank Will Thomson and Oz Huner for his input and help with my programming

Lastly and very importantly, I would like to thank my family for their constant support, encouragement, criticism, prayer and support, without them, it would have been very hard, infact almost impossible.

Table of Contents

The table of contents below is automatically generated from the paragraphs of style Heading N and Unnumbered N. To update this after revisions, right-click in the table and choose Update Field for the entire table.

RADAR 1

Abaniwonda David 1

Abstract i

Abstract i

Acknowledgements iii

Acknowledgements iii

Table of Contents iv

Table of Contents iv

List of Figures v

List of Figures v

1.2.Background xii

1.2.Background xii

List of Figures

Illustration of a Radar system viii

Representation of an unmodulated radio (carrier) wave xii

A simple magnetron xiii

Figure 2.1. Block diagram of a radar system xvi

Figure 2.2. A typical Radar screen being monitored by a human operator xvi

Project aims and Objectives

The Primary aim of this project is to understand the operation and principles "Radar". Although many readers may not be familiar with it, hence, a sketch of the background is provided.

The aim of the project has been complemented by the following objective

Understand the concept of Radio waves and how it is generated

Research how Radar works, it functions and application

Understand the Signal Processing techniques

Learn how to program using mat lab

Research and learn about the future development of the Radar system.

Introduction and Background

Introduction

What is Radar?

All objects in the world can be seen through light which comes from the sun, it reflects off the object into our eyes. Imagine walking in the dark with a torch light, the torch light is shined towards our direction, light then travel from the torch light unto an object in our direction, bounces into our eyes, and our brain then works out how far or near the object is, and then we can walk our way through it either by picking them up, or walk by it making sure we don't collide with them.

Radar basically works like the torch light, but this time, it uses radio waves instead of light waves. Radar using radio waves to detect the presence of an object, and find its location. The word "radar" is derived from "radio detection and ranging" and from that, it can be seen it does two main jobs, detection and location.

The modern radar goes further and is being developed to identify targets and even produce images of them.

The target

Transmitted signal

Returned signal

Transmitting and receiving dish

Illustration of a Radar system

.

Radio waves are generated with an instrument called the magnetron which basically generates high frequency radio waves and is transmitted with an antenna which bounces off any object in its path and reflects back, and with the reflected wave, the object can be detected and located. Properties of returned reflected signal can be used to determine certain properties about the object or its shape [3].

Radar further permits the measurement of the range of the object it detects and can also measure the instantaneous speed of such an object.

History of Radar

The complete history of where radar came from, its growth, is a long and complicated one, "but the British were the first one to fully exploit radar as a defence against aircraft attack.

This was spurred on by fears that the Germans were developing death rays. The Air Ministry asked British scientists in 1934 to investigate the possibility of propagating electromagnetic energy and the likely effect. Following a study, they concluded that a death ray was impractical but that detection of aircraft appeared feasible".

During the World War 1, a Scottish physicist named "Robert Watson-Watt (1892-1973)" went to work for the British weather forecasting company to use radio waves to detect upcoming storm. [4]

Before World War II started, Robert Watson-Watt and his assistant Arnold Wilkins realized they could use the same kind of technology to detect aircraft (enemies). It was proven it could work, so they constructed an elaborate network of ground-based radar detectors around the south and east of the British coastline. In the course of the war, Britain's radar defences (known as "Chain Home"(McGraw Hill 1947)) gave it an enormous edge over the German air-force which contributed to the ultimate allied victory. "The victory served as the basis for the Chain Home network of radars to defend Great Britain" (McGraw Hill 1947).

In April 1940, popular science showed an example of a radar unit using the Watson-Watt patent in an article on air defence, but not knowing that the U.S. Army and U.S. Navy were working on radars with the same principle, stated under the illustration, This is not U.S. Army equipment

The war was followed by research to find better resolution, more portability, and more features for radar, including complementary navigation systems like Oboe used by the RAF's Pathfinder .The years after the war has seen the radar technology being used in other fields such as air traffic control, speed control, weather monitoring, astrometry and other diversities [5].

Applications of a Radar system

The following examples give an idea to the various applications of a radar system. They will be divided into two subheadings; the first is the civil application which basically deals with how it affects the society around us, while the second subheading will list its application in the military that is the armed forces point of view.

Civil applications

The civil application of a radar system basically means how radar affects our society in general. Examples of the civil application of a radar system can be divided into the following:

Ground-based system

Air-borne system

Sea-borne system

Space-borne system

Ground-based system

Air traffic control

Sea traffic control

Weather forecasting

Speed traps

Intruder alarms

Radar astronomy

Ground probing

Industrial measurement

Air-borne system

Altimeters

Navigation

Weather

Sea-borne system

Navigation

Collision avoidance

Space-borne system

Studying earth resources

Sea sensing

Manipulating space craft

Mapping planet and minor bodies

Military applications

This section deals with how Radar is being applied in the armed force. Examples of its applications are

Detection of its own and external forces

Tracking of air, sea, land or space target

Guidance of own weapon system

Background

This section begins with the question: what are Radio waves? The concept of radio waves is introduced; its brief history, how it is generated, and finally, the present day applications of radar would be looked at.

The first part of this section talks about radio waves, while the rest of the section will place emphasis on the applications of radar

What are Radio waves

According to Wise geeks (2003), radio waves has one of the widest ranges in the electromagnetic spectrum of about a millimetre to 100,000 km, they vary in length and are usually not visible. "Radio" is a catch-all term describing all forms of electromagnetic radiation with a wavelength longer than a millimetre and a frequency above 300 GHz

Radio is mostly known for sending images, sounds and text in the form of signals. "Guglielmo Marconi and Nikola Tesla are both credited with being early pioneers in the area of radio" Its long wavelength permits it to go round or avoid obstacles and travel long distances, which is not the case with visible light and other spectra of higher frequencies [7].

Wikipedia describes it as a type of electromagnetic radiation with wavelengths in the eleHYPERLINK "http://en.wikipedia.org/wiki/Electromagnetic_spectrum"ctromagnetic spectrum longer than infrared light. Like all other electromagnetic waves, they travel at the speed of light. Naturally-occurring radio waves are made by lightning, or by astronomical objects. Artificially-generated radio waves are used for fixed and mobile rHYPERLINK "http://en.wikipedia.org/wiki/Radio_communication"adio communication, broadcasting, radar and other navigation systems, satellite communication, computer networks and innumerable other applications [8]

Representation of an unmodulated radio (carrier) wave

In summary, Radio waves are a type of electromagnetic radiation with long wavelengths, and travel at the speed of light.

"Radio waves" transmit music, pictures and data even conversations through the air over millions of miles daily. Although radio waves are not visible and completely undetectable to humans, its contribution has changed the society.

Its brief discovery

James Clerk Maxwell first predicted Radio wave which was done through experiments done and published in 1865. He noticed wave-light property of light and its similarities in his observations in electric and magnetic property. He then moved on to prose equation that describes described light waves and radio waves as waves of electromagnetism that travel in space [9].

(J.C Toomay & Paul J. hannen 2004) The first use of radar was for communication and the means for generating radio waves was with spark gaps generating short, intense pulses of current to achieve the needed electromagnetic radiation [10]

In 1886, Heinrich Hertz conducted a number of experiments showing that radio wave reflected, refracted, and were polarized, interfered with each other, and travelled at high velocity. Hertz is credited for proving Maxwell's theory. [Encyclopedia Britannica 1984, vol6, pp 647-648].

Several other inventions followed, practically enabling transferring information to and fro space a possibility

How its being generated

Radio wave is being generated by an instrument called a Magnetron. "It's a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field"

A simple magnetron

In 1921 Albert Wallace Hull invented the magnetron as a microwave tube. During World War II it was developed by John Randall and Henry Boot to a powerful microwave generator for Radar applications.

The way a magnetron generates radio waves is similar to the operation of a T.V set. Using the illustration below, the process at generating radio waves is explained.

There's a heated cathode (a solid metal rod) at the center of the magnetron. Here its colour orange.

A ring-shaped anode surrounds the cathode (colour red).

If you switched on a simple magnetron like this, electrons would boil off from the cathode and zip across to the anode in straight lines (shown by the black arrow) much like the electron beam in a TV set. But there are two added extra bits in a magnetron that change things completely.

First, the anode has holes or slots cut into it called cavities or resonant cavities. Second, a powerful magnet is placed underneath the anode to generate a magnetic field along the length of the tube (parallel to the cathode and, in this diagram, going directly into the computer screen away from you).

Now when the electrons try to zip from cathode to anode, they are travelling through an electric field (stretching between the anode and cathode) and a magnetic field (produced by the magnet) at the same time. So, like any electrically charged particles moving in a magnetic field, they feel a force and follow a curved path (blue circle) instead of a straight one, whizzing around the space between the anode and the cathode.

As the electrons nip past the cavities, the cavities resonate and emit microwave radiation. Think of the electrons passing energy to the cavities, making then resonate like someone blowing on the open end of a flute-only producing microwaves instead of sound waves.

The microwave radiation that the cavities produce is collected up and channelled by a kind of funnel called a waveguide, either into the cooking compartment of a microwave oven or beamed out into the air by an antenna or satellite dish in radar equipment[11].

Principles of operation

2.1. How Radar works?

The principles of Radar are very basic; they transmit, propagate and reflect. Radar works by sending out radio waves from a transmitter powerful enough so that measurable amounts of radio energy will be reflected from the objects to be seen by the radar to a radio receiver usually located, for convenience by the side of the transmitter" (McGraw-Hill 1947).

The components of a radar are very basic, and are as follows

Something to generate the wave (Magnetron)

Something to haul the wave into the air (Transmitter)

Something to receive the reflected signal (Transmitter)

Something to display the processed reflections (Radar Screen).

Figure 2.1 explains a basic radar system. A pulse of electromagnetic energy (radio waves) is oscillated at an already determined frequency f0 and a period Ï„. The pulse is then routed through a duplexer to an antenna. A duplexer performs a dual role of transmitting and receiving. The pulse is then hauled into free space by the antenna. The pulse then travels, scattering from objects it comes in contact with along its way, and part of the scattered signal then returns to the radar which is then collected by the antenna and routed through the duplexer to the receiver. Then returned signal can then be detected by the receiver because it resembles or imitates the frequency and duration of the transmitted signal.

Transmitter

Power

Supply

Control

Data signal processing

Storage

Antenna

Signal

Processor

Receiver

Duplexer

Figure 2.1. Block diagram of a radar system

While the received signal is enhanced, the noise or interference signal is reduced, making it possible for the measurement of the object, and this is done by a process called signal processing which would be looked at in the next section. The resulting detection of the returned signals is represented on display usually called a radar screen and is watch constantly by a human operator.

Figure 2.2. A typical Radar screen being monitored by a human operator

(Wikipedia) Furthermore, there is usually a time delay between the time it takes for the wave to be transmitted and received. Usually, it travels at the speed of light f which is approximately 186,000 miles per second or 3 x 108 meters per seconds [6]. The delay between the transmissions of pulse and receiving it from an object at x range will be

Where T is the period/delay

Factor 2 because the distance to the target has to be traversed twice, once out and once back

X stands for the range of the object in seconds

f stands for speed of light in meters per seconds

2.1.1 Range Measurement

If the radar transmission is a pure continuous wave with frequency f0, the backscattered wave will have the same frequency (if the relative velocity between radar and target is equal to zero), whatever the range, However, the greater the target range, and the lower the Radar Cross-section (RCS) of the target, the weaker the received signal. The RCS characterizes the backscattering coefficient of the target.

The target range can be obtained using one of several methods

By calculating the time between the detected target echoes and the transmitted wave

By calculating the difference in frequency between the received echo and the transmitted wave in the case of linear frequency modulation

By calculating the time between the detected target echoes and the transmitted wave

By calculating the difference in frequency between the received echo and the transmitted wave in the case of linear frequency modulation

By calculating the differential phase of the double detection of an echo obtained using two transmissions of different frequencies

The following sections give a rapid overview of the first two methods.

2.1.2. Time Measurement

In order to obtain the target range by calculating the time between the transmitted wave and the detected echo, the radar signal should be emitted in short pulses as shown below

Power Transmission

Pt Reception:

Target echo

t0

t

TR

TR

Figure 2.3. Pulse Modulation

A radar using this type of transmission is known as a "pulse radar". It periodically transmits microwaves with peak power Pt. The interval between two pulses is known as the interpulse period, TR. Under such conditions, measurement of time t0, equal to the wave propagation time on the two-way path between radar and target, gives the range R between the radar and the target.

Note that the frequency of the wave transmitted has no influence on this measurement:

2.1.3. Frequency Shift Measurement

In order to obtain the radar-target range by calculating the difference in frequency between the transmitted wave and the detected echo, transmission must be linearly frequency-modulated.

Ignoring the Doppler effect, the range between the radar and the target is given by

In which

fm = maximum modulation frequency

= difference between transmission and reception

c.TR/2 = range domain without range and ambiguity.

2.2. Graphical Illustration of a radar system operation

Figure 2.4. Graphical representation of the principles of radar

Description of the diagram above:

Magnetron generates high-frequency radio waves.

Duplexer switches magnetron through to antenna.

Antenna acts as transmitter, sending narrow beam of radio waves through the air.

Radio waves hit airplane/ship, e. t. c and reflect back.

Antenna picks up reflected waves during a break between transmissions.

Duplexer switches antenna through to receiver unit.

Computer in receiver unit processes reflected waves and draws them on a TV screen.

Air planes shows up on TV radar display with any other nearby targets.

Waveforms

The first generations of airborne radars almost exclusively used magnetron transmitters. Magnetrons are microwave "oscillator" tubes that deliver high peak power (of the order of 100kW), with a mean power approximately 1,000 times weaker (100W). For a magnetron to oscillate at its own frequency, it must be triggered by a modulator supplying it with a high power "rectangular" pulse. This pulse is generally 1µs. Given the magnetron "form factor," it can only be reproduced every 1ms. A radar fitted with this type of transmitter is known as Low Pulse Repetition Frequency (LPRF) and will be unambiguous in range if used exclusively within a 150km range domain (i.e., with a PRF of 1000 Hz).

As a first approximation, the waveform is determined by the transmitter. Receiver protection and CRT display sweeping ( as well as certain receiver and processing circuits) should function synchronously.

The Transmitter

Radar transmission can be obtained using either resonating microwave tubes such as magnetrons or amplifier tubes such as certain klystrons or Travelling wave tubes (TWT). Solid-state transmitters have recently been used; these deliver low peak power and can be used, for example, for missile homing heads and active radar antennas/

Returning to the magnetron transmitter, its main characteristics are as follows ( all other considerations being equal):

Low cost, bulk and weight

High peak power/, mean power ratio

Good efficiency levels

Low magnetron duty cycle(50 to 200 Hz)

Fixed frequency oscillations, linked to mechanical aspects but variable in temperature and with non-negligible phase and amplitude noise levels. This type of magnetron is known as a tunable magnetron. So called "coaxial" magnetrons have more stable frequencies. Some special types of magnetron can be frequency modulated by a few percent using a small motor

The Antenna

The first airborne antennas were composed of a set of dipoles, giving a certain amount of directivity. They were rapidly replaced by antennas fitted with parabolic reflectors with a feed at the center of the reflector

Parabola

Duplexer Feed

Equiphase

Plane

Figure. 2.5. Parabolic Antenna Dish

Energy transmitted from the duplexer via the feed, which can be a mini horn, illuminates the entire parabola, which, via reflection, forms a beam with parallel rays. This ensures optimal directivity. The feed which causes slight blockage, illuminates just the parabola. On reception, the waves backscattered by the target follow the same trajectory but in opposite direction. Antenna gain and directivity are thus doubly influential.

The gain and directivity of the antenna main lobe depend on the dimensions of the antenna in relation to the wavelength used (λ) and the efficiency (n). Gain (G) is defined as the ratio between the energy radiated along the radioelectric axis and that radiated by an omnidirectional antenna (isotropic). Where S is equal to the antenna surface area, the gain is as follows:

For a circular parabolic dish antenna with a 60cm diameter, λ = 3 cm, and = 70%

G = 2,800 = 34.5 dB.

Antenna directivity is characterized by the aperture of the main beam. The narrower the beam, the greater the directivity. Directivity plays a vital role in determining the direction of the target seen by the radar.

Two factors should be taken into consideration:

Total beamwidth measured between the two beam zeros (ѳnn)

Beamwidth at 3 dB (ѳ3dB). This beamwidth is by far the most frequently used

The Display

After the returned signal has been processed, there is a need to view the information by an operator, that's where the display system comes in. This brought about the Cathode Ray Tube (CRT) displays.

Radar Equation

For any mission/purpose whatsoever, the most important and fundamental determinant of radar performance is the radar equation.

An Engineer main concern is to determine the radar power budget which consists of three parts:

Transmission of energy to the target

Backscattering part of that energy back to the radar

Reception of the backscattered energy by the radar.

The power budget enables the calculation of the parameters that is required to ensure the required range performance. A major aspect in signal processing is the backscattering of the signal from the target; it is very unpredictable and must be dealt with statistically.

3.1. Signal Transmission and Reception

The Role of Antenna in transmission

The role of the antenna on transmission is to concentrate the energy transmitted along a chosen direction in space.

P1(k) is the power transmitted in direction k by a directive antenna, and p2(k) is the power transmitted by an omnidirectional antenna in that same direction. The transmission source, pt, is the same for both antennas.

By definition, the gain, Gt(k) =

Pt is the power transmission source.

The power transmitted within solid angle dΩ in the direction k by the omnidirectional antenna is

dp2 =

The power transmitted inside dΩ by the directive antenna is

dp1 = Gt (k) dp2 =

In free space, the energy is retained in angle dΩ. At range R, the area intercepted by dΩ on a sphere with a radius R is

dS = dΩR2.

The Power density, W, per area unit is therefore

Role of antenna on Reception

If an energy sensor with geometric area Sg normal to k, is placed at range R in solid angle dΩ, the power crossing Sg is as follows:

Pr = W . Sg

In reality, an antenna, with area Sg, only captures part of Pr(due to losses, weighting function, etc). By definition, the effective area Sef is an area such that

Pr = W . Sef.

Sef is the ideal geometric area of an antenna capturing Pr with a power density W.

For the same antenna, either transmitting or receiving, this gives the ratio

Where λ is the wavelength.

Reflection from the target

The target receives part of the transmitted energy. The incident EM field excites currents on the target, which then reradiates the energy in directions determined by its shape and material construction, and in a manner that depends (often very strongly) on the geometry and polarization of the incident field. In short the target acts very much like an "inefficient antenna" and usually does not reradiate most of the energy in the backward direction (toward the radar). This is called "target scattering".

On reception, the target acts as an antenna with an area Sef = σ aimed at the transmitter. The power captured by this antenna is radiated Omni directionally without loss.

The value of σ known as the Radar Cross Section (RCS) is such that the power captured by the radar receiver is the same as when the model is used in place of the real target.

This example is an ideal illustration of backscattering for this particular configuration. However, the value of σ represents the target for this configuration only. The slightest alteration of this configuration can cause major modifications to σ.

Radar equation in free space

Considering a radar transmitting power Pt in the direction of a target located at distance R with an antenna gain of Gt.

The power density at the target is as follows

The power received by the target is Pc = W . σ, where σ is the effective area of the target considered as a receiving antenna. The target diffused this power isotropically in accordance with the model.

The power received by the radar receiver antenna, which has an effective area Sef is

,

Giving the power budget

Replacing Sef withyields the power budget

Notes

For a monostatic radar( one that uses the same antenna for transmission and reception)

Gt = Gr = G.

Pe and Pr designate either peak power and mean power.

For ease of measurement, transmitted power is generally measured directly at the transmitter output; the received power is measured directly at the receiver input. Microwave elements between the transmitter and the antenna on the other hand, and the antenna and the receiver at the other, crate lose l (with l > 1) that must be taken into account.

The Radar equation is therefore generally written as:

Limiting factors

Limiting factors basically means factors that affect the quality of signal received. These factors are considered below.

4.1. Noise

Signal noise is an internal source of random variations in the signal, which is generated by all electronic components. Noise typically appears as random variations superimposed on the desired echo signal received in the radar receiver. The lower the power of the desired signal, the more difficult it is to discern it from the noise (similar to trying to hear a whisper while standing near a busy road). Noise figure is a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized.

Noise is also generated by external sources, most importantly the natural thermal radiation of the background scene surrounding the target of interest. In modern radar systems, due to the high performance of their receivers, the internal noises is typically about equal to or lower than the external scene noise. An exception is if the radar is aimed upwards at clear sky, where the scene is so "cold" that it generates very little thermal noise.

There will be also flicker noise due to electrons transit, but depending on 1/f, will be much lower than thermal noise when the frequency is high. Hence, in pulse radar, the system will be always heterodyne. See intermediate frequency.

4.2. 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.

In less technical terms, SNR compares the level of a desired signal (such as targets) to the level of background noise. The higher a system's SNR, the better it is in isolating actual targets from the surrounding noise signals.

4.3. 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.

Some clutter may also be caused by a long radar waveguide between the radar transceiver and the antenna. In a typical plan position indicator (PPI) radar with a rotating antenna, this will usually be seen as a "sun" or "sunburst" in the centre of the display as the receiver responds to echoes from dust particles and misguided RF in the waveguide. Adjusting the timing between when the transmitter sends a pulse and when the receiver stage is enabled will generally reduce the sunburst without affecting the accuracy of the range, since most sunburst is caused by a diffused transmit pulse reflected before it leaves the antenna.

While some clutter sources may be undesirable for some radar applications (such as storm clouds for air-defence radars), they may be desirable for others (meteorological radars in this example). Clutter is considered a passive interference source, since it only appears in response to radar signals sent by the radar.

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.

Constant False Alarm Rate (CFAR, a form of Automatic Gain Control, or AGC) is a method relying on the fact that clutter returns far outnumber echoes from targets of interest. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software controlled, and affected the gain with greater granularity, in specific detection cells.

Radar multipath echoes from a target cause ghosts to appear.

Clutter may also originate from multipath echoes from valid targets due to ground reflection, atmospheric ducting or ionospheric reflection/refraction (e.g. Anomalous propagation). 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.

4.4. 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 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. 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 is problematic to radar since the jamming signal only needs to travel one-way (from the jammer to the radar receiver) whereas the radar echoes travel two-ways (radar-target-radar) and are therefore significantly reduced in power by the time they return to the radar receiver. Jammers therefore can be much less powerful than their jammed radars and still effectively mask targets along the line of sight from the jammer to the radar (Mainlobe Jamming). Jammers have an added effect of affecting radars along other lines of sight, due to the radar receiver's sidelobes (Sidelobe Jamming).

Mainlobe jamming can generally only be reduced by narrowing the mainlobe solid angle, and can never fully be eliminated when directly facing a jammer which uses the same frequency and polarization as the radar. Sidelobe jamming can be overcome by reducing receiving sidelobes in the radar antenna design and by using an omnidirectional antenna to detect and disregard non-mainlobe signals. Other anti-jamming techniques are frHYPERLINK "http://en.wikipedia.org/wiki/Frequency_hopping"equency hopping and polarization. See Electronic counter-counter-measures for details.

Interference has recently become a problem for C-band (5.66 GHz) meteorological radars with the proliferation of 5.4 GHz band Wi-Fi equipment

Radar Signal Processing

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Exams can be one of the most stressful experiences you’ll ever have! Revision is key, and we’re here to help. With custom created revision notes and exam answers, you’ll never feel underprepared again.