Radar: Echoes of Silent Vigilance, Unveiling the Technological Sentinel

By Fouad Sabry

What is Radar

Radar is a system that uses radio waves to determine the distance (ranging), direction, and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track aircraft, ships, spacecraft, guided missiles, motor vehicles, map weather formations, and terrain.

How you will benefit

(I) Insights, and validations about the following topics:

Chapter 1: Radar

Chapter 2: Phased array

Chapter 3: Doppler radar

Chapter 4: Synthetic-aperture radar

Chapter 5: Direction finding

Chapter 6: Active electronically scanned array

Chapter 7: Pulse repetition frequency

Chapter 8: Imaging radar

Chapter 9: History of radar

Chapter 10: Pulse-Doppler radar

(II) Answering the public top questions about radar.

Who this book is for

Professionals, undergraduate and graduate students, enthusiasts, hobbyists, and those who want to go beyond basic knowledge or information for any kind of Radar.

• Language: English
• Publisher: One Billion Knowledgeable
• Release date: Jun 20, 2024

Book preview

Chapter 1: Radar

Radar is a radiolocation system that measures distance, azimuth, and radial velocity with the aid of radio waves. It can map weather patterns and terrain features as well as detect and track air, sea, and spacecraft as well as guided missiles and automobiles. A radar system consists of a transmitter that generates electromagnetic waves in the radio or microwave range, an antenna for sending and receiving signals (often the same antenna is used for both), and a receiver and processor for analyzing the data. The location and velocity of objects can be determined through the use of radio waves (either pulsed or continuous) sent from the transmitter and reflected back to the receiver.

Several countries worked on radar in secret for military use before, during, and after World War II. The cavity magnetron, invented in the United Kingdom, was instrumental in enabling the development of compact systems capable of sub-meter accuracy. Originally an acronym for radio detection and ranging, the term RADAR was created in 1940 by the United States Navy. Since then, radar has become a common noun in English and other languages, and its initial capitalization has been dropped.

Air and ground traffic control, radar astronomy, air defense, anti-missile systems, marine radars for locating landmarks and other ships, aircraft anti-collision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring, altimetry and flight control systems, guided missile target locating systems, autonomous vehicles, and ground-penetrating radars are just some of the many modern applications of radar. In order to extract useful information from extremely noisy environments, modern high-tech radar systems use digital signal processing and machine learning.

Similar systems that use different parts of the electromagnetic spectrum to radar exist. Lidar is one such technique, and it differs from radar in that it uses infrared laser light rather than radio waves. As autonomous vehicles enter the market, radar will likely be used to help the vehicle keep an eye on its surroundings and avoid any mishaps.

German physicist Heinrich Hertz demonstrated the reflectivity of radio waves in 1886. In 1895, Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, used a coherer tube to create a device that could detect lightning strikes from a great distance. A spark-gap transmitter was added the following year. In 1897, he was conducting trials of this technology in the Baltic Sea to facilitate communication between two ships when he noticed an interference beat brought on by the passage of a third ship. Popov mentioned the possibility of using this phenomenon for object detection in his report, but he didn't follow up on this idea. and in the 1920s, led the British research establishment to many breakthroughs using radio techniques, such as ionosphere probing and long-distance lightning detection. Before turning his attention to shortwave transmission, Watson-Watt became an expert in radio direction finding through his lightning experiments. Since he needed a good receiver for his research, he instructed Arnold Frederic Wilkins, the new boy, to look into the best shortwave receivers on the market. After reading about the fading effect (the common term for interference at the time) described in the manual of a General Post Office model, Wilkins would make his selection.

U.S. Navy scientists A. Hoyt Taylor and Leo C. Young made this discovery in 1922 across the Atlantic when they set up a transmitter and receiver on opposite sides of the Potomac River and observed that the received signal faded in and out as ships passed through the beam path. The Navy did not immediately pursue Taylor's report, which suggested this phenomenon could be used to detect the presence of ships in low visibility. This phenomenon was first noticed by Lawrence A. Hyland, a researcher at the Naval Research Laboratory (NRL), eight years later.

The modern form of radar was developed independently and in secrecy by scientists in the United Kingdom, France, Germany, Italy, Japan, the Netherlands, the Soviet Union, and the United States before World War II. Following Great Britain's lead in radar development before World War II, Canada, New Zealand, and South Africa did the same, and Hungary developed its own radar technology in the midst of the conflict.

The year 1934 in France, after extensive research into the split-anode magnetron, the research branch of the Compagnie générale de la télégraphie sans fil (CSF) headed by Maurice Ponte with Henri Gutton, To paraphrase, "Sylvain Berline and M.

Hugon, started working on a radio-based obstacle-detection system, in 1935, the ocean liner Normandie had some of these components installed.

Those of France and the Soviet Union, however, featured continuous-wave operation that fell short of providing the peak performance that has come to be associated with contemporary radar.

The first true radars were pulsed systems, Moreover, American Robert M. Wilson showed off the first such simple device in December 1934.

Page, in the Navy's Experimental Testing and Evaluation.

This design was followed by a pulsed system demonstrated in May 1935 by Rudolf Kühnhold and the firm GEMA [de in Germany and then another in June 1935 by an Air Ministry team led by Robert Watson-Watt in Great Britain.

In 1935, Watson-Watt delegated the task of evaluating reports of a German radio-based death ray to Wilkins. Wilkins sent back some numbers showing that the system couldn't possibly work. When Watson-Watt followed up with a question about the potential uses of this system, Wilkins mentioned aircraft as a possible source of radio interference. This discovery resulted in the Daventry Experiment, which took place on February 26, 1935, and involved a bomber flying around a field where a GPO receiver had been set up and a powerful BBC shortwave transmitter. Air Member for Supply and Research Hugh Dowding was so impressed by their system's potential after it successfully detected the plane that he immediately approved funding for its continued operational development.

When Watson-Watt was appointed superintendent of the British Air Ministry's new Bawdsey Research Station on September 1, 1936, at Bawdsey Manor, near Felixstowe, Suffolk, the progress of radar technology took off. Due to their efforts, the Chain Home air-detection and tracking stations along England's East and South coasts were ready for use when World War II broke out in 1939. Without this system, the Royal Air Force would have always needed a large number of fighter aircraft, which Great Britain did not have, in the air to respond quickly, losing the Battle of Britain. Great Britain may have lost the Battle of Britain if it had to rely solely on the observations of individuals on the ground to detect German aircraft. In the Dowding system, which compiled reports of enemy aircraft and coordinated a response, the radar played a key role in both processes.

When the group was given the resources they needed to develop and produce working radar systems, they began deploying them in 1935. The first five Chain Home (CH) systems went live in 1936, and by 1940, they covered the entire United Kingdom (UK) and Northern Ireland. CH was rudimentary even for its time; rather than transmitting and receiving from a focused antenna, it lit up the area in front of it with a signal and used one of Watson-own Watt's radio direction finders to pinpoint the source of the reflected signals. This necessitated more powerful and higher-quality antennas for CH transmitters than those of competing systems, but made for a speedy rollout thanks to already-existing infrastructure.

The cavity magnetron, developed in the United Kingdom, was instrumental in enabling the development of compact systems capable of sub-meter accuracy. During the 1940 Tizard Mission, Britain gave the United States this technology.

The need for higher resolution, portability, and functionality in radar during the war prompted the development of new technologies, such as the supplementary navigation system Oboe, which the Royal Air Force's Pathfinder aircraft relied on.

Radar can tell you where something is located by giving you its bearing and range from the radar scanner. As a result, it finds widespread application wherever accurate positioning is an absolute must. Radar was first put to use by the military to detect land, air, and sea-based threats. Applications for airplanes, ships, and cars emerged in the civilian sector as a result. Common security features include automatic door opening, lighting, and intruder detection.

The radar system's transmitter sends out signals of radio waves (radar signals) in a predetermined pattern. When these waves collide with an object, some of them will be reflected or scattered in all directions, while others will be absorbed by the object and travel deeper inside. Materials with high electrical conductivity, like most metals, water, and wet ground, are excellent reflectors of radar signals. Because of this, radar altimeters can be used in some situations. For radar detection to take place, the reflected radar signals must reach the radar receiver. Due to the Doppler effect, the frequency of radio waves will shift slightly depending on whether the object is moving toward or away from the transmitter.

It's common for radar receivers to be situated close to the radar transmitter, but this is not always the case. The radar signals that are reflected and picked up by the receiving antenna are typically very weak. Electronic amplifiers can be used to amplify them. Radar signals are recovered using more complex signal processing techniques.

Radar systems can detect objects at relatively long ranges because radio waves are only weakly absorbed by the medium through which they pass, unlike other electromagnetic wavelengths like visible light, infrared light, and ultraviolet light. Fog, clouds, rain, snow, and sleet are all examples of weather phenomena that can obscure visibility but allow radio waves to pass through. Water vapor, raindrops, and atmospheric gases (especially oxygen) absorb or scatter certain radio frequencies, so these are avoided when designing radars.

Instead of using natural light from the Sun or Moon, or electromagnetic waves (EMWs) emitted by the target objects themselves (such as infrared radiation), radar uses its own transmissions to detect and track them (heat). Although neither human eyes nor optical cameras can see radio waves, they can be used to artificially illuminate objects.

Electromagnetic waves will reflect or scatter at the boundary between materials if they are traveling through one and then encountering another with a different dielectric constant or diamagnetic constant. Radar (radio) waves are normally scattered from the surface of a solid object in air or a vacuum, or when there is a significant difference in atomic density between the object and its surroundings. Radar is especially effective at picking up on aircraft and ships because they are made of electrically conductive materials like metal and carbon fiber. Military vehicles have radar absorbing material, which is made up of resistive and sometimes magnetic substances, to reduce radar reflection. If you wanted to make something invisible at night with your eyes, this is what you'd do with radio waves.

The radar wave's size (wavelength) and the target's shape determine how the radar wave scatters. If the wavelength is smaller than the size of the target, the wave will reflect off of it like light bouncing off of a mirror. It's possible that you won't be able to see the target if the wavelength is significantly longer than its size. Resonances are essential to the detection, but not the identification, of targets by low-frequency radar technology. Rayleigh scattering, the phenomenon responsible for the coloration of Earth's skies and sunsets, describes this phenomenon. Resonances may occur if the two length scales are roughly the same. Many modern radar systems use shorter wavelengths (a few centimeters or less) that can image objects as small as a loaf of bread, whereas early radars used very long wavelengths that were larger than the targets and thus received a vague signal.

The reflection of short radio waves off of corners and curves is analogous to the sparkle off of a sphere of glass.

The most reflective targets for short wavelengths have 90° angles between the reflective surfaces.

A corner reflector has three parallel surfaces that meet in a right angle, much like the interior of a cube.

Waves that pass through the structure's opening will be reflected back in the same direction they came.

They are frequently used as radar reflectors, which improve the detectability of otherwise challenging targets.

Boats with corner reflectors, for example, increase their visibility to aid in rescue efforts and collision avoidance.

In a similar vein, Inside corners and surfaces/edges perpendicular to likely detection directions are not characteristics of objects designed to evade detection, because of which stealth aircraft end up looking odd.

Even with these measures, some reflection will still occur due to diffraction, the more so at larger wavelengths.

Long, conductive wires or strips, equal to half a wavelength, similar to chaff, reflect a lot of light but don't send any of that energy back where it came from.

Radar cross section measures how much an object reflects or scatters radio waves.

The power Pr returning to the receiving antenna is given by the equation:

{\displaystyle P_{r}={\frac {P_{t}G_{t}A_{r}\sigma F^{4}}{{(4\pi )}^{2}R_{t}^{2}R_{r}^{2}}}}

where

Pt = transmitter power

Gt = gain of the transmitting antenna

Ar = effective aperture (area) of the receiving antenna; this can also be expressed as {{G_{r}\lambda ^{2}} \over {4\pi }} , where

\lambda = transmitted wavelength

Gr = gain of receiving antenna

σ = radar cross section, coefficient of dispersion, of the target

F = factor of pattern propagation

Rt = distance from the transmitter to the target

Rr = distance from the target to the receiver.

When both the sender and the receiver are in the same physical location, Rt = Rr and the term Rt² Rr² can be replaced by R⁴, , where R denotes a range.

This yields:

P_{r}={{P_{t}G_{t}A_{r}\sigma F^{4}} \over {{(4\pi )}^{2}R^{4}}}.

Given that the received power falls off as the fourth power of the range, weak signals can only be detected from extremely far away targets.

To improve the detection range and decrease the transmit power of pulse-Doppler radars, the radar equation is slightly modified by adding filtering and pulse integration.

For interference-free transmission in a vacuum, the above equation can be simplified to F = 1. The propagation factor takes into account environmental factors like multipath and shadowing. Pathloss effects are also taken into account in the real world.

When the distance between the radar and the reflector changes, the frequency changes as a result. Depending on how this impacts detection, radar performance may suffer or improve. For instance, signal degradation occurs when moving target indication interacts with Doppler at specific radial velocities.

Doppler effect is used to improve the performance of sea-based radar systems, semi-active radar homing, active radar homing, weather radar, military aircraft, and radar astronomy. During the detection process, this yields data about the speed of the target. This also enables the detection of small objects in a setting with nearby, relatively slow-moving objects of much larger sizes.

The radar's active or passive setup determines the amount of Doppler effect. The signal from an active radar system is broadcast and then received after being reflected. For passive radar to work, an object must actively transmit a signal to a receiving antenna.

For operational radar, the Doppler frequency shift looks like this:, where F_{D} is Doppler frequency, F_{T} is transmit frequency, V_{R} is radial velocity, and C is the speed of light:

F_{D}=2\times F_{T}\times \left({\frac {V_{R}}{C}}\right) .

Both radio astronomy and electronic countermeasures can benefit from passive radar:

F_{D}=F_{T}\times \left({\frac {V_{R}}{C}}\right) .

The only part of the velocity that matters is the radial part. At a 90 degree angle to the radar beam, the reflector's relative velocity is zero. Doppler frequency shifts are largest for targets moving perpendicular to the radar beam.

When the transmit frequency ( F_{T} ) is pulsed, using a pulse repeat frequency of F_{R} , the resulting frequency spectrum will contain harmonic frequencies above and below F_{T} with a distance of F_{R} .

This means, the Doppler measurement is only non-ambiguous if the Doppler frequency shift is less than half of F_{R} , The Nyquist frequency, to give it its proper name, Given that the absence of this condition renders the returned frequency indistinguishable from the addition or subtraction of a harmonic frequency,, thus requiring:

{\displaystyle |F_{D}|<{\frac {F_{R}}{2}}}

Or when substituting with F_{D} :

{\displaystyle |V_{R}|<{\frac {F_{R}\times {\frac {C}{F_{T}}}}{4}}}

For instance, an aircraft traveling at 1,000 meters per second would be too fast for a Doppler weather radar with a pulse rate of 2 kilohertz and a transmit frequency of 1 gigahertz to accurately measure its radial velocity (2,200 mph).

The electric field, which is the wave's polarization, is always perpendicular to the direction of propagation in electromagnetic radiation. Radar signals sent through space can be altered in appearance by manipulating their polarization. Radars can detect a wide variety of reflections thanks to their ability to switch between horizontal, vertical, linear, and circular polarization. Circular polarization, for instance, is used to lessen the impact of precipitation on a signal. In most cases, metal surfaces can be identified by the linear polarization returns. Navigation radars use random polarization returns, which typically indicate a fractal surface like rocks or soil.

Due to differences in the refractive index of air, the radar horizon causes a radar beam to deviate slightly from its straight path in a vacuum. The beam still rises above the ground because of the Earth's curvature, even when it is emitted perpendicular to the ground. In addition, the beam spreads out and the signal weakens as it travels through the medium.

Several factors limit the maximum range of conventional radar:

Visibility, which is affected by altitude. If there is anything in the way of the beam's path, it will not go through.

Range at which no meaning is lost, limited only by the frequency of the pulses being sent. A pulse's maximum non-ambiguous range is the maximum distance it can travel to and back before another pulse must be emitted.

The radar equation for calculating radar sensitivity and the strength of the returned signal. This aspect takes into account variables like the weather and the target's dimensions (its radar cross section).

All electronic components contribute to what is called signal noise, or random fluctuations in the signal.

Noise limits the radar's range because reflected signals weaken exponentially with distance. Performance metrics like range are affected by both the noise floor and the signal-to-noise ratio. The signal from distant reflectors is too weak to be detected because it does not rise above the background noise level. To be detected, a signal must be at least as large as the signal-to-noise ratio above the background noise level.

In a radar receiver, noise manifests as erratic variations superimposed on the expected echo signal. Distinguishing a weak signal from background noise is more challenging. To achieve the best results, receiver noise, as measured by the noise figure, should be kept to a minimum as much as possible.

All detectors experience shot noise, which is caused by electrons passing through a gap or barrier. Most receivers' primary noise contribution comes from shot noise. Heterodyne amplification is used to lessen the flicker noise produced by electron transit through amplification devices. Another benefit of heterodyne processing is that the instantaneous bandwidth grows linearly with frequency for a given fractional bandwidth. The result is better range resolution. However, ultra-wideband radar is a notable departure from heterodyne (downconversion) radar systems in general. Like ultra-wideband (UWB) communications (for a list of UWB channels), a single wave cycle is used here.

There are also external sources of noise, especially the ambient heat from the environment around the object of study.

Current radar technology, There is usually as much or less noise coming from within as there is from without.

Unless the radar is pointed at the clear sky above, situations where there is little to no thermal noise because of how cold the scene is.

The thermal noise is given by kB T B, the temperature, denoted by T, B is bandwidth (post matched filter) and kB is the Boltzmann constant.

In a radar, this connection can be intuitively understood, which is a plus.

By using matched filtering, all of the incoming power from a target can be shoved into a single bucket, Doppler, elevation, Binocular Azimuth.

At first glance, it would appear that within a predetermined amount of time, perfect, error free, possibility of detection.

To achieve this, we concentrate all of our energy