RADAR TECHNOLOGY

WHAT IS RADAR?

     The word “Radar “is an acronym derived from the words Radio Detection And Ranging. It refers to the technique of using radio waves to detect the presence of objects in the atmosphere. Radar was designed shortly before World War II. Its primary purpose was to detect the presence of aircraft. Today, radar is used for a wide array of applications, but primarily to detect precipitation and other meteorological events.

Radar is an object-detection system that uses electromagnetic waves - specifically radio waves - to identify the range, altitude, direction, or speed of both moving and fixed objects.

If you want to walk at night, you can shine a torch in front to see where you're going. The light beam travels out from the torch, reflects off objects in front of you, and bounces into your eyes. Your brain instantly computes what this means: it tells you how far away objects are and makes your body move so you don't trip over things.

Radar works in much the same way.

The basic idea behind radar is very simple: a signal is transmitted, it bounces off an object and it is later received by some type of receiver.  This is like the type of thing that happens when sound echo's off a wall.  (Check out the image on the left)  However radars don't use sound as a signal.  Instead they use certain kinds of electromagnetic waves called radio waves and microwaves.  This is where the name RADAR comes from (RAdio Detection And Ranging).  Sound is used as a signal to detect objects in devices called SONAR (SOund NAvigation Ranging).  Another type of signal used that is relatively new is laser light that is used in devices called LIDAR (you guessed it...LIght Detection And Ranging).             Radio waves and microwaves are two types of electromagnetic waves.

HISTORY OF RADAR

The history of radar starts with experiments by Heinrich Hertz in the late 19th century that showed that radio waves were reflected by metallic objects. The name radar comes from the acronym RADAR, coined in 1940 by the U.S. Navy .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. Before the Second World War developments by the British, the Germans, the French, the Soviets and the Americans led to the modern version of radar.

World War II saw more rapid developments in radar technology. Both the British and the Germans were engaged in a race to produce larger and more sophisticated radars. However, the Germans were not able to fully harness it. It was the British that were able to utilize it more effectively

HOW DOES IT WORK?

The basic principle of operation of primary radar is simple to understand.

A basic radar system is spilt up into a transmitter, switch, antenna, receiver, data recorder, processor and some sort of output display.  Everything starts with the transmitter as it transmits a high power pulse to a switch which then directs the pulse to be transmitted out an antenna.  Just after the antenna is

finished transmitting the pulse, the switch switches control to the receiver which allows the antenna to receive echoed signals.  Once the signals are received the switch then transfers control back to the transmitter to transmit another signal.  The switch may toggle control between the transmitter and the receiver as much as 1000 times per second.            Any received signals from the receiver are then sent to a data recorder for storage on a disk or tape.  Later the data must be processed to be interpreted into something useful which would go on a display.

Radio waves are similar to light. They are made up of fluctuating patterns of electrical and magnetic energy, just like light waves, and they travel at the same speed—but their waves have much longer wavelengths and higher frequencies.

The antenna is usually curved so it focuses the waves into a precise, narrow beam, but radar antennas also typically rotate so they can detect movements over a large area.

The radio waves travel outward from the antenna at the speed of light (186,000 miles or 300,000 km per second) and keep going until they hit something.

TARGET DETECTION

Radars create an electromagnetic (EM) pulse that is focused by an antenna, and then transmitted through the atmosphere (Figure A).

Objects in the path of the transmitted EM pulse, called "targets" or "echoes," scatter most of the energy, but some will be reflected back toward the radar (Figure B). 

The receiving antenna (normally also the transmitting antenna) gathers back-scattered radiation and feeds it to a "receiver." 

An EM pulse encountering a target is scattered in all directions. The larger the target, the stronger the scattered signal (Figure C). 

Also, the more targets, the stronger the return signal, that is, the targets combines to produce a stronger signal (Figure D).

The radar measures the returned signal, generally called the "reflectivity."  Reflectivity magnitude is related to the number and size of the targets

encountered.

TARGET LOCATION

     The radar needs 3 pieces of information to determine the location of a target.

1. The "azimuth angle," the angle of the radar beam with respect to north.2. The "elevation angle," the angle of the radar beam with respect to the

ground.3. The distance (D) from radar to target.

     Distance is determined by measuring the time it takes for the EM pulse to make a round trip from the radar to the target and back using the relation: distance = time (t) * velocity     The pulse travels at the speed of light (c). Since the pulse travels to and from the target, the total distance is 2D. If t is the time it takes, then 2D = c*t or D = (c*t)/2. 

TARGET VELOCITY

Doppler radars, like NEXRAD*(next radar generation), can also measure "radial velocity," the component of target velocity moving toward or away from the radar. 

For example, at "time interval 1" (T1), an EM pulse transmitted by the radar is intercepted by a target at distance "D1".

At "time interval 2" (T2), another pulse returns a target distance "D2." Doppler radars measure the change in "D" from T1 to T2. These changes, the radar's wavelength, and the time interval between T1 and T2, are used to compute target velocity.

TYPES OF RADAR

Radars configurations include Monopulse radar, Bistatic radar, Doppler radar, Continuous-wave radar, etc.. depending on the types of hardware and software used. It is used in aviation (Primary and secondary radar), sea vessels, law enforcement, weather surveillance, ground mapping, geophysical surveys, and biolo

MONOPULSE RADAR:

Monopulse radar is an adaptation of conical scanning radar which sends additional information in the radar signal in order to avoid problems caused by rapid changes in signal strength. The system also makes jamming more difficult. Most radars designed since the 1960s are monopulse systems.

Conical scan systems send out a signal slightly to one side of the antenna's boresight , and then rotating the feed horn to make the lobe (projecting part) rotate around the bore sight line. A target centered on the boresight is always slightly illuminated by the lobe, and provides a strong return. If the target is to one side, it will be illuminated only when the lobe is pointed in that general direction, resulting in a weaker signal overall (or a flashing one if the rotation is slow enough). This varying signal will reach a maximum when the antenna is rotated so it is aligned in the direction of the target, by looking for this maximum and moving the antenna in that direction, a target can be automatically tracked.

One problem with this approach is that radar signals often change in amplitude for reasons that have nothing to do with beam position.

Jamming a conical scanner is also relatively easy.

MONOPULSE BASICS:

Monopulse radars are similar in general construction to conical scanning systems, but add one more feature. Instead of broadcasting the signal out of the antenna "as is", they split the beam into parts and then send the two signals out of the antenna in slightly different directions. When the reflected signals are received they are amplified separately and compared to each other, indicating which direction has a stronger return, and thus the general direction of the target relative to the boresight

Monopulse radar was extremely "high tech" when it was first introduced by Robert M. Page in 1943 in a Naval Research Laboratory experiment. As a result, it was very expensive and generally more difficult to maintain

BISTATIC RADAR:

Bistatic radar is the name given to a radar system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. Conversely, a radar in which the transmitter and receiver are collocated is called a monostatic radar

SPECIFIC CLASSES OF BISTATIC RADAR:

PSEUDO-MONOSTATIC RADAR:

Some radar systems may have separate transmit and receive antennas, but if the angle subtended between transmitter, target and receiver (the bistatic angle) is close to zero, then they would still be regarded as monostatic or pseudo-monostatic.

FORWARD SCATTER RADAR:

In some configurations, bistatic radars may be designed to operate in a fence-like configuration, detecting targets which pass between the transmitter and receiver, with the bistatic angle near 180 degrees. This is a special case of bistatic radar, known as a forward scatter radar.

Multistatic radar

A multistatic radar system is one in which there are at least three components - for example, one receiver and two transmitters, or two receivers and one transmitter, or multiple receivers and multiple transmitters. It is a generalisation of the bistatic radar system, with one or more receivers processing returns from one or more geographically separated transmitters.

Passive radar

A bistatic or multistatic radar that exploits non-radar transmitters of opportunity is termed a passive radar or passive coherent location system or passive covert radar.

DOPPLER RADAR:

A Doppler radar is a specialized radar that makes use of the Doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been altered by the object's motion. This variation gives direct and highly accurate measurements of the radial component a target's velocity relative to the radar. Doppler radars are used in aviation, sounding satellites, police speed guns [1] , and radiology.

Most modern weather radars use the pulse-doppler technique to examine the motion of precipitation, but it is only a part of the processing of their data. So, while these radars use a highly specialized form of doppler radar, the term is much broader in its meaning and its applications.

DOPPLER EFFECT:

The emitted signal toward the car is reflected back with a variation of frequency that depend on the speed away/toward the radar (160 km/h). This is only a component of the real speed (170 km/h).

The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the waves

It is commonly heard when a vehicle sounding a siren approaches, passes and recedes from an observer. The received frequency is increased (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is decreased during the recession. This variation of frequency is maximum when the emitted the wave travel parallel to the motion and diminish when the angle between the beam and the target increase to become null at right angle. Thus the Doppler shift gives only the radial component of the motion.

Since with electromagnetic radiation like microwaves frequency is inversely proportional to wavelength, the wavelength of the waves is also affected. Thus, the relative difference in velocity between a source and an observer is what gives rise to the doppler effect.[2]

U.S. Army soldier using a radar gun, an application of Doppler radar, to catch speeding violators.

There are three ways of producing the Doppler effect. Radars may be Coherent pulsed (CP), Continuous wave (CW), or Frequency modulated (FM).

CONTINUOUS-WAVE RADAR:

Continuous-wave radar system is a radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. The return frequencies are shifted away from the transmitted frequency based on the Doppler effect if they are moving.

The main advantage of the CW radars is that they are not pulsed and simple to manufacture

CW radars also have a disadvantage because they cannot measure range

The military uses continuous-wave radar to guide semi-active radar homing (SARH) air-to-air missiles

APPLICATIONS OF RADAR TECHNOLOGY:

Radar was originally devised as an instrument to detect    approaching    ships    or aircraft.    Practice    and experience in reading the scope soon showed that radar could do much more. By plotting successive positions of enemy ships and aircraft, you could determine their course and speed. Further experience made it possible to determine   whether   the   target   was   a   battleship, destroyer,   aircraft,   or   a group   of   targets.   Also,   an aircraft's altitude could be determined.

Use in Tactical Air Control

Both airborne and shipboard radar is a major link in an   operational  system. It directs   fighter   aircraft to a favorable position for intercepting enemy aircraft. The air control officer can determine the number of fighters so they can

successfully attack and destroy the enemy. Airborne early warning (AEW) aircraft, equipped with   high-powered   radars,   is   used   in   tactical   air control. These aircraft extend the range of air control radar  by  operating  in  areas  outside  the range  of  the ship board or land- based radar . The Aviation Electronics Technician   (AT)   rating   maintains   AEW equipment.

USE IN SHIPS:

Marine Radars are x-band or s-band radar to provide bearing and distance of ships and land targets in vicinity from own ship (radar scanner) for collision avoidance and navigation at sea.

In port or in harbour, Vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters.

Navigation

Marine radar systems can provide very useful radar navigation information for navigators onboard ships. Ship position could be fixed by the bearing and distance information of land target on radar screen.

Operation procedures

1. before turning on the radar, ensure no person standing near by the radar scanner and then switch the radar (OFF/STANDY/ON) to STANDBY (wait up to 4 minutes for the radars magnetron to warm up)

2. turn the radar to ON

3. turn the range scale to 3 or 6 n.m. depend on the coastal or open sea

3. adjust the brightness to rotation trace just dimly appeared

4. adjust the gain and tuning until background noise just dimly visible

5. adjust the anti-clutter sea/rain slightly if the sea clutters/rain clutters obstructs the area at sea (never over suppress the clutters completely as it reduce the target signal too)

RADAR GUN:

A radar gun or speed gun is a small Doppler radar unit used to detect the speed of objects, especially trucks and automobiles for the purpose of speed limit enforcement, as well as pitched baseballs, automatic door openers, runners or other moving objects in sports. A radar gun does not return information regarding the object's position. It relies on the Doppler effect applied to a radar beam to measure the speed of objects at which it is pointed. Radar guns may be hand-held, vehicle-mounted or static.

Radar guns are, in their most simple form, radio transmitters and receivers. They send out a radio signal, then receive the same signal back as it bounces off the objects. However, the radar frequency is different when it comes back, and from that difference the radar gun can calculate object speed.

Traffic radar comes in many models. There are hand held, stationary and moving radar instruments. Hand held units are mostly battery powered, and for the most part are used as stationary speed enforcement tools. Stationary radar is mounted in police vehicles, and may have one or two antennae. These are employed when the vehicle is parked. Moving radar is employed, as the name implies, when the police vehicle is in motion. These devices are very sophisticated, able to track vehicles approaching and receding both in front of and behind the patrol vehicle. They can also track the fastest vehicle in the selected radar beam, front or rear.

WEATHER RADAR:

Since the 1940s, meteorologists have used Doppler radar to detect storm intensity and movement, predict precipitation amounts and help give weather warnings and alerts.

A weather radar, or weather surveillance radar (WSR), is a type of radar used to locate precipitation, calculate its motion, estimate its type (rain, snow, hail, etc.), and forecast its future position and intensity.

Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather.

There are limits to using radar to predict and detect weather and precipitation. Radar cannot detect the height of precipitation. Precipitation that occurs but doesn’t reach the ground, called virga, is detected and recorded by radar.

Mountains, trees or buildings can block the radar waves. Sea and ground clutter occur when radar waves reflect off the ocean or birds, planes and insects. Doppler radar also looses its ability to detect precipitation with increased distance. The curvature of the Earth plays a part in limiting weather detection by radar.

Radar Can Measure Pressure :

Radar Can Measure Pressure The strength of the echo received from the ionosphere measures the number of electrons able to scatter radio waves or what we call electron pressure

Radar Can Measure Temperature :

Radar Can Measure Temperature Some electrons are moving due to heat - In this case the echo is scattered The echo will contain a range of frequencies close to the transmitter frequency As the temperature increases, the electrons move faster So radar can act like a thermometer and measure the temperature of the ionosphere

Radar Can Measure Wind Speed :

Radar Can Measure Wind Speed When an electron is removed from an atom, the remaining charged atom is called an ion The ion gas can have a different temperature from the electron gas The electron/ion mixture is known as a plasma and is usually in motion (like our wind) So incoherent scatter radar can also measure wind speed

A ground-penetrating radargram collected on an historic cemetery in Alabama, USA. Hyperbolic reflections indicate the presence of reflectors buried beneath the surface, possibly associated with human burials.

Ground-penetrating radar ( GPR ):

Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. This non-destructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum, and detects the reflected signals from subsurface structures. GPR can be used in a variety of media, including rock, soil, ice, fresh water, pavements and structures. It can detect objects, changes in material, and voids and cracks.[1]

GPR uses transmitting and receiving antennas or only one containing both functions. The transmitting antenna radiates short pulses of the high-frequency (usually polarized) radio waves into the ground. When the wave hits a buried object or a boundary with different dielectric condielectric constants instead of acoustic impedances.

The depth range of GPR is limited by the electrical conductivity of the ground. As conductivity increases, the penetration depth also decreases. This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in signal strength at depth. Higher frequencies do not penetrate as far as lower frequencies, but give better resolution.

Ground-penetrating radar antennas are generally in contact with the ground for the strongest signal strength; however, GPR air launched antennas can be used above the ground.

GPR is used to study bedrock, soils, groundwater, and ice. Military uses include detection of mines, unexploded ordnance, and tunnels.

Ground penetrating radar survey of an archaeological site in Jordan.

SYNTHETIC APERTURE RADAR (SAR):

What is Synthetic Aperture Radar?

In synthetic aperture radar (SAR), microwave pulses are transmitted by an antenna towards the earth surface.

The microwave energy scattered back to the spacecraft is measured. The SAR makes use of the radar principle to form an image by utilizing the time delay of the backscattered signals.

IT IS USED IN Environmental monitoring, earth-resource mapping, and military systems require broad-area imaging at high resolutions. Many times the imagery must be acquired in inclement weather or during night as well as day. Synthetic Aperture Radar (SAR) provides such a capability.

Synthetic-aperture radar (SAR) is a form of radar in which multiple radar images are processed to yield higher-resolution images

In a typical SAR application, a single radar antenna is attached to the side of an aircraft or spacecraft. A single pulse from the antenna will be rather broad (several degrees) because diffraction requires a large antenna to produce a narrow beam

The advantage of a single moving antenna is that it can be easily placed in any number of positions to provide any number of monostatic waveforms. For example, an antenna mounted on an airplane takes many captures per second as the plane travels.

SAR requires that echo captures be taken at multiple antenna positions. The more captures taken (at different antenna locations) the more reliable the target characterization.

Multiple captures can be obtained by moving a single antenna to different locations

A radar pulse is transmitted from the The radar pulse is scattered by the

antenna to the ground ground targets back to the antenna.

In real aperture radar imaging, the ground resolution is limited by the size of the microwave beam sent out from the antenna. Finer details on the ground can be resolved by using a narrower beam. The beam width is inversely proportional to the size of the antenna, i.e. the longer the antenna, the narrower the beam.

The microwave beam sent out by the antenna illuminates an area on the ground (known as the antenna's "footprint"). In radar imaging, the recorded signal strength depends on the microwave energy backscattered from the ground targets inside this footprint. Increasing the length of the antenna will decrease the width of the footprint.

It is not feasible for a spacecraft to carry a very long antenna which is required for high resolution imaging of the earth surface. To overcome this limitation, SAR capitalises on the motion of the space craft to emulate a large antenna (about 4 km for the ERS SAR) from the small antenna (10 m on the ERS satellite) it actually carries on board.

Limiting factors:

Beam path and range

Echo heights above ground

The radar beam would follow a linear path in vacuum but it really follows a somewhat curved

Noise

Signal noise is an internal source of random variations in the signal, which is generated by all electronic components.

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.

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 is considered an active interference source, since it is initiated by elements outside the radar and in general unrelated to the radar signals.

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