Radar: Principles and applications

Vivek srivastava

• Radar: Radio detection and ranging• Radio stands for microwaves and range means

distance• Radar sensors were originally developed and

used by millitary but now they are used in civil applications widely such as environmental monitoring

What is Radar?

• RADAR (Radio Detection And Ranging) is a way to detect and study far off targets by transmitting a radio pulse in the direction of the target and observing the reflection of the wave.

• It’s basically radio echo

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What is RADAR?

• A Radar system has three primary functions:

- It transmits microwave (radio) signals towards a scene

- It receives the portion of the transmitted energy backscattered from the scene

- It observes the strength (detection) and the time delay (ranging) of the return signals.

• Radar is an active remote sensing system & can operate day/night

RADARRAdio Detection And Ranging

Radar observables:• Target range• Target angles • Target size • Target speed • Target features

Antenna

TransmittedPulse

TargetCross

Section

Propagation

ReflectedPulse

(“echo”)

The Range

• Distance from the radar

• Measured from time delay between transmitted pulse and returned signal received

The Range

• Remember, in general v=d/t and d=vt

• The range is just a distance

• Since radio waves travel at the speed of light (v = c = 300,000 km/sec )

range = c•time/2• Why divided by 2?

The Range

• The “2” is because the measured time is for a round trip to and from the target. To determine the range, you only want the time to the object, so you take half!

Radar Range Measurement

Transmitted

Pulse

Reflected

Pulse

Range

Target

• Target range = cτ2

where c = speed of lightτ = round trip time

Why microwaves

• Microwaves are electromagnetic waves with wavelength between 1 cm to 1 m

• Used by radars because can penetrate clouds and haze or fog

• Microwave remote sensing is considered as an active remote sensing but passive sensors are also used which detect naturally emitted microwave energy

• Mainly used in meteorology, hydrology and oceanography

• In radar, antenna transmits microwave signals to the earth’s surface and they are backscattered

• The part of the electromagnetic energy which is scattered into the direction of the antenna is detected by the sensor

• Active sensors are divided into two groups: imaging and non imaging

• Radar is an imaging microwave sensor

• The non-imaging microwave instruments include altimeters which collect distance information such as sea surface height and scatterometers which acquire information about object properties such as wind speed

• Radar is an active sensor means it is having its own energy source

• Since radar is an active sensor so data can be acquired any time

• Another advantage of radar is that signal characteristics (wavelength, polarization, incidence angle etc.) can be easily controlled

Principles of imaging radar

• The main systems of an imaging radar includes: transmitter, receiver, an antenna and a recorder

• Transmitter is used to generate microwave signals and transmit the energy to the antenna from where it is emitted towards the earth surface

• Antenna accepts the backscattered signals and transfer it to the receiver which amplifies the signals for recording purpose

• Recorder then stores the received signals• Bistatic radar and monostatic radar

• Radar acquires an image according to the strength of the backscattered energy that is received from the ground

• The energy received from each transmitted radar pulse can be expressed as radar equation

• Pr = G2λ2Pt σ/(4π)3R4

• Pr = G2λ2Pt σ/(4π)3R4

• Pr : received energy• G: antenna gain(measure of the ability of the

antenna to focus the radiation in particular direction )

• λ: wavelength of radiation• Pt: transmitted power• σ: backscattering constant (measure of the

strength of the radar signals reflected )• R: Distance (Range between target and sensor)

• Transmitting antenna: How well the antenna

converts input power into radio waves headed

in a specified direction

• Receiving antenna: How well the antenna

converts radio waves arriving from a specified

direction into electrical power

• Some radar transmitters do not transmit constant,

uninterrupted electromagnetic waves. Instead, they transmit

rhythmic pulses of EM waves with a set amount of time in

between each pulse. The pulse itself would consist of an EM

wave of several wavelengths with some dead time after it in

which there are no transmissions. The time between each

pulse is called the pulse repetition time (PRT) and the number

of pulses transmitted in one second is called the pulse

repetition frequency (PRF)

• Once the radar receives the returned signal, it calculates useful information from it such as the time taken for it to be received, the strength of the returned signal, or the change in frequency of the signal. This information is then translated to reveal useful data: an image, a position or the velocity of the speeding car.

Radar images

Earth Science and RF Radiometery

Microwave Radiometry

Applications.

Ocean surface windSoil moisture

Sea surface temperature/Sea surface salinity

Atmospheric temperature, humidity, and clouds

Precipitation

Atmospheric chemistry