Classification of RADARS

                   Classification of RADARS

When we start reading about radar, we come across various terms which are explained differently. There are various kinds of Radar classified in different ways. This article explains the various radar types in a lucid manner

RADAR CLASSIFICATIONS

1.         Classification based on specific function

Classification based on the primary function of radar is shown in the following figure

 

Primary Radar:

A Primary Radar transmits high-frequency signals toward the targets. The transmitted pulses are reflected by the target and then received by the same radar. The reflected energy or the echoes are further processed to extract target information.

Secondary Radar:

Secondary radar units work with active answer signals. In addition to primary radar, this type of radar uses a transponder on the airborne target/object.

A simple block diagram of secondary radar is shown below

 

The ground unit, called interrogator, transmits coded pulses (after modulation) towards the target. The transponder on the airborne object receives the pulse, decodes it, induces the coder to prepare the suitable answer, and then transmits the interrogated information back to the ground unit. The interrogator/ground unit demodulates the answer. The information is displayed on the display of the primary radar.

The secondary radar unit transmits and also receives high-frequency impulses, the so called interrogation. This isn't simply reflected, but received by the target by means of a transponder which receives and processes. After this the target answers at another frequency.

Various kinds of information like, the identity of aircraft, position of aircraft, etc. are interrogated using the secondary radar. The type of information required defines the MODE of the secondary radar.

Pulsed Radar:

Pulsed radar transmits high power, high-frequency pulses toward the target. Then it waits for the echo of the transmitted signal for sometime before it transmits a new pulse. Choice of pulse repetition frequency decides the range and resolution of the radar.

Target Range and bearings can be determined from the measured antenna position and time-of-arrival of the reflected signal.

Pulse radars can be used to measure target velocities. Two broad categories of pulsed radar employing Doppler shifts are

•           MTI (Moving Target Indicator) Radar

The MTI radar uses low pulse repetition frequency (PRF) to avoid range ambiguities, but these radars can have Doppler ambiguities.

•           Pulse Doppler Radar

Contrary to MTI radar, pulse Doppler radar uses high PRF to avoid Doppler ambiguities, but it can have numerous range ambiguities.

Doppler Radars make it possible to distinguish moving target in the presence of echoes from the stationary objects. These radars compare the received echoes with those received in previous sweep. The echoes from stationary objects will have same phase and hence will be cancelled, while moving targets will have some phase change.

If the Doppler shifted echo coincides with any of the frequency components in the frequency domain of the received signal, the radar will not be able to measure target velocity. Such velocities are called blind speeds. 

 

Where, fo = radar operating frequency.

Continuous Wave Radar:

CW radars continuously transmit a high-frequency signal and the reflected energy is also received and processed continuously. These radars have to ensure that the transmitted energy doesn’t leak into the receiver (feedback connection). CW radars may be bistatic or monostatic; measures radial velocity of the target using Doppler Effect.

CW radars are of two types

1.         Unmodulated

An example of unmodulated CW radar is speed gauges used by the police. The transmitted signal of these equipments is constant in amplitude and frequency. CW radar transmitting unmodulated power can measure the speed only by using the Doppler-effect. It cannot measure a range and it cannot differ between two reflecting objects.

2.         Modulated

Unmodulated CW radars have the disadvantage that they cannot measure range, because run time measurements is not possible (and necessary) in unmodulated CW-radars. This is achieved in modulated CW radars using the frequency shifting method. In this method, a signal that constantly changes in frequency around a fixed reference is used to detect stationary objects. Frequency is swept repeatedly between f1 and f2.  On examining the received reflected frequencies (and with the knowledge of the transmitted frequency), range calculation can be done. 

 

 

If the target is moving, there is additional Doppler frequency shift which can be used to find if target is approaching or receding.

Frequency-Modulated Continuous Wave radars (FMCWs) are used in Radar Altimeters.

 2.         Classification based on Scan Pattern

According to scan patterns, radars are classified as

•           Conical Scan

The radar rotates its main lobe of the beam in a circle about the boresight line. When the target is on the boresight line, maximum power is returned and amplitude variation is zero. When the target is away from the boresight line, returned signal will be of sinusoidal shape whose amplitude is proportional to the distance the target is away from the boresight. Using the location of maximum power received (& monitoring location of the scanning beam), target location can be determined. More accurate is the target tracking, smaller the amplitude of sine wave; zero amplitude implies radar is boresighted at the target.

The disadvantage of these radars is that they are not able to see the target outside their narrow scan patterns, hence they require another radar to provide initial target fix. Also, the target can easily escape from these radars. Another major problem with conical scan is that power returned fluctuates irrespective of target position in the beam due to several other factors. Since the target position is determined based only on the received power, variation in received power due to other factors gives misleading results.  

 

•           Track-While-Scan(TWS) Radars

These radars overcome the disadvantage of Conical Scan Radars. TWS radars scan their beam over larger areas and hence, they are able to see the target even if the track has been broken. In TWS radars also, returned power as a function of beam location is measured and accordingly, tracking is done.

However, large area scanning makes TWS radars highly vulnerable to jamming.

•           Monopulse Scan Radars

Scanning can be done by sequentially pulsing several antenna or sections of a large antenna. Monopulse scan radars uses single reflector and four feed horns. These horns illuminate different sections of the reflector thereby forming two overlapping antenna beams for two orthogonal axes. 

 

Front view of the beam pattern is as shown below: 

 

Sum pattern of the four horns is used for range measurement.

These techniques provide higher scan rates, but require extra hardware. It is normally used on ships.

•           Electronic Scanning

Mechanical scan systems' potential remains under-utilised because of the factors like antenna inertia, inflexibility, etc. Since they need to be positioned mechanically, they are inherently slow and require large amount of power. While manoeuvring high speed targets, they are not able to position the radar beam optimally.

To overcome these advantages, electronically steered phased array radars are deployed. They do not have any issue in terms of inertia, delays or problems related to mechanical motion controls.

Principle governing electronically steering a beam is the constructive and destructive interference of the electromagnetic energy.

Phased array antennas use number of radiating elements (in an array). Phase of the excitation signal to these elements decide which way a beam would steer. Hence by electronically changing the phase of the excitation signal, beam can be steered precisely in a desired direction.

Typical phase array antenna implementation (on a microstrip) is shown here. 

 

3.         Classification based on applications

1.         Surveillance Radars

Primary application of radar is surveillance. These radars typically use high power, scanning antenna and have moderate resolution. They are deployed for

•           Detection and Tracking of Aircraft, Missiles or Space Objects

•           Detection of Fixed or Moving Surface Targets

•           Moderate precision tracking of multiple targets

Some of the important applications of Surveillance radars are

a.         Air Traffic Management Radars

Radars commonly used for air traffic management are

• En-route radar systems

These radars usually operate in L-Band, detect and determine the position, course, and speed of air targets in an area up to 250 nautical miles.

• Air Surveillance Radar systems

These radars usually operate in E-Band, and are used to detect and display an aircraft's position in the terminal area. They can reliably detect and track aircrafts at altitudes below 25,000 feet and within 40-60 nautical miles of the airport

• Precision Approach Radar (PAR) systems

This radar helps the aircraft to land in bad weather. Using Precision Approach radar, the guidance information is obtained by the radar operator and passed to the aircraft.

• Surface movement radars,

Surface Movement Radar (SMR) uses very narrow pulse widths and is used to scan the airport surface and locate the positions of aircraft as well as ground vehicles.

b.          Air-defence Radars

Air-Defence Radars are employed to detect air targets and to determine target range, velocity, etc. in a relatively large area. They are able to detect threats at great distances and hence act as early warning devices. Typical range of an Air-Defence Radar is 300 miles, and azimuth coverage is full 360 degrees.

Range and bearing information provided by these radars are used to initially position tracking radars.

2.         Tracking Radars

Also called fire control radars, they are used to provide range and bearing information of a single target continuously. These radars use very high PRF, very narrow pulsewidth as well as beamwidth. This allows these radars to have high accuracy, limited range, and initial detection of the target a bit difficult.

They typically take range and bearing information from the search radars. Until a target is located, they remain in acquisition phase searching for the target. Once a target is located, they enters track phase and automatically follow target motions.

3.         Meteorological Radars

Also known as weather radars, they are primarily used to observe hydrometeors in the atmosphere. Radar is probably the only way to map the spatial distribution of precipitation over large areas. Radar can be used to forecast flash flooding and severe thunderstorms.

 

Weather radars help to determine the movement and trend of thunderstorms, variability and concentration of precipitation. The amount of energy scattered back from a target to the radar helps to estimate the intensity of storms and the amount of precipitation. Velocity of a target relative to the radar helps to estimate air motions and circulations within clouds.

Weather radars use various frequency bands. W & K are high frequency, short-wavelength bands, useful for detecting clouds and aerosols. X, C, S and L are useful for detecting precipitation. With longer wavelengths, the attenuation is less, but it also cannot detect smaller targets. L-band radar will detect heavy rain and hail, but not clouds, snow, or light rain. S-band radars are most widely used because S-band offers fair compromise between sensitivity and attenuation.

Weather radars listen for the scattered signal from the target (snow, hail, rain, etc.). Received signal is related to the diameter of particles.

 

The term is called reflectivity (Z) in radar meteorology and can be related to the rainfall rate.

Apart from measuring the magnitude of reflected signal, weather radars often measure frequency shifts produced by the motion of the precipitation particles. The frequency shift is used for measuring wind speed. Weather radars use Doppler Effect to detect storm circulations (e.g., tornados and hurricanes), air flow boundaries created by storms (e.g., outflows and microbursts).

4.         Imaging Radar

In contrast to non-imaging radars(which create linear 1D measurement), imaging radar sensors measure two dimensions of co-ordinates to create a map-like picture of the observed object or area. Imaging radars have been used to map the Earth, other planets, and celestial bodies and also to classify military targets. Imaging radar, monostatic radar, is an active illumination system and is mounted on airborne objects like aircraft, satellites.  Radar transmits a signal towards the Earth's surface and then waits for the reflected signal or echo. Backscattered signal from the surface is processed to construct the image.

A Synthetic Aperture Radar SAR is a coherent, active, microwave imaging method. SAR produce a two dimensional (2D) image. One dimension in the image is called range (cross track) range and is a measure of the "line-of-sight" distance from the radar to the target. Another dimension is called azimuth (along track) and is perpendicular to range.

Contrary to real aperture radars (where only magnitude (not the phase) of reflected signal is processed and azimuth resolution, a function of antenna dimensions, is affected by antenna beamwidth), SAR improves radar resolution by focusing the image through synthetic aperture processing. Focusing is the reconstruction of the contribution of each range resolution cell (pixel). Digital signal processing is used to focus the image and obtain a higher resolution than achieved by conventional radar systems. The improvement of resolution could be approximately a thousand times of a Real Aperture Radar. The effect of this processing is to synthesize a very large aperture, hence the name synthetic aperture radar. The ability of SAR to produce relatively fine azimuth resolution differentiates it from other radars.

SAR uses Doppler history of the radar echoes generated by the forward motion of the spacecraft to synthesize a large antenna (see Figure).  

 

This allows high azimuthal resolution in the resulting image despite a physically small antenna. As the radar moves, a pulse is transmitted at each position. The amplitude as well as phase of the return echoes is recorded in an echo store throughout the time period in which the objects are within the beam of the moving antenna.

Return signals from the centre portion of the beamwidth are discriminated by detecting Doppler frequency shifts. Within the wide antenna beam, echoes from features in the area ahead of the platform will have higher frequencies resulting from the Doppler Effect. Conversely, echoes from features behind the platform will lower frequencies. Echoes from features near the centre-line of the beamwidth will experience no frequency shift. By processing the return signals according to their Doppler shifts, a very narrow effective antenna beamwidth can be achieved, even at far ranges, without requiring a physically long antenna or a short operating wavelength.

Range resolution of pulsed radar is governed by the pulse width. Shorter pulse width provides longer ranges but shorter pulse reduces average transmitted power and hence radiometric resolution. SAR systems use longer pulses with linear frequency modulation, i.e., chirp. The chirp bandwidth is defined by required range resolution & length of chirp used is defined by radiometric resolution.  

These radars find applications for terrain mapping, remote sensing, etc and are deployed on airborne vehicles or satellites.

Pros and Cons of a SAR are

Pros:

•           Extremely large effective aperture.

•           Outstanding resolution.

•           Microwave energy propagates through clouds and hence these radars can be an all weather sensing system.

Cons:

•           Requires massive storage capacity to accumulate returns while radar (aircraft or satellite) moves.

•           Requires massive computing power to process returns as if they were received simultaneously.

5.         Radar Altimetry

FMCW radars are used for measurement of altitude above ground level. Principle of operation of FMCW radars has already been discussed. They provide very precise altitude information.