Radar Systems Tutorial: Types and Applications

Radar systems have evolved into various types, each designed to perform specific functions. Knowing the different radar types helps in selecting the right system for the desired application, whether it’s navigation, weather monitoring, or defense. At their core, radars use radio waves to detect and track objects, but they differ in their methods and purposes.

Radar Basics

As we know, RADAR is the short form derived from RAdio Detection And Ranging. It is mainly used to determine or detect that some object lies at a certain distance away. This object is one which cannot be observed visually by the eye.

In a radar system, the radar unit or radar station transmits high-frequency RF signals, which get reflected from the target. This reflected signal (referred to as ‘echo’) is received by the radar station and helps determine the distance of the target from the radar station. It also helps determine the direction, azimuth, and elevation of the target.

The same concept is illustrated in the figure below.

Equation for the distance to the target is:

$D = T/12.36$

Where:

• $D$ is the distance between the radar station and the object in nautical miles.
• $T$ is the total time from the transmission of the signal and the reception of the signal in microseconds.

In most radar applications, the Nautical mile is used in place of statute miles for measurement purposes. A Nautical mile is equal to 6076 ft.

The radar can see through mediums consisting of fog, snow, rain, darkness, clouds, etc. Radar signals can penetrate and see through insulators.

It can help find out the following parameters of an object or target:

• Range
• Angular Position
• Location of Target
• Velocity of Target

It can distinguish fixed as well as moving target types.

Radar Classification

Radars fall into two major categories referred to as primary radar and secondary radar.

The main type of radar is primary radar. These radars transmit Continuous Wave (CW) as well as pulses. The reflected CW and pulses received are processed by the radar to determine range, velocity, elevation, and azimuth. Examples of primary radar include those used for ground-penetrating radar, air-based search radar, and weapon control radar.

Primary radars are further categorized into CW radar and pulse radar. Unmodulated CW radar and FMCW radar systems fall under CW radar. MTI (Moving Target Indicator) pulse radar and pulse Doppler radar are pulse radars.

Secondary radars are used to provide identity and altitude-related information. An example of secondary radar is IFF radars (Identification Friend or Foe). These radars respond when they are interrogated by other radars using encrypted signals. Ground radars or airborne radars are used for this purpose.

There are basically two main types of radar systems: pulsed radar and CW (continuous wave) radar. These pulse and CW radars have been explained below in brief. There is another radar by the name of phased array radar, which uses multiple dipoles instead of a single horn/parabolic reflector.

Types of Radar

Radars are also classified based on their functionalities, such as air traffic radar, police radar, weather radar, defense radar, marine radar, ground-penetrating radar, and so on. The function of these radars are outlined in the table below. These types depend upon the techniques used for the measurement of various radar parameters.

Pulse Radar

In pulse radar, signals are transmitted in short pulses (bursts). The duration or width of the pulse is very short and will be from microseconds to several microseconds. The distance of the object is determined by the measurement of the time delay from the time the pulse is transmitted to the time the echo is received.

PRT is the time between transmitted pulses, called Pulse Repetition Time.

$PRF (Pulse Repetition Frequency) = 1/PRT$

$Duty Cycle = w(100)/PRT$, unit of percentage.

CW Radar

Let us understand CW radar basics. In CW radar, a constant amplitude continuous sine wave in the microwave frequency range is transmitted. Amplitude variations of the echo will not help determine the object, but as the object moves, there will be a frequency change observed between the transmitted and received echo. This frequency shift between the transmitted signal and the received echo is referred to as the Doppler effect. This helps determine the relative speed between the radar unit and the target object.

$V(m/s) =1.1 \times f \times \lambda$

This Doppler effect provides frequency modulation of the carrier. A variation to this concept is the FM continuous wave radar, where the carrier is continuous but is frequency modulated by a sawtooth or triangular wave.

Doppler Radar

• Description: Doppler radar uses the Doppler effect to measure the velocity of objects by observing changes in the frequency of the reflected waves. It distinguishes between stationary and moving targets.
• Applications: Essential for weather forecasting, tracking storms, and monitoring precipitation rates.

Synthetic Aperture Radar (SAR)

• Description: SAR uses the motion of the radar antenna to simulate a larger aperture (or antenna) than is physically present. This creates high-resolution images of the ground.
• Applications: Used in Earth observation, mapping, and military reconnaissance.

Phased Array Radar

• Description: Phased array radar uses an array of antennas to electronically steer the radar beam without moving the physical antenna. This allows for rapid scanning and tracking of multiple targets.
• Applications: Widely used in military radar systems, air traffic control, and advanced weather radar systems.

Ground Penetrating Radar (GPR)

• Description: GPR emits pulses of radio waves into the ground and analyzes the reflected signals to detect objects or layers below the surface. It is used for subsurface exploration.
• Applications: Used in geology, archaeology, and civil engineering to locate underground features.

Inverse Synthetic Aperture Radar (ISAR)

• Description: ISAR creates high-resolution images of moving targets by using the movement of the target relative to the radar. It is often used for imaging of airborne or maritime targets.
• Applications: Used in military applications for target recognition and tracking.

Tracking Radar

• Description: Tracking radar is designed to continuously follow a moving target. It adjusts the radar beam to keep the target within the beam and provides updated position information.
• Applications: Used in missile guidance, air traffic control, and military tracking systems.

Weather Radar

• Description: Weather radar measures precipitation and its intensity by sending out pulses and analyzing the returned signal. It can detect storm systems and rainfall.
• Applications: Used in meteorology for weather forecasting and storm tracking.

Bistatic Radar

• Description: Bistatic radar has separate transmitter and receiver locations. This configuration allows for different radar geometries and can provide different types of information compared to monostatic radar.
• Applications: Used in some defense applications and for remote sensing.

Radar Applications

The main applications are defense surveillance systems, navigation, and locating enemy targets. They are used as altimeters. High-frequency radars are also used to plot or to map the terrain in the area. There are many, as mentioned above in the table.

Radar Range Equation

As per the basic principle, an RF burst is transmitted towards the target, and the receiver is turned on to listen for the echo reflected from the target. This pulse will have a very short pulse duration of width ($T$) and a long receiver time between pulses. The number of pulses per second is referred to as the Pulse Repetition Rate (PRR) or Pulse Repetition Frequency (PRF). The time between the onset of consecutive pulses is referred to as Pulse Repetition Time (PRT).

$PRF = 1/PRT$

$Range\ R = c \times t /2$

Where:

• $R$ is the range to the target in meters.
• $c$ is the speed of light ($3 \times 10^8$ m/s).
• $t$ is the time in seconds between the original transmitted pulse and the arrival of the echo from the target.

For example, a target return echo in 98 microseconds after the time of the transmitted pulse. Calculate the radar range to the target.

Radar range Equation is:

$R = c \times t/2$

$R = 3 \times 10^8 \times 98 \times 10^{-6} /2$

$R = 14700$ meter

Summary

Different types of radar systems serve various purposes based on their operational principles. Whether for detecting and tracking objects, mapping, or weather monitoring, each radar type has its unique applications and advantages. Understanding these types helps in selecting the appropriate radar system for specific needs.