RADAR (Radio Detection and Ranging)

Radar is something that is in use all around us, although it is normally invisible. 

• Air traffic controllers radar to track planes both on the ground and in the air, and also to guide planes in for smooth landings.
•  Police use radar to detect the speed of passing motorists. 
• NASA uses radar to map the Earth and other planets, to track satellites and space debris and to help with things like docking and manoeuvring.
• The military uses it to detect the enemy and to guide weapon.

So in this article i would discuss;

• What's RADAR
• Its basic principle
• And Its two types i.e PSR and SSR.

So, What is RADAR? Radar is an acronym for Radio Detecting And Ranging. The name itself suggests that the radars are used to detect the presence of object and determine its range, i.e distance and bearing, using radio frequency waves.

Radars are being used to measure different parameters

1.      Range                    Using Pulse Delay

2.      Velocity                  From Doppler Frequency Shift

3.      Angular Direction     Using Antenna Pointing

4.      Target Size              From magnitude of reflected energy

5.      Target Shape           Analysing reflected signal as a function of direction

6.      Moving Parts            Analysing modulation of the reflected signal

Cost and complexity of radar is dependent upon the number of functions it performs. Radars are used for various applications like Surveillance, imaging, remote sensing, altitude measurement, etc.

BASIC PRINCIPLE OF RADAR

Basic principle governing the functionality of radar is due to the properties of radiated electromagnetic energy.

·         The electromagnetic energy travels through space in a straight line, at a constant speed (approximately the speed of light). The propagation of these waves differs slightly because of atmospheric effects.

·         When the electromagnetic waves strike an electrically conductive surface, a part the energy is reflected back towards the source, rest of the reflected energy gets radiated in different directions.

·         Receipt of reflected energy towards the source is an indication of the obstacle in the direction of propagation.

These basic principles are utilised in Radar to determine distance, and bearings of the target, i.e., a reflecting object.

The block diagram of a primary Radar is shown below:

·         Transmitter

The radar transmitter produces microwave signal, which is typically short duration high-power RF- pulses of energy for a pulsed radar.

·         Duplexer

Duplexer acts as a switch; it switches the antenna between the transmitter and receiver. This obviates the need for separate transmitting and receiving antennas. Duplexer prevents high power energy to go into receiver (high power pulses can damage the receiver) while transmission and prevents reflected signal to be fed to the transmitter during reception.

·         Antenna

The transmitting antenna radiates the transmitting energy to signals in space, in desired directions.

The radiated energy propagates with constant velocity. When it finds the target, the energy is scattered, a part of which is reflected towards the transmitting antenna. The antenna receives the reflected energy and feed it to the duplexer. The duplexer directs this energy towards the receiver.

·         Receiver

The receiver demodulates the received reflected energy and analyses the signal to find target parameters.

·         Display

The receiver sends the output to display, which shows the analysed signal in an easily understandable user friendly manner.

The two general types of radars are 

•         Primary Surveillance Radar (PSR) and
•         Secondary Surveillance Radar (SSR).

Primary surveillance radar (PSR)

Primary radar is what you conventionally think of as radar, a transmitter sends out pulses and tries to detect the reflected pulse off an object, such as an airplane.

A phenomena of signals correlation has been used widely in primary radars, a radio pulse is send through the antenna, which provides the bearing of the aircraft from the ground station. Time taken for the pulse to reach the target and return provides a measure of the distance of the target from the ground station. This measured distance now displayed on the screen or meters of ATC air traffic controllers.

Secondary surveillance radar (SSR)^[1] is a radar system used in air traffic control (ATC), that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude. Unlike primary radar systems that measure only the range and bearing of targets by detecting reflected radio signals, SSR relies on targets equipped with a radar transponder, that replies to each interrogation signal by transmitting a response containing encoded data.