Radio Detection And Ranging

Radar

Introduction

Radar was developed by several nations during World War II. The term RADAR was coined in 1940 by the US Navy which is an acronym for RAdio Detection And Ranging.

A radar system usually operates in the ultra-high-frequency (UHF) or microwave part of the radio-frequency (RF) spectrum, and is used to detect the position and/or movement of objects, So it can be said as an object-detection system that uses radio waves to determine the range, altitude, direction, or speed of objects.

It works on the principle of echo i.e. the radar dish or antenna transmits pulses of radio waves or microwaves that bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna that is usually located at the same site as the transmitter.

 

 Basic Components of the Radar

Radars in their basic form have four main components:

• ·         A transmitter, which creates the energy pulse.
• ·         A switch that tells the antenna when to transmit and when to receive the pulses.
• ·         An antenna to send these pulses out into the atmosphere and receives the reflected pulse.
• ·         A receiver, which detects, amplifies and transforms the received signals into video format.

The received signals are displayed on a display system.

 

The Science of Radar

Reflectivity

The physics behind radar has its roots in wave theory. The German Heinrich Hertz discovered the behavior of radio waves in 1887. He showed that the invisible electromagnetic waves radiated by suitable electrical circuits travel with the speed of light, and that they are reflected in a similar way. This is why data received from the radar is called reflectivity.

Doppler

In 1842 the Austrian physicist Christian Doppler had discovered what is now called the Doppler effect. This is the theory that sound waves will change in pitch when there is a shift in the frequency.

An example of this would be an ambulance siren, which has a higher pitch when it is approaching, but a lower pitch if it is travelling away.

This theory is used by Doppler weather radar to determine the speed of precipitation in the atmosphere, toward or away from the radar. Since precipitation as it falls generally moves with the wind, you can determine the wind velocity with Doppler technology.

 

The Types of Radar

There are many types of radars but the two main kinds as used by ATC(Air Traffic Controllers) are:

Primary Radars

Primary radars are "classical" radars which works on reflection i.e. reflect all kind of echoes, including aircraft, clouds etc.

Secondary Radars

Secondary radar emits pulses and listens for special answer of digital data emitted by an Aircraft Transponder as an answer. Transponders emit different kind of data like a 4 octal ID (mode A), the onboard calculated altitude (mode C) or the Call sign (mode S).

 

Applications of Radar

The modern uses of radar are highly diverse. They include Air traffic control, Air-defense systems, and Antimissile systems.

Nautical Radars

it is used to locate landmarks and other ships; ocean-surveillance systems, outer space surveillance, etc.,

Aviation Radars

Aircrafts are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings

Marine Radars

They are used to measure the bearing and distance of ships to prevent collision with other ships, to navigate and fix their positions at sea when within range of shore or other fixed references such as islands, buoys and light ships

Weather-sensing Radars

It is an important tool in weather forecasting and helps make the forecasts more accurate

Detection and search Radar

It is the “early warning radar”, which is used for long-range detection of objects

Target Acquisition (TA) Radar systems

It is used to locate surface-to-air-missiles (SAM). These types of radar are often used in the military and in coastal surveillance, as well as for detecting car speed in high way patrol.

Missile Guidance Systems

This radar is used to locate the target of missile often present in Military aircraft

Radar for Biological Research

Bird and Insect radar are used frequently by scientists to track the migration patterns of animals. Bird radar is also being used in NASA’s Kennedy Space Center in Florida to track the presence of birds, especially Vultures, near launching pads.

Air traffic control and navigation Radar

This radar is used by airport to ensure the safety of planes. This type of radar detects the proximity of an aircraft and identifies the identity and altitude of the plane.

 

 

Air traffic control & RADAR

For air traffic control radars, the beam is shaped like a fan, narrow in the horizontal direction, and wide in the vertical direction, to accommodate high-flying planes. This beam scans around in a circle once every two or three seconds and echoes are displayed on a circular display called a plan-position indicator.
The air traffic controller can track the echoes or 'blips' on the display to determine where the aircraft is heading with help of primary radar.
These days, secondary radar is also used, in which a coded pulse sequence is sent to the aircraft and a transponder on the plane generates a coded return, containing a lot of information about the aircraft. This used to be called identification friend or foe, or IFF.
Air traffic controllers mostly use secondary radar to track commercial aircraft and only use real radar in the case where transponders are not fitted, are turned off or are broken.
If the aircraft transponder is switched off, it can be difficult to identify which one of the many primary radar "blips" on the air traffic control display corresponds to the aircraft you are interested in.

 

Limits to RADAR

Most people will have heard the expression 'flying below the radar', This is named after a true phenomenon.
It is caused by the interaction of the radar beam with the ground, which results in the beam 'lifting' off the horizon. If an aircraft is flying low enough, the beam hardly illuminates it and the range at which it can be seen is limited.
There are also limits to the distance over which radar can be used. The main problem with radar for long distance operation is the fact that the amount of power required to send and receive the signal is dependent on the distance to the aircraft raised to the power of four.
Therefore if you want to double the range at which you can detect an aircraft, the amount of transmitted power must increase by a factor of 16.
For really long-range operation, the peak power required to send out the radar pulses become prohibitively large.
This has resulted in the development of innovations such as phased arrays that consist of a large number of smaller transmitters and receivers on a planar surface that operate in unison and pulse compression, which allows longer and lower power encoded pulses to be generated while still maintaining good range accuracy.
Another limitation to long-range radar is caused by attenuation through the atmosphere — even in clear air, but worse in the rain. This is inversely related to the wavelength of the signal, so long range radars operate at low frequency.

 

Hiding from radar

Electromagnetic waves "bounce" off objects that conduct electricity, so old-fashioned aircraft made from wood and canvas didn't produce big radar echoes. The same applies to modern planes made from carbon fiber composites. Aluminum skinned planes make the best targets.
The shape of the aircraft is also important, and metal aircraft made from flat plates, sharp corners and edges generally produce strong echoes, so if you want to make an aircraft invisible, you can either make it from flat plates or facets that are aligned in such a way that the radar signals reflect away from the receiver. The F-117 stealth attack aircraft is an example of this.
Alternatively aircraft can be made without any right angles so that wings are blended into the body and external features are eliminated. Making an aircraft skin that absorbs radar energy using "radar absorbent materials" is another method to minimize the echo size.
The B-2 stealth bomber is probably the state-of-the-art, which uses most of these techniques, and provides an echo about as big as that produced by a bumble bee.