An Introduction to Radar - The Basic Principle
Created | Updated Feb 9, 2007
The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures
The term 'radar' was coined in the 1930s and is an abbreviation for RAdio Detection And Ranging. The initial purposes of detecting objects and range measurements nowadays are only part of a radar's function, as radars also serve for tracking an object's movements and whereabouts, its identification and the determination of certain properties.
The Principle In A Nutshell
Radars consist of a transmitter, a receiver, one or two antennae and lots of signal-processing circuitry. This is a basic overview of how the principle of radar works:
The transmitter produces a signal which is radiated through the antenna.
The signal is an electromagnetic wave which is capable of propagating through space.
If an object happens to be in the way then part of the signal is reflected and finds its way back to the radar.
The receiver picks up this echo, using the transmit antenna or a dedicated receiving antenna.
The signal processor detects the echoes, sorts them out from among noise and other types of interference, takes measurements with regard to the object's location in space, its speed and perhaps some more characteristics, and prepares them for output to a display or some remote command post.
The principle can be compared to finding one's way through a forest at night with the help of a torchlight. The torchlight contains the light bulb (the transmitter) and a reflector (the transmit antenna) which is used to shape the beam and direct it into the area of interest. The 'signal' is the beam of light which is capable of propagating through the atmosphere. Objects like trees and grass reflect the light, and part of it is picked up by the eye (the receiver). Finally, the brain controls the whole process, performs the signal processing and avoids its owner tripping over a fallen tree.
Behind the scenes, radars are rather complex devices which bring together scientific areas like mathematics, electronics, optics, mechanics, fluid dynamics and many more. Main application fields for radars include weather forecasting, air surveillance, navigation and collision avoidance, air traffic control, law enforcement and warfare, astronomy, geodesy and ecology.
Within a single radar, the units of measure usually stretch over tens of orders of magnitude. The transmitter may operate at megawatts of power for the duration of a microsecond whereas the receiver measures picowatts. The signal in the air can have a frequency of several hundred GigaHertz while distances in the millimetre range are being measured.
The radar principle is also used in optics and acoustics (eg: LIDAR, laser radar) and sonar (acoustic-sounding). Lidar finds its most prominent application in detecting chemical substances in the atmosphere. Sonar1 is used by navies and commercial fleets all over the world for finding underwater targets and fish. Ultrasonic devices used in medical diagnostics also employ the radar principle in order to produce images of a baby within the mother's womb or a Doppler profile of the bloodstream within a vessel.
The signals used in radars are electromagnetic waves just like light. There are only two major distinctions which make it hard for the lay person to gather the similarity:
The wavelengths are different. Light has a wavelength of some minute fraction of a millimetre2, whereas radars use wavelengths between 1mm and 100m.
Humans don't have sensors for these wavelength bands. Our senses only cover the optical band (which is perceived as light) and the far infrared which is perceived as heat. Radar wavelengths are some 10 orders of magnitude greater than those of infrared radiation. In order to be able to see a radar signal, our eyes would have to grow by the same factor. Eyes this big might prove a little cumbersome - even perhaps impractical.
However, one distinctive difference between light and typical radar signals is that longer wavelengths can easily propagate through precipitation. Hence, radars are able to look through fog, clouds and snow. Some wavelengths do exhibit significantly stronger attenuation in the atmosphere, especially when molecular resonances of oxygen or water molecules are excited. But even these areas of the electromagnetic spectrum are used by radars: weather radars and wind-profiling radars use them in order to look out for clouds and to track hurricanes. Radars can also take a look into the earth - although not very deep - to detect buried mines.
An Outline of this Project
The project is organised in the following sections, with the numbers in brackets denoting the number of Entries in the section:
- History (2)
- Radar Technology (9)
- Theoretical Basics (5)
- Civilian Applications (5)
- Military Applications (4)
- Electronic Combat (7)
Even though all entries are cross-linked, is recommended to start reading each section with the entry listed first because they are sorted according to complexity. However, mathematical formulae can only be found in the 'Theoretical Basics' section.
Anyway, radar can be understood with no more mathematical knowledge than some understanding of the concepts of addition, subtraction, multiplication and division. Even small animals like bats know what 'range ambiguity' is and how to avoid it, and no-one has ever seen them using a scientific pocket calculator. Try Electronic Combat in Wildlife for a start.
History: Overview | Isle of Wight Radar During WWII
Technology: Basic Principle | Main Components | Signal Processing | Antennae | Side Lobe Suppression | Phased Array Antennae | Antenna Beam Shapes | Monopulse Antennae | Continuous Wave Radar
Theoretical Basics: The Radar Equation | Ambiguous Measurements | Signals and Range Resolution | Ambiguity and PRFs
Civilian Applications: Police Radar | Automotive Radar | Primary and Secondary Radar | Airborne Collision Avoidance | Synthetic Aperture Radar
Military Applications: Overview | Over The Horizon | Low Probability of Intercept | How a Bat's Sensor Works
Electronic Combat: Overview | Electronic Combat in Wildlife | Range Gate Pull-Off | Inverse Gain Jamming | Advanced ECM | How Stealth Works | Stealth Aircraft