The term "radar" is based on radio detection and ranging, which means that radio waves are used to detect distant objects and to discover how far away the objects are located. However, range finding can be done by using many other signals. Nature has incorporated this scheme, very successfully, in bats. These nocturnal animals hunt for insects in the dark of the night, while in flight. Bats are not blind, but even the best vision would be useless for this task. This is because for a successful hunt the animal must not only locate the prey, but determine its flight direction and speed. Swallows and other birds do this, but in day time when the light levels are millions of times brighter. Instead of radio waves, bats use pulses of sound. These sound pulses reflect from insects. By measuring the intensity and direction of these reflections the bats' brains can determine the position and flight information of the insects. If you have had a bat inside the house, you know that these animals also rely on their sound pulses for navigation. This not because you could hear their "chipper" when they are nearby - their sound wave is not audible to humans- but that the best way to catch them for release outdoors is with a fish or butterfly net! (The standard use of a tennis racket is cruel and unnecessary.) This scheme that uses sound waves for navigation and ranging is the so called sonar.
For just detection purposes any signal that reaches a detector is good enough. For example, we could close our eyes and listen for sounds that people themselves make (from their speech to the noise that they make as they walk, etc.) and know that there are people in the room and about where they are located. But to know their location with greater precision we need to do ranging, i.e. send a known signal and detect its reflection. For this we need to supply our own "signal". Some of the basic considerations are: 1- signal type (radio or sound wave, or light, etc.), 2 - signal strength, and 3 - pulsed or continuous. But more refined considerations are necessary when the transmitted signal used is a wave - from its frequency (wavelength) to its absorption by the medium, etc.
Electromagnetic Waves revisited
Radio waves are just part of the spectrum of electromagnetic waves. Their wavelengths range from several tens of meters to tenths of a meter. There is really no rule or regulation to separate out different wavelengths of electromagnetic radiation into different categories, so there are areas of casual overlap! For example, electromagnetic waves that are used to heat your food in a microwave oven have wavelengths that are about a centimeter long (1/100 of a meter, or roughly about a 0.5 of an inch). These are of course microwaves. Most radars also operate in the microwave region, even though the term "radar" is derived from "radio waves"! Below I list a selection of frequency bands typically used by radar systems. As an exercise, please calculate the corresponding wavelength ranges, for each of these frequency ranges. Please remember that speed of light, C, is 300,000,000 meters per second (i.e. C = 3 x 108 m/s), and that wavelength = C / frequency = 3 x 108 / f (in Hz).
Another wave feature that we need to remember is that when we "chop" a wave that is continuously traveling (CW) to turn it into pulses, the chopping rate (pulse rate) cannot exceed the frequency of the wave itself! In fact, for a meaningful pulse, every pulse must contain many oscillations of the wave itself. The frequency range "containing" these number of oscillations is referred to as the band width. More specifically, then, the inverse of the pulse width (1/duration time of the pulse) is referred to as the band width. Please note that the shorter the pulse duration, then, the wider is the needed band width. This, of course, can only be satisfied for faster and faster waves, i.e. higher and higher frequencies.
Please check the web site arranged by US Government's NOAA for an excellent primer on doppler radars. Another excellent primer is at the NRL site on Radars.
Basically, radars are instruments that generate electromagnetic waves, mostly in the HF or VHF bands, using an oscillator. This generated wave is directed to a region of interest using an antenna. Reflected waves (from the target) are collected by the same emitting, or a separate, antenna and is processed using electronics and computing devices to get information about the target. The information may range from size to location to speed.
The physics of the generation of the electromagnetic wave is based on the fact that when a charged object accelerates, it emits electromagnetic radiation. To generate a wave train, then, the object needs to undergo successive accelerations. Clearly, an easy way to do this is through acceleration followed by a deceleration, followed by an acceleration. This is exactly what an oscillator, such as a pendulum, does: the pendulum bob accelerates while descending and decelerates while ascending. A magnetron was the first truly useful electromagnetic oscillator that was designed for generating high frequency waves. It is made of a wire cathode axial in a cylindrical anode with an axial external magnetic field. Another device whose invention advanced radar technology was a klystron. This is an amplifier used for increasing the power (strength) of the high frequency waves.
Most basic radar systems operate on a CW basis, while others use pulsed waves. For target locations radars just measure the time it takes the emitted signal to return after reflecting off the target. They could determine target speed by repeated range measurement, but a better method is to determine target's velocity using information from the Doppler shift of the wave. Just as a moving whistling train changes pitch, reflected wave changes frequency if the target is moving toward or away from the radar. The amount of this frequency shift depends directly on the target velocity. (For a visual demonstration of Doppler effect see the applet developed Davidson College.)
Some of the important features of a radar system are its frequency of operation, its power output, and its form of operation - i.e. pulsed, CW, Doppler, etc. These factors are not independent. For example, the frequency of a radar's operation can affect its useful value for its power output. For example, radars operating in the lower frequencies of HF have the capability of being very long range, based on the fact that these radio waves get reflected from the earth's ionosphere. But to make these capability a reality, high power outputs are necessary. Such radars are called Over The Horizon (OTH) systems. It is important to note that the frequency of transmission, and therefore the wavelength, is what determines the antenna size. A typical efficient antenna has a physical dimension of about one-half of the wavelength. So, radars operating in the HF band require antennas that are 10 to 100 meters long , roughly the length of a football field! (Array antenna systems for these radars may in fact be even many 100's of meters long.) Different applications, then, require different radar systems. Other factors that that play major roles in radar detection and are note mentioning are background noise (with atmospheric changes) effects and robust character of a radar system (detection sensitivity, etc).
To avoid radar detection targets (flying airplanes or missiles) could fly too low to reflect (scatter) radar signals. The body of the target could also be designed so as not to reflect back the radio signal. A third method of evasion is to jam radar signals. For a W.W.II research work by a former Union College Physics Professor on radar jamming device see: Why Chaff, by A. T. Goble
Some of current applications of radars are:
Other applications of range finding include scientific investigations (see Lunar Range Finding ) and mm-radio astronomy.
Last Modified: Friday, November 2, 2007 malekis@union.edu
