Radar

Radio detection and ranging has many practical uses, from aviation, ship navigation, weather prediction and scientific research. By firing certain electromagnetic waves and listening for echoes, one can obtain a picture of the surroundings.

In 1887, German physicist Heinrich Hertz experimented with Maxwell's electromagnetic theory. By generating electric waves with an oscillator, Hertz was able to detect and prove that radio waves were all around us. However, he found no use for the radio waves and it was up to other scientists to make use of this new evidence of radio waves. The first real implementation of radar was in 1904, when Christian HÑŒlsmeyer detected a ship in the fog. However, it was not until 1935 that a British physicist, Sir Robert Alexander Watson-Watt put radar into aircraft. At that time, he was able to locate aircraft within a 27km radius. However this radar was very unreliable and had little sensitivity.

During World War II, real advances started to come forth. The British handed research over to the Americans and they were able produce much higher power outputs, greater receiver sensitivity, and all the while improving the interpretation software required to translate the echoes into pictures we can understand. The power outputs were improved through the use of magnetrons, which is a vacuum tube in which the flow of electrons is controlled by an external magnetic field. The magnetron will generate power at microwave frequencies. Microwaves, having smaller wavelengths than regular radio waves, are able to travel farther, allowing the radar to have a greater "seeing distance."

Radar works on the basis of reflection of radio waves sent out. In your typical radar, we have a transmitter, which provides the radio waves to be pulsed out. The next piece of equipment is the antenna, which directs where the radio waves are to be pulsed. Once the radio waves are reflected and scattered off objects and the echoes return, the receiver picks them up.

Another concept that radar has worked upon is called the Doppler shift. The Doppler shift applies to all waves. For example, when an emergency vehicle passes you, the sound of the siren starts out high pitched, and then the pitch lowers as the vehicle passes. This happens because of the changes in frequency of the sound waves traveling to the observer. As the vehicle comes closer to the observer each consecutive sound wave travels less and the frequency goes up. Once the vehicle passes, the sound waves have to travel farther and are therefore less frequent, thus the pitch is lower. The same concept occurs in radio. Radar guns measure this difference in frequency to detect the speed of a moving object.

Sometimes, radar detects more objects (and sometimes objects that are not there) that the operator is not interested in. These things are called interferences, such as signal noise, clutter, and jamming. Signal noise is usually generated by electronic equipment. This interference generally shows up as random variations that are pretty easy to discern from regular echoes. Clutter refers to the echoes of objects that are actually present but uninteresting to the radar operator. These are usually objects such as the ground, rain, sand storms, animals, buildings and atmospheric turbulence. Radar jamming is radio signals that come from sources outside the radar itself. Jamming can be intentional, or unintentional (where a friendly source uses radio within the same frequency range). However, jamming is quite problematic because jamming signals only need to travel one-way while radar echoes travel two-ways, making them reduced in power as they travel back to the radar receiver. Thus, jamming signals do not need to be very powerful to mask radar echoes.

The amount of power returning to the radar's receiver after reflecting off an object is given by the radar equation:

Pt = transmitter power

Pt = transmitter power

Gt = gain of the transmitting antenna

Ar = effective aperture (area) of the receiving antenna

σ = radar cross section, or scattering coefficient, of the target

R = range

Avoiding radar is something that is rather tricky. Stealth is the process of ideally making a