The Theory of Nothing

How does radar work?

How does radar work?

Radar has been around since World War II. It saved Britain from the blitz by allowing the RAF to detect incoming German bombers and intercepting them, thus averting a possible invasion. Radar has been in the news lately because of the United States Air Force stealth fighter and bomber. Radar serves as the principle means of tracking commercial air traffic and it's also used to help ships at sea avoid collision with other ships and icebergs. Everyone is familiar with radar being used to detect weather. Military radar uses are many and include missile guidance and tracking. And, radar is also used in space exploration. It allowed NASA to land on the moon and map the surface of Venus.

But, how does radar actually work?

Radar stands for Radio Detection and Ranging. This pretty much tells you how it works. You send out radio waves and they bounce off an object and come back to you. Radio waves travel at the speed of light like all electromagnetic radiation, so it's easy to time the delay in the wave that goes out and the waves that bounces back. Like most amazing inventions this was an accidental discovery that goes back to 1886 when some German physicist found that radio waves could be reflected from solid objects. That wasn't what he was trying to do. He was trying to detect distant lightening strikes. It wasn't long before someone realized the utility of this idea and by World War II, they had devices to detect and track airplanes. Think of the radar unit near Pearl Harbor in 1941 at the time of the Japanese attack that got the U.S. into the war. The unit was new and when the operators called back to base that they were detecting a large number of aircraft approaching, their report was ignored. That never happened again.

Radar signals are primarily in the microwave region and are pulse modulated. This type of modulation is used in order to allow the receiver a period of time to detect the returning signal. The amplifiers in the receiver system are blanked during the transmitted pulse to avoid being swamped by the more powerful signal. This pulse modulation scheme limits the radar signal's range both in the case of close and distant targets. These continuous modulation pulses must be adjusted in frequency to allow for the echo signal to be painted or integrated into the signal processor every time a pulse is sent out. This is a balancing act that depends upon the type of targets that the radar is designed to detect.

Of course, it didn't take the military long to realize the essential utility of radar as a means of detecting enemy aircraft and ships. All that it required is putting together some electronic equipment. Foremost is the radar transmitter. This sends out a radio signal in a specific direction. In many cases the direction is changed by antenna rotation, giving the radar unit a 360-degree field of view. Radar also depends on the object being tracked bouncing enough of the radar signal back. As you might expect, an object reflects the radar signal and scatters it. The best return signals are from material that has electrical conductivity such a metal. Other objects, such as ground, sea and precipitation, cause clutter or static that makes it harder to detect a target, especially if it's low. This static idea is used by military aircraft to fool radar guided air to air and ground to air missiles by releasing chaff, essentially metal fragments that deflect the missile's course.

A receiver antenna detects radar signals that are reflected off of a target, but since the signal is much weaker than the transmitted signal an amplifier must be employed to boost the signal strength. As you might have guessed, the further away the target is the weaker the signal. Also, some materials scatter radar waves better and are therefor harder to detect.

Water droplets also reflect back radar signals. This is why radar can detect rain, snow, sleet and storms. Combine radar with computers and you have a means to detect tornados, and increased resolution allows the differentiation of hail and sleet from rain and snow.

The latest weather radar units employ Doppler radar. This specialized radar equipment is able to detect motion in a target by analyzing how the frequency of the reflected signal is altered by the target. Doppler shift is what happens when something is moving away or toward an observer. Einstein predicted the effect in the case of electromagnetic radiation, and it's how the speed gun units that police use can determine the velocity of a vehicle. This same principle is how weather radar can detect rotation of tornados in a return from a storm.

Of course the military began working on a method of making an aircraft transparent to radar. By using sharp angled edges and special paint, the stealth fighter was designed to avoid radar detection, but it came with a price. The plane was so unstable that it had to employ fly-by-wire technology to stay up in the air. No pilot can fly it without this technology. The other problem is that the plane cannot use radar itself. The transmitted signal would give it away. So, they designed it to use high resolution infrared to help the pilot see at night. As with most military secrets, there are ways around this problem, but they're secret.

Radar has become one of the most useful ways that radio signals are used. So, don't speed. You never know when some cop is aiming a radar gun at you.

Thanks for reading.