RADAR: How It Works?

      The basic idea behind radar is very simple: a signal is transmitted, it bounces off an object and it is later received by some type of receiver.  This is like the type of thing that happens when sound echo's off a wall.  (Check out the image on the left side) However radars don't use sound as a signal. Instead they use certain kinds of electromagnetic waves called radio waves and microwaves.  This is where the name RADAR comes from (RAdio Detection And Ranging). 

     Radar technology began as top secret work by the British military during World War II, but has now become a very commonplace technology. Meteorologists use radar to detect cloud formations, military pilots use radar to pinpoint enemy aircraft, geologists use ground-penetrating radar to outline buried objects and coaches use radar to time a pitcher's throwing speed. The word 'radar', much like snafu or scuba, has gone from technical acronym to common usage. But how does radar work?

     If you've ever shouted at a distant building or hillside and heard an echo, you've experienced the basic principle behind radar. Sound waves from your mouth moved through the air at approximately 600 mph. If there were nothing substantial in front of you, these sound waves would have traveled a certain distance and basically dissipated. But if those sound waves struck the side of a building or the surface of a lake or a cliff wall, a portion of those sound waves would be reflected back to the source. Your ears would pick up the words you shouted a few seconds earlier. If you had a very sensitive stopwatch and good hearing, you could have clocked the time between your shouted words and the instant you heard the echo. This is essentially what radar does, only on a more complicated way.

Radar is an acronym for Radio Detection and Ranging. In order to understand the more complicated principles of radar, it might be useful to examine each element by itself:

 

Figure: Block Diagram for RADAR

1. Radio. Most of us are bombarded by some form of radar every day and we rarely notice. This is because radar units use high-frequency radio waves which cannot be heard by human ears. These waves could be as high as 40,000 Mhz in the case of microwave transmitters, but the specific frequency must be known to the radar operator. A radio transmitter is used to send out a series of electronic pulses into the air. This transmitter is often mounted on a rotating device in order to sweep the entire area around the radar tower. Instead of moving at the speed of sound, these pulses move at nearly the speed of light. Once a pulse is sent out, it is tracked by a receiver system. If the pulse is not blocked by an object, it will continue outward until it dissipates. Consequently, the receiver will not pick up a return signal and the radar screen will remain blank in that particular area. But if the pulse strikes an object, some of the wave will be reflected back to the tower. The receiver will pick up this signal and computer-aided modules will assign relatively strength to it. The radar screen will contain a visible dot at this location. Once all of the individual pulses have been received and measured, the entire screen will show where all the signals were reflected. A trained operator can then use this information to determine what objects were encountered. 

2. Detection. The first thing a basic radar system can do is detect a solid object in the area it scans sonically. The transmitter sends out a pulse of radio waves and eventually they strike an object. By measuring the time it takes to send out the signal and receive its echo, a radar operator can calculate the distance of the object from the radar array. Since radio waves move so fast, this time is often measured in microseconds. If a solid object continues to be picked up in the same place, the radar operator can assume the object is not moving. Many weather radar systems will pick up reflections from the ground which will appear to be cloud formations. Trained meteorologists call this phenomenon 'ground clutter' and they will generally ignore it. Military pilots also learn to recognize enemy aircraft by their specific patterns on the radar screen. This helps pilots discriminate between an enemy threat and a flock of birds. 

3. Ranging. This is where radar technology gets very interesting. A pulse of radio waves can detect a solid object simply by measuring reflectivity towards the tower, but this reading alone does not indicate movement. In order to estimate the range and movement of an object, more sophisticated readings must be taken. The basic principle behind these readings is called a Doppler shift. Understanding Doppler shift is key to calculating the movement and speed of any object in the radar's range.

One common example of Doppler shift occurs when a police car approaches from a distance. A listener may hear the siren grow louder as the car becomes closer, but something else happens at the same time. The tone of the siren will also become higher. This apparent change in pitch is caused by a Doppler shift. The siren's true pitch and volume never changed from the moment it was activated, but the listener receives a higher concentration of soundwaves as the car grew closer. The tone changed because the frequency of these concentrated waves became higher. As the car moves away, the siren's soundwaves slow down and the tone is lower. 

In order to calculate an object's speed on radar, the system must take this Doppler shift into account. Remember that the original radio pulse which detected the object was sent out at a specific frequency. If the reflected signal came back at that same frequency, the object is most likely stationary. But an object moving towards the receiver is essentially shortening the distance of the reflected radio wave. The frequency becomes higher because of the Doppler shift. By calculating how much higher the reflected frequency has become, the relative speed of the object can be determined. As more readings are taken, the motion of the object can be seen on the radar screen. This is how law enforcement can calculate the speed of drivers as they pass by the radar unit. An initial electronic pulse becomes the base measurement. As the car moves towards the radar gun, the Doppler shift is measured and calculated. Only an object moving in the narrow beam of the radar gun can be timed in this manner, which eliminates the possibility of trees or street signs being clocked at 115 mph. Doppler shift measurements also allow meteorologists and pilots to determine the speed of an approaching storm system.