Making Waves with Radar

Have you ever been in a cave or an empty room and shouted loudly? If so, you probably heard an echo of your voice. Echoes happen when sound waves reflect off objects, like the walls of a cave. The farther you stand from the wall, the longer it takes for your voice to echo back. If you knew how long the sound took to reflect, and the speed of sound, you would know how far the wall was away from you.

This is similar to how radar works. But instead of sound waves, radar uses radio waves to detect the location of objects. Radio waves are at the low energy end of the electromagnetic spectrumelectromagnetic spectrum.

Radar is used in many different ways. At airports, air-traffic controllers use radar to keep track of airplanes and direct them onto their flight paths. Weather forecasters use radar to predict weather conditions and locate severe weather systems like hurricanes.

How it Works

A radar system is made up of five main parts. These are the transmitter, antenna, duplexer, receiver and display.

1. The transmitter uses a magnetron to produce short pulses of radio waves.
2. The antenna sends or transmits the waves out into the air. When the waves hit an object, they reflect, or bounce back, to the same antenna. The antenna picks up reflected waves during breaks or pauses between transmissions.
3. Because the antenna has the job of both sending and receiving radio waves, radars have a part called a duplexer. A duplexer helps to switch the antenna back and forth between transmitting and receiving. Just as you pause to listen to your voice echo back after you shout something in a cave, radars have to do the same thing.
4. Information from the antenna is sent to the receiver. A computer in the receiver processes the reflected waves to make sense of them. By measuring how long it takes the waves to return, the receiver can tell how far away the object is. The receiver can also identify how fast an object is moving and where a moving object is headed. Sometimes radars are also able to detect how big the object is. For example, an S-band radar can detect rain drops but not clouds whose droplets are too small.
5. The information from the receiver is then shown on a display, or screen, for people to see.

Early Radar

In 1886, Heinrich Hertz discovered that radio waves can be reflected off solid objects. At the time, Hertz could not think of any practical uses for his discovery. Later, in 1897, Alexander Popov discovered that radio waves could be used to detect a ship at sea, but he too didn’t do anything about it.

In 1904, German electrical engineer Christian Hülsmeyer invented a device using radar that he called a Telemobiloskop. He was inspired to invent it when, as a child, he witnessed two ships colliding on a foggy night. He was determined to find a way to prevent similar tragedies that resulted from poor visibility. His Telemobiloskop transmitted radio waves that would reflect off metal objects, like the hulls of ships. He demonstrated his invention to the German Navy, but they were not interested at the time.

Although many scientists contributed to the development of radar, best known among them is Scottish physicist Robert Watson-Watt. His radar system was originally designed for Britain's Meteorology Office to help them detect approaching storms.

In 1935, he wrote to the British government explaining how radio waves could be used to detect aircraft. By then he was able to demonstrate that his radar system could locate aircraft consistently at a distance of up to 140 km. The government saw the great potential in this, and by World War II (1939–45), were using his radar system as part of a series of radars called Chain Home. By detecting enemy aircraft, Chain Home radar played an important role in helping Britain, France, and the United States defeat Germany. A similar system was developed at the same time in the United States. The system there was able to detect the approach of Japanese airplanes over Pearl Harbor, in Hawaii, in December 1941. Sadly, no-one figured out the seriousness of so many approaching planes until it was too late.

Did you know?

At the beginning, these systems were not called radar. The word "radar" was an acronym coined in the 1940s by the United States Navy. It stands for radio detection and ranging.

Types of Radar

There are two main types of radar. These are pulse radar and continuous wave (CW) radar.

Pulse Radar

As explained above, a pulse of radio waves is transmitted by the radar antenna and reflects from the surface of the target. The returning signal or "echo" is received by the radar antenna to help measure distance to the target.

The effect is very similar to flashing strobe lights you see at a dance party. The quick flashes of light allows you glimpses of people or objects across the room at intervals.

The advantage of pulse radar is that they do better over long distances than CW radar.

Image - Text Version

A colour animated graphic of pulse radar. There is a stop watch on the upper left corner. It shows how long the pulse takes to be sent and reflected. A radar antenna on the lower left sends a signal (blue dot) diagonally across to a grey aircraft in the upper right corner. This is reflected back to the antenna as a light blue ripple. A green graph in the lower right corner tracks the time it takes for the wave to be sent and reflected.

Image - Text Version

Shown is a graph with signal intensity on the vertical axis and time on the horizontal axis. A blue line oscillates up and down in one small section of the graph. The line is horizontal on either side of the oscillation.

 

Continuous Wave Radar

In a CW radar system, there is no pulsing of waves. Radio waves are transmitted non-stop.

In a CW system, a wave with a specific frequencyfrequencyis transmitted. When the transmitted signal hits a moving object, the reflected signal changes in frequency. The frequency change is used to calculate the object’s speed. A benefit of CW radars is that they can provide continuously updated information about a target.

Image - Text Version

Shown are two graphs stacked on top of each other. Signal intensity is on the vertical axis and time is on the horizontal axis. The top graph shows high frequency where the wave pattern (blue) appears more compressed. The bottom graph shows low frequency where the wave pattern (blue) appears more stretched out.

 

Uses of Radar

Airport Surveillance Radar (ASR)

ASR is a radar system used to detect and show the location of aircraft in the airspace around airports. This is the main Air Traffic Control (ATC) system used around the world. The purpose of ATC is to prevent collisions between aircraft. It does this by showing the location and movement of aircraft so that air traffic controllers can tell them where to go. This system typically controls traffic within a 96 km radius of the airport for aircraft flying below an altitude of 7.62 km (25,000 feet).

Large airports often combine two different radar systems. Primary surveillance systems use a rotating dish to send out pulses of radio waves to detect the location of aircraft. The waves bounce back to the antenna from the aircraft's surface.

A rectangular
secondary surveillance radar is usually mounted on top of the primary radar. This type of radar only acts as a receiver. It receives signals from transmitters, known as transponders, located on aircraft. These signals allow air traffic controllers to identify aircraft and get specific information about the aircraft such as speed and altitude.

Image - Text Version

Shown is a colour photo of an airport surveillance radar. Seen in the foreground is an orange curved dish (primary radar). A horizontal rectangular metal grid (secondary radar) is attached on top of the dish. The radar system is mounted on a base atop a tower (not seen in the photo). There is grey railing around the perimeter of the radar system. In the background, blue skies, wisps of white clouds and the tops of trees can be seen.

Image - Text Version

Shown is a radar display of traffic at an airport. Runways and taxiways are black and the surrounding land is coloured green. Aircraft appear as small white shapes. Beside these are information about each aircraft including its designation, speed and altitude.

 

Doppler Weather Radar

Doppler radar is a specific type of radar that uses the Doppler Effect to gather information from a moving object. Doppler effect describes how sound changes as the source of the sound moves towards or away from the receiver.

Imagine a police car with its siren on. If the car is not moving, the frequency of the waves is the same in all directions. The siren will sound the same if you are listening to it from in front of or behind the car.

When the car is moving, the waves start to bunch up in front of it. The waves are compressed, meaning they have a greater frequency. The greater the frequency, the higher the pitch of the sound. Behind the car, the waves are stretched out, meaning they have a lesser frequency. Here the pitch is lower.

Doppler radar is able to use the frequency of reflected waves to measure speeds very accurately. This is very important when tracking weather systems. In the case of weather radar, the objects that the waves are reflecting off of are water droplets.

Image - Text Version

Shown is a colour illustration showing the difference in sound waves between a stationary police car vs. a moving police car. The sound waves around the stationary car (top) radiate evenly in all directions. The sound waves around the moving car are closer together in front of the car and further apart behind the car. The direction of motion of the car is indicated with a green arrow which points to the right.

 

Air-Borne Radars

Air-borne radars are often used by the military for air defence purposes. They are used as early-warning systems to detect enemy aircraft or missiles.

Search radar is one type of air-borne radar. It can scan great volumes of space with pulses of short radio waves. They normally scan a space two to four times a minute.

When search radars detect an aircraft, they display them as a "blip" on the radar display shaped like a compass. The radar sweeps around, like the hands of a clock.

Image - Text Version

Shown is a black and blue coloured image of a search radar display. Numbers can be seen faintly on the outer circumference of the compass-like display. Aircrafts (blue dots) are scattered on the left side of the display. One brighter dot appears on the upper right close to an illuminated area caused by the sweeping radar arm that moves around the display clockwise like the hand of a clock.

 

Targeting radars work in a similar way, but they scan smaller volumes of space more frequently. They usually scan an area several times a second. Target tracking radars are even more precise. They allow a pilot to target an aircraft using the Missile lock-on. This is a feature of some radars that allows them to automatically follow an object. Once a targeting radar has locked onto a target, the firing system can calculate a path for a missile to the target.

For aircrafts to be able to escape radar detection, they need stealth technology. This can include technology for letting them fly very fast, shapes that reflect radar in unusual ways and radar absorbent materials.

More Uses for Radar

Radars do not just point up at the sky! Sometimes they are pointing at us - or more specifically, the cars we are in.

Police use radar systems in devices used to detect vehicles that are travelling over the speed limit. They may be hand-held or in fixed locations. Similar devices are also used to measure the speed of baseball pitches and soccer kicks.

Image - Text Version

Shown is a colour photo of a hand held radar device being used at the side of a highway. In the foreground is the display part of the device and the top of a person's hand. On the display are numbers for the speed and range of an approaching transport truck. A concrete overpass can be seen behind the truck.

 

Did you know?

In some cases, radar is being replaced with LIDAR. LIDAR stands for LIght Detection And Ranging. It uses pulses of light instead of pulses of radio waves. Scientists also use a form of this visible radar to measure air pollution with lasers.

Archaeologists and geologists also use radar. They use a type of radar called ground penetrating radar. They use it to study the Earth’s layers as well as find mineral deposits and objects of historical interest. Radar helps them pin-point where to dig so that they can avoid damaging important cultural remains.

Image - Text Version

Shown is a greyscale image of radar data. Depth measurements are on the vertical axis and position measurements are on the horizontal axis. Several white coloured wavy lines of different sizes are visible. Yellow and red arrows point to these waves. These indicate the possible locations of human remains that reflected the radar signals.

 

The United States National Aeronautics and Space Administration (NASA) uses radar for many things. They use it to map the Earth and other planets, to track satellites and space debris as well as help with docking and manoeuvring space modules.

With so many practical uses for radar, even Heinrich Hertz would be amazed!