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Lesson

Functional components of radar systems

RADAR stands for RAdio Detection And Ranging.  It uses a radar transmitter to send out a radio signal burst.  The it turns off the transmitter and turns on the receiver. If the signal bounces off something and comes back to the radar receiver, the system can calculate how long the echo took and the direction it came from.   The system cycles between transmitting and receiving at very high speeds.  

Information collected can be used to plot the position of the object on a screen.  Since the radar signal can be sent in a controlled beam, the direction to the object can also be determined, as well as direction of travel and the speed of the object if it is moving. 

On ships and airplanes, radar is used to track other ships and planes as well as other objects in real time.  This allows the operators to avoid collisions and stay on course.  Portable radar devices are used to determine the speed of moving objects such as baseballs and vehicles.  Radar is also used to track weather systems by detecting the movement of wind and precipitation.  Radar is also used to create maps.

Two key concepts are important to understanding radar

  • echo
  • Doppler shift

Echo

Echo is, as stated above, what occurs when a sound or radio wave hits an obstruction and is reflected.  Echoes from sound waves are familiar.  They occur in big empty buildings and outside on still, calm days when there is a large hill or cliff to reflect the sound.  The diagrams illustrate what is happening

  • Waves propagate out from a signal source

 

  • The waves strike a reflective surface

  • Waves are reflected back.  The time, and therefore the distance can be calculated.  The direction to the reflector can also be calculated.

Doppler shift

Doppler shift is a phenomena that occurs when the signal source and the receiver are moving relative to one another.  The source may be moving, the receiver may be moving, or both may be moving.  In either case the effect is the same.  

When they are converging (moving towards each other) the waves (sound or electromagnetic) get compressed together.  When they are moving apart, the waves get stretched out.  This is the effect you hear when a vehicle approaches you at high speed.  The pitch of the sound increases as the vehicle approaches, reaches its maximum just as it gets near you, and then drops rapidly as it passes and moves away.  The following sequence of ../images illustrates this process for sound waves.  It is similar for electromagnetic waves, including light and radio frequencies.

  • Neither the signal source or the observer is moving.  The observer hears the correct pitch.

  • The source and observer are converging (moving towards each other.  The motion of the source and the observer towards each other causes the waves to compress. The observer hears a higher pitch than normal

  • The source and the observer are moving away from each other.  The motion of the source and the observer away from each other causes the waves to stretch out.  The observer hears a lower pitch than normal.

Since the speed that waves propagate (sound or radio through air, for example) is known, it is possible to use the information collected about the amount of Doppler shift in order to perform calculations.  These calculations enable plotting of information in real time and the ability to make predictions.  This is exactly the case with Doppler weather radar.  By measuring the Doppler shift from precipitation and air masses it is possible to determine the location, direction of movement, and speed of weather fronts.

A more detailed description of how radar works can be found at

Be sure to follow the next page links at the bottom of each screen.

Activity

Assigned activities

  • Develop a set of diagrams showing the function of a radar system.  Include
    • the transmitter
    • the transponder in send mode
    • a pulse being sent
    • the transponder in receive mode
    • an echo being received
    • the receiver
    • a representation of how information might be displayed
  • Describe how Doppler shift indicates if the radar source and the target are moving towards each other, or away from each other. 
  • Determine what additional information you would need to know to in order to track an object moving over time on a radar display screen.

Test Yourself

There is no self test for this lesson.