How Radar Works Mars_leadin


(photos: JPL/NASA/MSSC)

Terrestrial analogs of Martian
radar targets from the Dry Valleys, Antarctica

 



How Radar Works

The Basic Idea:

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)  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).  Sound is used as a signal to detect objects in devices called SONAR (SOund NAvigation Ranging).  Another type of signal used that is relatively new is laser light that is used in devices called LIDAR (you guessed it...LIght Detection And Ranging). 

            Radio waves and microwaves are two types of electromagnetic waves.  Electromagnetic waves, which I will call EM waves, like all waves transport energy but can do so through a vacuum.  Sound waves and ocean waves require matter to transport energy but EM waves can do so without the presence of matter.  Because of this, satellites can use radars to work on projects outside of the Earth's atmosphere and on other planets.  Another useful thing about EM waves is that they travel at a constant speed through a vacuum called the speed of light abbreviated by the letter "c" (299,792,458 meters per second).  This is very useful to know to when doing ranging calculations.  To learn more about EM waves or waves in general, please visit The Physics Classroom.  Radio waves have wavelengths that are 10 cm and greater and microwaves have wavelengths that range from 10 cm to 1/10 of a mm.  (Check out the Electromagnetic Spectrum) Once the radar receives the returned signal, it calculates useful information from it such as the time taken for it to be received, the strength of the returned signal, or the change in frequency of the signal.  This information is then translated to reveal useful data: an image, a position or the velocity of your speeding car.


Your Basic Radar System:


A basic radar system is spilt up into a transmitter, switch, antenna, receiver, data recorder, processor and some sort of output display.  Everything starts with the transmitter as it transmits a high power pulse to a switch which then directs the pulse to be transmitted out an antenna.  Just after the antenna is finished transmitting the pulse, the switch switches control to the receiver which allows the antenna to receive echoed signals.  Once the signals are received the switch then transfers control back to the transmitter to transmit another signal.  The switch may toggle control between the transmitter and the receiver as much as 1000 times per second.

            Any received signals from the receiver are then sent to a data recorder for storage on a disk or tape.  Later the data must be processed to be interpreted into something useful which would go on a display.

Pulse Width and Bandwidth:

            Some radar transmitters do not transmit constant, uninterrupted electromagnetic waves.  Instead, they transmit rhythmic pulses of EM waves with a set amount of time in between each pulse.  The pulse itself would consist of an EM wave of several wavelengths with some dead time after it in which there are no transmissions.  The time between each pulse is called the pulse repetition time (PRT) and the number of pulses transmitted in one second is called the pulse repetition frequency (PRF).  The time taken for each pulse to be transmitted is called the pulse width (PW) or pulse duration.  Typically they can be around 0.1 microseconds long for penetrating radars or 10-50 microseconds long for imaging radars (a microsecond is a millionth of a second). 

In math language, the above can be said...

PRT = 1 / PRF

or

PRF = 1 / PRT

And for all you visual learners out there, this is what it looks like...

RT means repetition time.

However, the above diagram is not quite realistic for several reasons.  One reason why it is not realistic is that the frequency in waves of the pulses are the same.  In real life the frequency of the waves are not the same and they change as time goes on.  This is called frequency modulation which means the frequency changes or modulates.

It looks something like this...

Think of this as one pulse.  All the pulses will look something like this.

On the above diagram, the frequency of the wave is low on the left and it slowly increases as you look right.  The different frequencies of the wave will lie in a range called bandwidth.  Radars use bandwidth for several reasons regarding the resolution of a data image, memory of the radar and overuse of the transmitter.  For instance, a high bandwidth can yield a finer resolution but take up more memory.

Backscatter:




 

 

When an EM wave hits a surface, it gets partly reflected away from the surface and refracted into the surface.  The amount of reflection and refraction depends on the properties of the surface and the properties of the matter which the wave was originally traveling through.  (To find out more on reflection and refraction please visit The Physics Classroom)  This is what happens to radar signals when they hit objects.  If a radar signal hits a surface that is perfectly flat then the signal gets reflected in a single direction (the same is true for refraction).  If the signal hits a surface that is not perfectly flat (like all surfaces on Earth) then it gets reflected in all directions.  Only a very small fraction of the original signal is transmitted back in the direction of the receiver.  This small fraction is what is known as backscatter.  The typical power of a transmitted signal is around 1 kilo-watt and the typical power of the backscatter can be around 10 watts.

To determine the range of a distant object that reflected a radar signal, the receiver must record the time when the signal was received and compare it to when that signal was transmitted.  This time is the time taken for the radio wave to propagate to the object and back to the antenna.  Since all EM waves travel at the speed of light in a vacuum, 299,792,458 meters per second (Air is not quite a vacuum but EM waves still travel through it at approximately this speed) it is very easy to determine how far away the object is (just multiply the speed of light by the time for the signal to get received).  Another thing the radar does when it receives a signal is determine how strong it is.  For ground penetrating radars the strength of the signal can tell how much the beds under the surface have different properties.  A higher received power indicates a larger difference between neighboring beds.