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Radar101

Updated May 8, 2012

Click here to go to our three-page tutorial radar absorbing materials

Click here to go to our page on antennas

Click here to go to our page on phased array antennas

Click here to go to our page on duplexers

Click here to go to our page on Radar Love

What microwave text book could be complete without a discussion of radar basics? None that are worth reading in our estimation. This microwave topic in particular is going to take a long time to get any depth, unless we get some help from the radar industry (hint, hint...)

We now have a page on the CASA project, an exciting program that will vastly improve our ability to monitor bad weather!

Radar stands for radio detection and ranging, even though its function has been expanded to include range-rate (velocity) measurements. Thanks to Murat for the correction! Radar is one of the few acronyms that is so cool that it is now truly a word, like when Pinocchio becomes a real boy.

 

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Books on radar

Perhaps the best reference on the topic of radar is Merrill Skolnik's Introduction to Radar Systems first published in 1962.

Links to sites with outstanding content on radar

Here's some links to some of the best content on radar on the worldwide web.

Here's a cool web page that provides history of radar!

http://www.vectorsite.net/ttwiz.html

The best website we know of to learn about radar is owned by the NAVAIR Point Mugu:

https://ewhdbks.mugu.navy.mil/

Here's a link to a U.S. Navy website that will provide you with free software for investigating radar propagation in different environments:

http://sunspot.spawar.navy.mil/

History of radar

We have two "radar guys" in our Microwaves101 Hall of Fame! Radar's history goes back farther than you think... Spallazani was the first to investigate how bats are able to "see" using sound waves (sonar).

Christian Andreas Doppler was the mathematician that noticed that objects moving toward or away from an observer shifted their apparent sound frequency. Doppler's work was all done with audio waves, but he postulated that the shift in electromagnetic spectrum would allow astronomers to determine whether stars are approaching or receding Earth (the so-called "red shift" or "blue shift").

The Mark 53 VT fuze was a miniature Doppler radar that helped the Allies target all manner of airborne targets by the end of World War II! Now we have a separate page on the topic!

Types of radar

Monostatic radar
Monostatic implies that the transmit and receive antennas are collocated. Most radar is monostatic.

Bi-static radar
Bi-static means that the transmit and receive antennas are NOT collocated

Doppler radar
Doppler is used to measure the velocity of a target, due to its Doppler shift. Police radar is a classic example of Doppler radar. The price of Doppler radar has come down recently, so you can buy one for 100 bucks just to play with. We grabbed one of these Bushnell Speedster units from Radio Shack, and found it to have remarkable accuracy, but not much range:

The Unknown Editor's racquetball serve was recently clocked at 83 miles per hour!

FMCW radar
Frequency modulated/continuous wave implies that the radar signal is "chirped", or its frequency is varied in time. By varying the frequency in this manner, you can gather both range and velocity information.

Synthetic aperture radar (SAR)
SAR uses a moving platform to "scan" the radar in one or two dimensions. Satellite radar images mostly done using SAR.

Radar range equation

Below is the equation for range in a two-way (round-trip) monostatic radar:

(1)

That equation was fixed on May 6, 2012, thanks to Brian!

The radar range equation can take many forms, in terms of energy, antenna diameter, receive noise figure, etc. And the "range" will be different in a bi-static radar.

Let's examine the range equation from the physical size of an aperture that is shared by transmit and receive. The gain of an antenna is:

(2)

For a circular aperture the area is:

(3)

Substituting (2) and (3) into (1) we get:

(4)

From the equation it is easy to see that in order to double range, you must increase power by 16 times (12 dB!) if everything else is kept constant. Or you could quadruple the frequency (one fourth the wavelength), or double the radius of the aperture. In radar range, aperture area is the most important thing to consider! Note that if you increase the frequency or antenna area, the change in gain means it takes more beams to search a given volume.

Doppler shift equations

Moved to a new page.

Example radar cross sections at microwave frequencies

Most of the examples below are from Skolnik. RCS is further discussed on this page. RCS is often expressed in dBsm, or decibels relative to a square meter.

 
Square meters
dBsm
Conventional missile
0.5
-3
Small single engine aircraft
1
0
Small fighter or four-passenger jet
2
3
Large fighter
6
6.8
Medium bomber or medium jet airliner
20
13
Large bomber or large jet airliner
40
16
Jumbo jet
100
20
Small open boat
0.02
-17
Small pleasure boat
2
3
Cabin cruiser
10
10
Ship
Really big!
Pick-up truck
200
23
Automobile
100
20
Bicycle
2
3
Man
1
0
Iron man*
2
3
Bird
0.001
-30
Insect
10E-5
-50

 

*estimated, before Tony Stark became interested in stealth technology.

 

More to come!


 
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