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Radars. Sandra Cruz-Pol Professor Electrical and Computer Engineering Department University of Puerto Rico at Mayagüez CASA- C ollaborative A daptive S ensing of the A tmosphere 2006. What is a Radar? Ra dio d etection a nd r anging. How does a radar work? Radar Concepts Games.

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Radars l.jpg


Sandra Cruz-Pol


Electrical and Computer Engineering Department

University of Puerto Rico at Mayagüez

CASA- Collaborative Adaptive Sensing of the Atmosphere


What is a radar ra dio d etection a nd r anging l.jpg
What is a Radar?Radio detection and ranging

  • How does a radar work?

  • Radar Concepts

  • Games

Slide3 l.jpg

The frequency of the em wave used depends on the application. Some frequencies travel through clouds with virtually no attenuation.

ALL em waves move at the speed of light

Slide8 l.jpg

Acoustic Echo-location



Slide9 l.jpg

Hi !!


Hi !!

t = 2 x range / speed of sound

Example:range= 150 m

Speed of sound≈340 meters/second

t= 2 X 150 / 340 ≈ 1 second

Radar echolocation radar ra dio d etection a nd r anging microwave echo location l.jpg
RADAR Echolocation(RADAR ~ RAdio Detection And Ranging)“Microwave Echo-Location”






Target range l.jpg
Target Range




t = 2 x range / speed of light

measure t, then determine Range

Example:t = .001 sec

Speed of light = c = 3x108 meters/second

Range = .001 x 3x108 / 2 = 150,000 m = 150 km

Thresholding l.jpg

Threshold Voltage


  • Measure time elapsed between transmit pulse

  • and target crossing a threshold voltage.

  • Then calculate range.

  • Don’t “report back” any information from targets that

  • don’t cross the threshold

Range gating l.jpg

Range Gates


Slide14 l.jpg

  • … of the microwave echo in each range gate

  • Target size l.jpg
    Target Size

    Scattered wave amplitude

    conveys size of the scattering objects. Measure amplitude, determine size.


    Target radial velocity l.jpg
    Target Radial Velocity




    ft+ fd

    Target radial velocity17 l.jpg
    Target Radial Velocity




    ft+ fd

    Zero velocity for crossing targets l.jpg
    Zero Velocity for “Crossing Targets”



    Doppler Frequency


    ft+ fd

    Target spatial orientation l.jpg

    Closer look at Large drop

    Target Spatial Orientation

    Large Drops



    Small Drops



    Example weather echoes l.jpg
    Example: Weather Echoes




    Echo versus range range profile l.jpg
    Echo versus Range(range profile)

    Transmitted Pulse #1

    Cloud Echo


    Slide22 l.jpg

    In summary, radars work by…

    Transmitting microwave pulses….

    and measuring the …

    … of the microwave echo in each range gate

    • Time delay (range)

    • Amplitude (size)

    • Frequency (radial velocity)

    • Polarization (spatial orientation & “oblateness”)

    Colors in radar images l.jpg
    Colors in radar images

    • The colors in radar images indicate the amount of rain falling in a given area.

    • Each raindrop reflects the energy from the radar. Therefore, the more raindrops in a certain area, the brighter the color in the radar image of that area.

    • The bright red color around the eye of a hurricane radar image indicates the area of heaviest rainfall. The green colored area has a moderate amount of rain, while the blue areas represent the least amount of rain.

    Hurricane Andrew, 1992

    Slide25 l.jpg

    QPE – Quantitative Precipitation Estimation

    0.1 mm/hr

    1 mm/hr

    15 mm/hr

    100 mm/hr

    >150 mm/hr

    Why radar can t usually see tornadoes l.jpg
    Why Radar Can't (Usually) See Tornadoes

    • The network of WSR-88D Doppler radars across the US has certainly proven itself for the ability to detect severe weather. Tornado warnings, in particular, are much better now that National Weather Service forecasters have this fantastic new (new as of the early 1990s) tool.

    • But did you know that Doppler radar (usually) can't see an actual tornado? When Doppler radar is cited in a tornado warning it is generally because meteorologists see evidence the storm itself is rotating. It is a supercell thunderstorm or at least contains an area of rotation called a mesocyclone.

    • When can and when can't Doppler radar see a tornado? It's math! Let's figure it out. We'll be looking into two factors:

      • 1) the first is something you learned in school a loooong time ago - the earth is curved, and

      • 2) the radar "beam" is 1 degree wide.

    Casa radars will complement nws radars l.jpg
    CASA radars will complement NWS radars

    Water spout at Mayaguez Beach, PR- Sept 2005 –unseen by NEXRAD

    Radar beamwidth l.jpg
    Radar "Beamwidth"

    • The geometry of the dish and a few other factors help determine the pulse volume, which can be specified in degrees.

    • NEXRAD radar sends discrete pulses (and spends 99.57% of the time listening for return echoes)

    • Meteorologists like to use the convenient terms "beam" and "beamwidth" to describe where the radar is pointing and the effective resolution of the air being sampled.

    Antennas l.jpg

    • Antenna is a transition passive device between the air and a transmission line that is used to transmit or receiveelectromagnetic waves.

    Antenna beamwidth l.jpg
    Antenna Beamwidth


    D is the antenna diameter

    λ is the wavelength of signal in air


    Small wavelengths (high frequencies) = small antennas

    But small wavelengths attenuate more

    Beamwidth size vs object size l.jpg
    Beamwidth Size vs. Object Size


    • What can a radar see? Beamwidth is one consideration. Earth curvature and height of the feature is another (addressed on the next page).

    • For the moment, we'll keep the problem in two dimensions and ignore height above ground.

    • The geometry is an isosceles triangle. Be sure to note which beamwidth you are calculating for (i.e. 1 degree).

    Beamwidth l.jpg

    0.7 mi

    1.4 mi

    2.1 mi

    2.8 mi

    Object size l.jpg
    Object Size

    How wide and tall are various things we want to see?

    Width of Meteorological Objects (i.e. Storms, Tornadoes)

    Earth curvature l.jpg
    Earth Curvature

    Fill in the table with values you calculate

    0.17 mi

    0.35 mi

    0.52 mi

    0.70 mi

    7 mi

    16 mi

    23 mi

    31 mi

    Play the games to learn the basics l.jpg
    Play the gamesto learn the basics

    • http://whyfiles.org

    • http://meted.ucar.edu/hurrican/strike/index.htm

    • http://meted.ucar.edu/hurrican/strike/

    • http://meted.ucar.edu/hurrican/strike/info_3.htm#

    • http://www.nws.noaa.gov/om/hurricane/index.shtml

    • http://www.nws.noaa.gov/om/edures.htm

    More games for kids 4 104 l.jpg
    More Games for Kids 4-104


    References l.jpg

    • The COMET project [http://www.comet.ucar.edu/]


    • NCAR (National Center for Atmospheric Research) - University Corporation for Atmospheric Research (UCAR)

    • NOAA Educational Page [http://www.nssl.noaa.gov/edu/ideas/radar.html]

    • Dave McLaughlin Basics of Radars presentation

    • NWS [http://www.crh.noaa.gov/fsd/soo/doppler/doppler.htm]