Radars

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Radars

• Sandra Cruz-Pol
• Professor
• Electrical and Computer Engineering Department
• University of Puerto Rico at Mayagüez
• CASA- Collaborative Adaptive Sensing of the
  Atmosphere
• 2006

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What is a Radar?Radio detection and ranging

• How does a radar work?
• Radar Concepts
• Games

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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
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• How does a radar work?

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(No Transcript)
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Compare to Acoustic Echo-location
hello
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Acoustic Echo-location
hello
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Acoustic Echo-location
hello
distance
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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
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RADAR Echolocation(RADAR RAdio Detection And
Ranging)Microwave Echo-Location
Tx
Rx
Microwave Transmitter
Receiver
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Target Range
Tx
Rx
time
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
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Thresholding
Threshold Voltage
time

• Measure time elapsed between transmit pulse
• and target crossing a threshold voltage.
• Then calculate range.
• Dont report back any information from targets
  that
• dont cross the threshold

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Range-Gating
Range Gates
time
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• We will see that Radars work by
• Transmitting microwave pulses.
• and measuring the
• Time delay (range)
• Amplitude
• Frequency
• Polarization
• of the microwave echo in each range gate

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Target Size
Scattered wave amplitude conveys size of the
scattering objects. Measure amplitude, determine
size.
time
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Target Radial Velocity
Frequency ft
Frequency ft fd
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Target Radial Velocity
Frequency ft
Frequency ft fd
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Zero Velocity for Crossing Targets
Frequency ft
Doppler Frequency
Frequency ft fd
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Target Spatial Orientation
Large Drops
Polarization Pt
Small Drops
Polarization Ps
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Example Weather Echoes
Microwave Transmitter
Receiver
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Echo versus Range(range profile)
Transmitted Pulse 1
Cloud Echo
time
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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)
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Other concepts of Radars
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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
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QPE Quantitative Precipitation Estimation
0.1 mm/hr
1 mm/hr
15 mm/hr
100 mm/hr
gt150 mm/hr
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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.

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NEXRAD System Today
Gap
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May 3, 1999 Tornado Outbreak in Oklahoma
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NWS has 150 NEXrad radars in US1 in Cayey, PR
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Proposed CASA radar network
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CASA radars will complement NWS radars
Water spout at Mayaguez Beach, PR- Sept 2005
unseen by NEXRAD
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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.

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Antennas

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

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Antenna Beamwidth
radians D is the antenna
diameter ? is the wavelength of signal in air
Tradeoff Small wavelengths (high frequencies)
small antennas But small wavelengths attenuate
more
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Beamwidth Size vs. Object Size

• Beamwidth
• 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).

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Beamwidth
0.7 mi 1.4 mi 2.1 mi 2.8 mi
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Object Size

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

Width of Meteorological Objects (i.e. Storms,
Tornadoes)
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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
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Play related games
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Play the games to 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

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More Games for Kids 4-104

• http//www.nws.noaa.gov/om/reachout/kidspage.shtml

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References

• The COMET project http//www.comet.ucar.edu/
• NASA TRMM
• NCAR (National Center for Atmospheric Research) -
  University Corporation for Atmospheric Research
  (UCAR)
• NOAA Educational Page http//www.nssl.noaa.gov/ed
  u/ideas/radar.html
• Dave McLaughlin Basics of Radars presentation
• NWS http//www.crh.noaa.gov/fsd/soo/doppler/doppl
  er.htm