radars sandra cruz-pol professor electrical and computer engineering department university of puerto...
TRANSCRIPT
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RadarsRadarsSandra Cruz-PolSandra Cruz-Pol
Professor Professor Electrical and Computer Engineering DepartmentElectrical and Computer Engineering Department
University of Puerto Rico at MayagüezUniversity of Puerto Rico at MayagüezCASA- CASA- CCollaborative ollaborative AAdaptive daptive SSensing of the ensing of the AAtmospheretmosphere
20062006
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What is a Radar?Radio detection and ranging
1. How does a radar work?
2. Radar Concepts
3. 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?How does a radar work?
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hello
Compare to: Acoustic Echo-location
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hello
Acoustic Echo-location
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hello
distance
Acoustic Echo-location
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Hi !!
Hi !!
time
t = 2 x range / speed of soundExample: range = 150 mSpeed of sound ≈ 340 meters/secondt = 2 X 150 / 340 ≈ 1 second
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RADAR Echolocation(RADAR ~ RAdio Detection And Ranging)
“Microwave Echo-Location”
MicrowaveTransmitter
Receiver
Tx
Rx
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Target Range
time
t = 2 x range / speed of lightmeasure t, then determine Range
Example: t = .001 secSpeed of light = c = 3x108 meters/secondRange = .001 x 3x108 / 2 = 150,000 m = 150 km
Tx
Rx
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Thresholding
time
•Measure time elapsed between transmit pulseand target crossing a threshold voltage.
•Then calculate range.
•Don’t “report back” any information from targets thatdon’t cross the threshold
Threshold Voltage
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Range-Gating
time
Range Gates
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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
time
Scattered wave amplitudeconveys size of the scattering objects. Measure amplitude, determine size.
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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”
t
rd
vf
2
Frequency ft
Frequency ft+ fd
Doppler Frequency
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Target Spatial Orientation
Polarization Pt
PolarizationPs
Large Drops
Small Drops
Closer look at Large
drop
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Example: Weather Echoes
MicrowaveTransmitter
Receiver
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Echo versus Range(range profile)
time
Transmitted Pulse #1
Cloud Echo
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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 Other concepts of of RadarsRadars
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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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0.1 mm/hr
1 mm/hr
15 mm/hr
100 mm/hr
>150 mm/hr
QPE – Quantitative Precipitation Estimation
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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 TodayNEXRAD System Today
Gap
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May 3, 1999 Tornado Outbreak in Oklahoma
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NWS has ~150 NEXrad radars in US;1 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
λ is the wavelength of signal in air
Tradeoff: Small wavelengths (high frequencies) = small antennasBut small wavelengths attenuate more
D
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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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BeamwidthDistance from
radarWidth of the
"beam"
20 mi
40 mi
60 mi
80 mi
d
badjopp
2//tan
0.7 mi
1.4 mi
2.1 mi
2.8 mi
tan2db
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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)
Object Width Height or Depth
Supercell thunderstorm 10-30 mi 28,000-55,000 ft
Circulation within the supercell thunderstorm
2-8 mi 2,000-55,000 ft
Tornado 0.1 - 1.0 miCloud base - 0.5 - 1.5
mi*
Individual storm cell within a squall line
2-8 mi 4,000-55,000 ft
Circulation embedded within a squall line
2-5 mi 4,000-40,000 ft
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Earth CurvatureFill in the table with values you calculate
Elevation Angle
Distance from radar
Height above ground
0.5 degrees
20 mi
40 mi
60 mi
80 mi
19.5 degrees
20 mi
40 mi
60 mi
80 mi
d
heighttan
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 gamesPlay 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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ReferencesReferences
The COMET project [The COMET project [http://www.comet.ucar.edu/]http://www.comet.ucar.edu/] NASA TRMMNASA TRMM NCAR (National Center for Atmospheric Research) - NCAR (National Center for Atmospheric Research) -
University Corporation for Atmospheric Research University Corporation for Atmospheric Research (UCAR)(UCAR)
NOAA Educational Page NOAA Educational Page [http://www.nssl.noaa.gov/edu/ideas/radar.html][http://www.nssl.noaa.gov/edu/ideas/radar.html]
Dave McLaughlin Dave McLaughlin Basics of RadarsBasics of Radars presentation presentation NWS NWS
[http://www.crh.noaa.gov/fsd/soo/doppler/doppler.htm[http://www.crh.noaa.gov/fsd/soo/doppler/doppler.htm]]