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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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Presentation Transcript

Sandra Cruz-Pol

Professor

Electrical and Computer Engineering Department

University of Puerto Rico at Mayagüez

CASA- Collaborative Adaptive Sensing of the Atmosphere

2006

• How does a radar work?

• Games

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

Compare to: Acoustic Echo-location

hello

Acoustic Echo-location

hello

Acoustic Echo-location

hello

distance

Hi !!

time

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

Tx

Rx

Microwave

Transmitter

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

Thresholding

Threshold Voltage

time

• 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

Range Gates

time

• 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

• Target Size

Scattered wave amplitude

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

time

Frequency

ft

Frequency

ft+ fd

Frequency

ft

Frequency

ft+ fd

Zero Velocity for “Crossing Targets”

Frequency

ft

Doppler Frequency

Frequency

ft+ fd

Closer look at Large drop

Large Drops

Polarization

Pt

Small Drops

Polarization

Ps

Microwave

Transmitter

Echo versus Range(range profile)

Transmitted Pulse #1

Cloud Echo

time

Transmitting microwave pulses….

and measuring the …

… of the microwave echo in each range gate

• Time delay (range)

• Amplitude (size)

• Polarization (spatial orientation & “oblateness”)

• 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

QPE – Quantitative Precipitation Estimation

0.1 mm/hr

1 mm/hr

15 mm/hr

100 mm/hr

>150 mm/hr

• 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.

Gap

May 3, 1999 Tornado Outbreak in Oklahoma

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

• 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

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

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

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).

0.7 mi

1.4 mi

2.1 mi

2.8 mi

Object Size

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

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

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 related games

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

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

References

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

• NASA TRMM

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