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

What is a Radar?Radio detection and ranging

1. How does a radar work?

2. Radar Concepts

3. 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

How does a radar work?How does a radar work?

hello

Compare to: Acoustic Echo-location

hello

Acoustic Echo-location

hello

distance

Acoustic Echo-location

Hi !!

Hi !!

time

t = 2 x range / speed of soundExample: range = 150 mSpeed of sound ≈ 340 meters/secondt = 2 X 150 / 340 ≈ 1 second

RADAR Echolocation(RADAR ~ RAdio Detection And Ranging)

“Microwave Echo-Location”

MicrowaveTransmitter

Receiver

Tx

Rx

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

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

Range-Gating

time

Range Gates

                                                                         

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

time

Scattered wave amplitudeconveys size of the scattering objects. Measure amplitude, determine size.

Target Radial Velocity

Frequency ft

Frequency ft+ fd

Target Radial Velocity

Frequency ft

Frequency ft+ fd

Zero Velocity for “Crossing Targets”

t

rd

vf

2

Frequency ft

Frequency ft+ fd

Doppler Frequency

Target Spatial Orientation

Polarization Pt

PolarizationPs

Large Drops

Small Drops

Closer look at Large

drop

Example: Weather Echoes

MicrowaveTransmitter

Receiver

Echo versus Range(range profile)

time

Transmitted Pulse #1

Cloud Echo

                                                                         

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”)

Other concepts Other concepts of of RadarsRadars

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

0.1 mm/hr

1 mm/hr

15 mm/hr

100 mm/hr

>150 mm/hr

QPE – Quantitative Precipitation Estimation

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.

NEXRAD System TodayNEXRAD System Today

Gap

May 3, 1999 Tornado Outbreak in Oklahoma

NWS has ~150 NEXrad radars in US;1 in Cayey, PR

Proposed CASA radar network

CASA radars will complement NWS radars

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

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

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

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

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).

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

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

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

Play related gamesPlay related games

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

More Games for Kids 4-104

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

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]]