Basic RADAR PrinciplesBasic RADAR Principles

Prof. Sandra Cruz-Pol, Ph.D.

Electrical and Computer Engineering UPRM

What is a Radar?radio detection and ranging

How does a radar work?

Microwaves = have air Microwaves = have air wavelengthswavelengths in the mm – cm scales in the mm – cm scales

with with frequenciesfrequencies in the Giga Hertz [GHz] range in the Giga Hertz [GHz] range

Light is an electromagnetic wave.

Radar Band

s

Electromagnetic Spectrum

Microwave Remote Sensing

• Percentage transmission thought the earth’s atmosphere, along the vertical direction, under clear sky conditions.

Cloud and Precipitation Remote Sensing

• Atmospheric gases absorption spectrum at the ground in various humidity conditions indicated by the specific humidity values.

Types of Doppler Radar• Continuous Wave (CW)

– Simple– No range information

• Frequency Modulated CW, (FMCW)– Fine range resolution– Artifacts from target motion

• Pulse Doppler– Range and Doppler– No artifacts (except when pulse compression used)

                                                                         

We will see that Radars work by…

Transmitting microwave pulses….

and measuring the …•Time delay (range)•Amplitude•Polarization•Frequency

… of the microwave echo in each range gate

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

                                                                         

We will see that Radars work by…

Transmitting microwave pulses….

and measuring the …•Time delay (range)•Amplitude•Polarization•Frequency

… of the microwave echo in each range gate

Pulse Doppler Radar

22

Range sUnambiguou

maxScT

PRF

cR

Range Resolution

Top View: 2D

DCAS advantages• Elimination of multiple echoes

False Echo

Radar 2 (Area B)

Radar 1 (Area A)

A&B

Unambiguous range of 1

radar

•Example illustrates

DCAS method for identifying real targets and false

targets that are aliased in

range

DCAS advantages (cont)

• Resolution optimization

Radar 1

Has signal

Radar 2

Has signal

x ’y ’

New resolution:

RX

Target

Range gate

Beam

                                                                         

We will see that Radars work by…

Transmitting microwave pulses….

and measuring the …•Time delay (range)•Amplitude•Polarization•Frequency

… of the microwave echo in each range gate

Polarization

Describes the way the electric field of the wave moves through space as seen from behind along time.•V = vertical lineal•H = Horizontal lineal

Sizes for cloud and rain dropsSizes for cloud and rain drops

Raindrops symmetryRaindrops symmetry

Differential Reflectivity

Zdr

Polarimetric radars, also called dual-pol radars, transmit radio wave pulses that have both H and V orientations. [NOAA]

TropiNET radars are the first Polarimatric Doppler radars in PR.

28

Reflectivity Factor, Reflectivity Factor, ZZ

• Is defined as

so that

• And expressed in dBZ to cover a wider dynamic range of weather conditions.

dDDNZ )(D6 ZKwo

vc2

4

5

||

ZdBZ log10

36

1-

24

512

/mmmin expressed is

and cmin is where

||

10

Z

ZKwo

vc

WHAT VARIABLES ARE MEASURED w/ Dual-pol radar?

• Differential Reflectivity –ratio of the reflected H & V power returns. Indicator of drop shape & good estimate of average drop size.

• Linear Depolarization Ratio –ratio of a V power return from a H pulse or a H from V. indicator of regions where mixtures of precipitation types occur.

• Specific Differential Phase –returned phase difference between the H V pulses caused by the difference in the number of wave cycles (or wavelengths) along the propagation path for horizontal and vertically polarized waves. It’s a "propagation effect.” very good estimator of rain rate.

Benefits of polarimetric radarsMeteorologists:•can significantly improve the accuracy of the estimates of amounts of precipitation•can tell the difference between very heavy rain and hail, which will improve flash flood watches and warnings•can identify types of precipitation in winter weather forecasts, improving forecasts of liquid water equivalent or snow depth•is more accurate than conventional radar, saving the forecasters the step of having to verify radar data•can contribute to increased lead time in flash flood and winter weather hazard warnings.Hydrologists:•provides critical rainfall estimation information for stream flow forecasts and river flooding

Raindrop shapes

Average within sample volume

                                                                         

We will see that Radars work by…

Transmitting microwave pulses….

and measuring the …•Time delay (range)•Amplitude•Polarization•Frequency

… of the microwave echo in each range gate

Doppler Effect

Target Radial Velocity

Frequency ft

Frequency ft+ fd In Weather radars, the

Doppler frequency shift, is caused by the motion

of the cloud and precipitation particles

Target Radial Velocity

Frequency ft

Frequency ft+ fd

Zero Velocity for “Crossing Targets”

t

rd

vf

2

Frequency ft

Frequency ft+ fd

Doppler Frequency

0.1 mm/hr

1 mm/hr

15 mm/hr

100 mm/hr

>150 mm/hr

QPE – Quantitative Precipitation Estimation

Radar reflectivity (intensity)

Doppler effect: shows vortex

Cloud and Precipitation Remote Sensing

Type of data collected by the millimeter-wave radar.

• Observations were made through the melting region of a stratiform cloud previously named “bright band” because of a systematic maximum of echo intensity observed just below the 0º isotherm.

Source: Dr. Steve Sekelsky 2004

Melting Layer at Mayaguez-Jun2011 (data from Doppler Pol radar at CID UPRM)

43

Radar equation for MeteorologyRadar equation for Meteorology• For weather applications

• for a volume

2

43

22

4 e

R

GPP oot

r drR

o

epceg

22

2pcR

V

vpoot

rR

ecGPP

2

2222

432

Vv

44

Radar EquationRadar Equation

• For power distribution in the main lobe assumed to be Gaussian function.

22

2

22

2ln1024 RL

LcGPP vrpoooot

r

22

as here defined are losses catmospheriway - two theAndeL

lossesreceiver and

tyreflectiviradar

where,

r

v

L

45

Radar EquationRadar Equation222

22

2ln1024 RLL

cGPP v

recatm

pooootr

RLLc

GPP

atmrec

prad

orad

oodBo

dBt

dBr

log20)2ln1024log(10log20log10log10log10

log10)log(10)log(10)log(2022

RRPP dBcp

dBt

dBr log20log10log10

RcdB=radar constant (including atmospheric attenuation)

For calibrated target

References• The COMET project [http://www.comet.ucar.edu/]• NASA TRMM• NCAR (National Center for Atmospheric Research) - University

Corporation for Atmospheric Research (UCAR)• NOAA http://www.nssl.noaa.gov/research/radar/dualpol.php• 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]• http://www.radartutorial.eu/07.waves/wa04.en.html