Basic 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?
Light is an electromagnetic wave.
Microwaves = have air wavelengths in the mm – cm scales
with frequencies in the Giga Hertz [GHz] range
Radar
Bands
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
Compare to: Acoustic Echo-location
hello
Acoustic Echo-location
hello
Acoustic Echo-location
hello
distance
Hi !!
Hi !!
time
t = 2 x range / speed of sound
Example: range = 150 m
Speed of sound ≈ 340 meters/second
t = 2 X 150 / 340 ≈ 1 second
RADAR Echolocation
(RADAR ~ RAdio Detection And Ranging)
“Microwave Echo-Location”
Tx
Rx
Microwave
Transmitter
Receiver
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
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
Unambiguou
s Range
c
cT S
R max 
2 PRF

2
Range Resolution
Top View: 2D
Range Resolution
ct
DRr =
2
q
Azimuth Range Resolution
DRa = R × q
DCAS advantages
• Elimination of multiple echoes
Radar 1
(Area A)
A&B
Radar 2
(Area B)
False Echo
Unambiguous
range of 1
radar
•Example
illustrates
DCAS
method for
identifying
real targets
and false
targets that
are aliased in
range
DCAS advantages (cont)
DX
• Resolution optimization
Target
Range gate
Beam
Radar
1
Radar
2
Has
signal
Has
signal
New
resolution:
Dx’
Dy’
DR
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 drops
Raindrops 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.
Reflectivity Factor, Z
• Is defined as
Z 
D
6
N ( D ) dD
so that
 vc 

5
4o
2
| Kw | Z
• And expressed in dBZ to cover a wider dynamic
range of weather conditions.
dBZ  10 log Z
   vc  10
 12

5

4
where  is in cm
2
| Kw | Z
o
-1
and
28
Z is expressed
6
in mm /m
3
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 radars
Meteorologists:
• 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”
Frequency
ft
Doppler
Frequency
Frequency
ft+ fd
fd 
 2vr
t
QPE – Quantitative
Precipitation Estimation
0.1 mm/hr
1 mm/hr
15 mm/hr
100 mm/hr
>150 mm/hr
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)
Radar equation for Meteorology
• For weather applications
Pr 
Pt G 
2
o
3
 4 
2
o
R
4
R
e
Pr 
   vV
Pt G   c  p e
2
o
2
32  4 R 
 
 
o
• for a volume
2
o
 2
2
 2
v
g
  e c   ep dr
 R    c p 

V 
 

 2   2 
2

43
Radar Equation
• For power
distribution in the
main lobe assumed
to be Gaussian
function.
Pt G o  oq o  o c  p L r
2
Pr 
2
1024  ln 2
L e
2
L
v
R
2
where,
 v    radar reflectivi ty
and L r  receiver
And the two - way atmospheri
2
2
losses
c losses are defined
here as
 2
44
Radar Equation
P 
Pt G o  oq o  o c 
r
Pr
dB
 Pt
dB
 2G
dB
o
 20 log(  o )  10 log( q o
rad
2
2
p
v
2
1024  ln 2 L atm L rec R
2
)  10 log(  o
rad
2
 10 log 
)
p
 10 log   10 log c  10 log L rec  20 log L atm  10 log( 1024  ln 2 )  20 log R
2
Pr
dB
 Pt
dB
 10 log  p  10 log   R
dB
c
 20 log R
For calibrated target
RcdB=radar constant (including
atmospheric attenuation)
45
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