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Radar: Acronym for Radio Detection and Ranging
Radar is a remote sensing technique: Capable of gathering
information about objects located at remote distances from the
sensing device.
Two distinguishing characteristics:
1. Employs EM waves that fall into
the microwave portion of the
electromagnetic spectrum
(1 mm < l < 75 cm)
2. Active technique: radiation is
emitted by radar – radiation
scattered by objects is detected
by radar.
Why microwaves?
Microwaves can penetrate haze, fog and snow readily, and
rain and hail less readily, so radar can “see through” these
conditions.
An elementary radar system
What does a conventional radar measure?
1. Distance to an object or collection of objects
Determined by the time it takes energy to travel to
the objects and return at the speed of light.
ct
r
2
r = 1 km t = 6.67 ms
r = 100 km t = 0.667 ms
2. Azimuth and elevation angle to the object(s)
Determined by the pointing angles of the antenna.
3. Physical properties of the object(s)
Determined by the magnitude of the backscattered
power.
Meteorological radars send out pulses of energy with relatively long periods
of “listening” between pulses. Pulses are required, rather than continuous
waves, to determine the distance to the target.
Pulse duration (t, ms) and pulse length (h, meters)
Pulse repetition period (msec) and pulse repetition frequency (s-1)
Duty Cycle (= t/Tr)
Resolution along the direction of the beam:
half the pulse length (h)
The back of the pulse at “a” will arrive at “b” at the
same time that radiation scattered from objects at the
front end of the pulse at “c” will arrive back at “b”.
When energy arrives back at the radar, an
instantaneous sample will include all radiation
scattered between locations b and c: the sample
volume is half the pulse length (h/2).
Second Trip Echo: an echo from a pulse that is not the most recent pulse
Definitions
Pulse repetition frequency (PRF): The frequency that pulses are
transmitted, measured in hertz (s-1)
Pulse repetition period (Tr): The time between pulses (typical
value 1 ms)
Maximum Unambiguous Range (rmax): The maximum distance that
an object can be located such that a pulse arriving at the object can
return to the radar before another pulse is emitted.
rmax
cTr
c


2
2( PRF )
Maximum unambiguous range vs. pulse repetition frequency
Note weird velocities
(characteristic of distant
storm)
Second trip echoes
Why not use a low PRF, insuring a large rmax?
1. Measurements are not made with a single pulse, but rather
with the average of many pulses – since the antenna is rotating,
dwell time (observing the same location) is an issue.
2. Measurement of Doppler velocities require a high PRF
How can you eliminate second trip echoes automatically?
1) Change the PRF
2) Use a different PRF every 2-3 pulses, if echo moves, get rid of
it! This is the methodology employed by the 88-Ds
Other quantities used to describe the transmitted signal:
Wavelength (l, cm, mm) and Frequency (ft, Ghz, Mhz)
Band designation
Frequency
range
Wavelength
range
Common
Frequency
Common
Wavelength
(Ghz)
(cm)
(Ghz)
(cm)
UHF
0.3-1.0
30-100
0.42
71
L
1.0-2.0
15-30
1.3
23
S
2.0-4.0
7.5-15
2.8
10.7
C
4.0-8.0
3.75-7.5
5.5
5.5
X
8.0-12.0
2.5-3.75
9.4
3.2
Ku
12.0-18.0
1.67-2.5
15.5
1.94
K
18.0-27.0
1.11-1.67
24
1.25
Ka
27.0-40.0
0.75-1.11
35
0.86
Millimeter
40-300
0.1-0.75
94 (W band)
0.3
Major wavelength choice issues:
1. Size of equipment
2. Attenuation
3. Size of scatterers relative to
wavelength (Rayleigh vs Mie
scattering)
4. Peak power (without arcing in
waveguide – e.g., 3 MW in
unpressurized waveguide for
S band, 0.4 MW for K band)
S (10 cm) band radar antenna
K (0.8 cm) band radar antenna
Modulator
Stores power
Between pulses
Magnetron
Generates
Microwaves
when high
voltage pulse
sent from
Modulator
Frequency
Determined
by characteristics
of magnetron
Duplexer
Fast acting
Switch that
protects
sensitive
receiver from
high energy
pulse from
magnetron
STALO
Oscillator
Generates
a steady
frequency
COHO
Oscillates
at lower
frequency
with same
phase as
transmitted
pulse
Quantities used to describe weather echoes
Wavelength (l   l, cm, mm) and Frequency (ft  fD) Ghz, Mhz)
fD is the Doppler shift, the change in frequency that occurs
because scatterers are moving toward or away from the
radar.
Doppler shift is typically no more than a few kilohertz, while
The transmitted frequency is typically gigahertz!
3,000,000,000
3,000,001,000
Quantities used to describe weather echoes
Received Power: typical value: nanowatts
Compare the received power with the transmitted power:
Peak transmitted power: 106 watts
Received power:
10-9 watts
Receiver must be very sensitive, and must be protected from
main pulse of energy transmitted by the radar!
Modulator
Stores power
Between pulses
Klystron
Amplifier
that creates
microwaves at
frequency
determined
by STALO
and COHO
Duplexer
Protects
Sensitive
Receiver from
High energy
Pulse from
magnetron
STALO
Oscillator
Generates
a steady
frequency
COHO
Oscillates
at lower
frequency
with same
phase as
transmitted
pulse
A0 A1

cos(d t   )
2
A0 A1

sin( d t   )
2
Amplitude determination:
Phase determination:
A0 A1
 I 2  Q2
2
Q

I
d    tan 1 
Dynamic range of a receiver
Ratio (db) of input power that causes the video output to
reach its maximum (saturation) level, to the lowest power
that produces a detectable input.
 Pmax
D.R.  10 log 
 Pmin



Dynamic range of precipitation echoes
Ratio (db) of maximum echo power received from a very
intense storm close to the radar to the minimum power
received from the weakest cloud that can be detected at the
greatest range of interest.
 PBig close storm 

D.R.  10 log 
 PLittle cloud at far range 


WOULD LIKE THESE TO BE THE SAME!
Dynamic range of a receiver
 Pmax
D.R.  10 log 
 Pmin



Dynamic range of precipitation echoes
 PBig close storm 

D.R.  10 log 
 PLittle cloud at far range 


But PBig close storm  105 watts and PLittlecloud at far range  1013 watts
DR precip  10  80 db
8
Linear receivers:
Linear receivers: Output voltage is linear with input power
Receiver 1:
Strong echoes
Receiver 2:
Weak echoes
Single linear receivers
Typically have only half of
the dynamic range of
precipitation echoes, so two
receivers are often used in
tandem with automatic
switching depending on the
magnitude of the returned
signal
Other types of receivers:
Logarithmic and Square law receivers:
Output voltage is non-linear with input power
Have worse
resolution than
linear receivers
but cover full
dynamic range of
weather echoes
Additional components of radars
Power supplies: provide power
Servo amplifiers and/or drive motors: position antenna
Selsyns or potentiometers: measure angular coordinates of antenna
Waveguides, rotary joints, slip rings: transmit microwaves from
transmitter to antenna while antenna is rotating
Directional couplers: allow sampling of transmitted signal or to inject test
signals into receiver
Radomes: to protect antenna from weather and wind
Other electronic components