Transcript Introduction to Radar

Introduction to Radar
Shaohua Li
Graduate Student
Department of Electrical and Computer
Engineering
Functions of Radar
RADAR is a method of using
electromagnetic waves to remote-sense
the position, velocity and identifying
characteristics of targets.
History of Radar
Radar was developed for military
purposes during W. W. II.
The British and US Military used radar
to locate ships and airplanes.
History of Radar
History of Radar
During the war, radar operators found
annoying blips continually appearing on
the radar screen. Scientists had not
known that radar would be sensitive
enough to detect precipitations.
Today, radar is an essential tool for
predicting and analyzing the weather.
Weather Radar
Weather
Surveillance
Radar,
designed in
1957. It
became the
primary
radar for the
weather
service for
nearly 40
years.
Weather Radar
NSSL's first
Doppler
Weather Radar
located in
Norman,
Oklahoma.
1970's research
using this radar
led to NWS
NEXRAD WSR88D radar
network.
The expensive
radar equipment is
protected by the
sphere shaped
cover. On the
inside it looks
similar to this:
Two Basic Radar Types
Pulse Transmission
Continuous Wave
Pulse Diagram
PRF
Resting
Time
PW
Carrier
Wave
Pulse Radar Components
Synchronizer
Transmitter
Power
Supply
Display Unit
Duplexer
Receiver
Antenna Control
ANT.
Pulse Transmission
Pulse Repetition Time (PRT=1/PRF)
Pulse Width (PW)
 Length or duration of a given pulse
 PRT is time from beginning of one pulse to the
beginning of the next
 PRF is frequency at which consecutive pulses are
transmitted.
PW can determine the radar’s minimum range
resolution.
PRF can determine the radar’s maximum detection
range.
Continuous Wave Radar
Employs continual RADAR transmission
Relies on the “DOPPLER EFFECT”
Doppler Frequency Shifts
Motion Away
Echo Frequency Decreases
Motion Towards
Echo Frequency Increases
Continuous Wave Radar
Components
Transmitter
CW
RF
Oscillator
Discriminator
AMP
Mixer
Antenna
OUT
IN
Antenna
Indicator
Pulse Vs. Continuous Wave
Pulse Echo
Single Antenna
Gives Range & Alt.
Susceptible To
Jamming
Physical Range
Determined By PW
and PRF.
Continuous Wave
Requires 2 Antennae
No Range or Alt. Info
High SNR
More Difficult to Jam
But Easily Deceived
Amp can be tuned to
look for expected
frequencies
Classification by Primary Radar
Mission
Search radars and modes
•
•
•
•
Surface search
Air search
Two-dimensional search radars
Three-dimensional search radars
Tracking radars and modes
Track-while-scan
AN/FPS-24 Search Radar
AN/SPS-49 Very Long Range Air
Surveillance Radar
AN/TPS-43
3-D Air Search Radar
The AN/TPS-43 radar
system, with a 200
mile range, was the
only Air Force
tactical ground based
long range search and
warning radar for
nearly two decades.
Most of the AN/TPS43 radars are being
modified to the
AN/TPS-75
configuration.
AN/TPS-75
Tracking Radar
Tracking radars dwell on individual targets
and follow their motion in azimuth,
elevation,range and Doppler.
Most tracking radars can follow only a single
target.
A few radars can track multiple targets
simultaneously. An electronically steered
array antenna is used so that beam
positions can be moved quickly from one
target to another.
AN/APG-66 in the F-16
http://www.tpub.com/neets/boo
k18/79j.htm
Types of Antenna
Introducing two types of antenna
reflector mirror antenna
array antenna
Reflector Antenna
Parabolic Reflector
Basic paraboloid reflector; Truncated paraboloid;Orange-peel
paraboloid;Cylindrical paraboloid
Array Antenna
An array antenna is composed of multiple
element arrays for example, linear array, area
array or nonformal array. The element
antennas are half-wavelength dipoles,
microstrip patches and wave guide slot. The
advantages of array antenna are to enable
beam scanning without changing the looking
angle of each array antenna and to generate
an appropriate beam shaping by selective
excitation of current distribution of each
element.
An Example of Array Antenna
Edgewall Slot Array-AN/APY-2
on E-3D Aircraft
The E-3 Sentry is an airborne warning and control system (AWACS) aircraft that
provides all-weather surveillance, command, control and communications needed by
commanders of U.S. and NATO air defense forces. As proven in Desert Storm, it is
the premier air battle command and control aircraft in the world today.
AN/FPS-115 PAVE PAWS Early
Warning Radar Array Antenna
PAWS stands for Phased Array Warning System. The radar is used primarily to detect
and track sea-launched and intercontinental ballistic missiles.
It can search over long distance(to 5000 km or more). Each system has two array faces
72.5 feet in diameter with 2677 element positions.
To provide surveillance across the horizon, the building is constructed in the shape of a
triangle. The two building faces supporting the arrays, each covering 120 degrees, will
monitor 240 degrees of azimuth.
Radar Performance and
Frequency Bands
Bandwidth
The bandwidth determines the range resolution and frequency agility capabilities of the radar.
Antenna
For a given gain, low frequency antennas are larger than high frequency. Low frequency are favored for long-range
search applications, because of the larger effective area associated with a given gain, allowing more effective
capture of echoes.
Transmitter
In general, more radio frequency power can be produced at low frequency than at high.
Receiver
There is no clear choice between high and low frequencies.
Propagation
The attenuation at high frequency is dramatic. A given raindrop has over three orders of magnitude more scattering
cross-section at X-band(10 GHz) than at L-band(1.3 GHz), producing far more clutter and signal at the higher
frequency.
Targets
If the wavelength is long compared to the target extent, targets are Rayleigh scatterers, and have small, non-fluctuating
RCS.
Summary
In general, the longer the range at which the radar must detect targets, the lower the frequency of the
radar.
Reference
www.fas.org/man/dod-01/sys/ac/equip/
http://www.tpub.com/neets/book11/46a.htm
http://www.tpub.com/neets/book18/79j.htm
etc.