Introduction to Radar

Shaohua Li

Graduate StudentDepartment 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 RadarWeather Surveillance Radar, designed in 1957. It became the primary radar for the weather service for nearly 40 years.

Weather RadarNSSL's first

Doppler Weather Radar

located in Norman,

Oklahoma. 1970's research using this radar

led to NWS NEXRAD WSR-

88D 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

PRFPRF

PWPW

RestingRestingTimeTime

CarrierCarrierWaveWave

Pulse Radar Components

SynchronizerSynchronizer TransmitterTransmitter

Display UnitDisplay Unit ReceiverReceiver

PowerPowerSupplySupply

ANT.ANT.DuplexerDuplexer

RF Out

Ech

o In

Antenna Control

Pulse TransmissionPulse 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

Motion Towards

Echo Frequency Decreases

Echo Frequency Increases

Continuous Wave Radar Components

Discriminator AMP Mixer

CW RFOscillator

Indicator

OUTOUT

ININ

Transmitter Antenna

Antenna

Pulse Vs. Continuous Wave

Pulse EchoSingle AntennaGives Range & Alt.Susceptible To JammingPhysical Range Determined By PW and PRF.

Continuous WaveRequires 2 AntennaeNo Range or Alt. InfoHigh SNRMore Difficult to Jam But Easily DeceivedAmp 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 modesTrack-while-scan

AN/FPS-24 Search Radar

AN/SPS-49 Very Long Range Air Surveillance Radar

AN/TPS-43The 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/TPS-43 radars are being modified to the AN/TPS-75 configuration.

3-D Air Search Radar

AN/TPS-75

Tracking RadarTracking 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/book18/79j.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

BandwidthThe bandwidth determines the range resolution and frequency agility capabilities of the radar.

AntennaFor 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.

TransmitterIn general, more radio frequency power can be produced at low frequency than at high.

ReceiverThere is no clear choice between high and low frequencies.

PropagationThe 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.

TargetsIf the wavelength is long compared to the target extent, targets are Rayleigh scatterers, and have small, non-fluctuating

RCS.

SummaryIn 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.htmhttp://www.tpub.com/neets/book18/79j.htmetc.