Pulse - RADAR transmits a series of pulses separated by non-transmission intervals during which the radar “listens” for a return. Continuous Wave - Constantly emitting radar. Relative motion of either the radar or the target is required to indicate target position. Frequency shift.
Second major type of radar. Produces a constant stream of energy. Can’t distinguish distances (range) because no interval between pulses. Can distinguish between moving and non-moving targets by using Doppler frequency shifts .
1. Doppler frequency shift describes the effect that motion has on a reflected frequency. 2. Use the diagram to show: a. If the wall is moving away a ball will have to travel farther than the previous ball so the reflected balls are further apart. b. If the wall is moving toward, a ball will have to travel a shorter distance than the previous ball so the reflected balls are closer together. 3. If you assume that each ball represents the top of a wave so the distance between each ball represents a wave cycle then you find: a. The frequency of the echo is lower if the target is moving away. b. The frequency of the echo is higher if the target is coming towards. ** This is why the sound of a passing train or airplane goes from higher pitch to lower pitch. 4. Key Points: a. Frequency expansion is the lowering of the echo frequency caused by an opening target (target moving away). DOWN DOPPLER b. Frequency compression is the raising of the echo frequency caused by the closing target (target moving closer). UP DOPPLER c. The moving of the transmitter can also cause frequency shifts (it’s relative motion that produces the effect). d. The faster the relative motion change the greater the frequency shift.
Make copies for distribution. 1. Transmit/Receive Antennas. Since must operate simultaneously, must be located separately so receiving antenna doesn’t pick up transmitted signal. 2. Oscillator or Power Amplifier. Sends out signal to transmit antenna. Also sends sample signal to Mixer. (used as a reference) 3. Mixer. a. A weak sample of the transmitted RF energy is combined with the received echo signal. b. The two signal will differ because of the Doppler shift. c. The output of the mixer is a function of the difference in frequencies. 4. Amplifier. Increases strength of signal before sending it to the indicator. 5. Discriminator. a. Selects desired frequency bands for Doppler shifts, eliminates impossible signals. b. The unit will only allow certain frequency bands so won’t process stray signals. 6. Indicator. Displays data. Displays velocity or the component directly inbound or directly outbound. Range is not measured . 7. Filters. Used to reduce noise, used in amp to reduce sea return, land clutter, and other non-desirable targets.
Discuss Slide Range for CW: (p. 106) Frequency Modulated Continuous Wave. Altitude for CW: Slant range (see coming slide)
Pg. 72 Fig 2-39 1. Discuss displays, time permitting.
What is RADAR an acronym for?What is RADAR an acronym for?
Radio Detection and Ranging.Radio Detection and Ranging.
Radio wave is generated, transmitted,Radio wave is generated, transmitted,
reflected, and detected.reflected, and detected.
RADAR unimpaired by night, fog,RADAR unimpaired by night, fog,
clouds, smokeclouds, smoke
RADAR is good for isolated targetsRADAR is good for isolated targets
against a relatively featurelessagainst a relatively featureless
The components of a radar system.The components of a radar system.
4. Display unit
5. Power supply
6. Duplexer( improved radar).
Continuous Wave RadarContinuous Wave Radar
Employs continualEmploys continual
RADAR transmissionRADAR transmission
Separate transmit andSeparate transmit and
receive antennasreceive antennas
Relies on theRelies on the
“DOPPLER SHIFT”“DOPPLER SHIFT”
Continuous Wave RadarContinuous Wave Radar
Continuous wave (CW) radarsContinuous wave (CW) radars
typically determine target velocity,typically determine target velocity,
and can achieve considerable rangesand can achieve considerable ranges
without the high peak power. Thesewithout the high peak power. These
radars are typically simpler, moreradars are typically simpler, more
compact and less costly.compact and less costly.
Doppler EffectDoppler Effect
that the Doppler effect is the change inthat the Doppler effect is the change in
frequency that occurs when a sourcefrequency that occurs when a source
and a target are in relative motion.and a target are in relative motion.
The Doppler affect can be used in aThe Doppler affect can be used in a
CW radar in order to determineCW radar in order to determine
Doppler effect theoryDoppler effect theory
FFd =d = doppler shiftdoppler shift
VVrr = relative velocity of target with= relative velocity of target with
respect to radar.respect to radar.
Doppler Frequency ShiftsDoppler Frequency Shifts
Echo Frequency Decreases
Echo Frequency Increases
FM-CW radarFM-CW radar
An unmodulated CW radar isAn unmodulated CW radar is
incapable of detecting range, as thereincapable of detecting range, as there
is no reference point in the transmittedis no reference point in the transmitted
or returned signal for measuringor returned signal for measuring
elapsed time.elapsed time.
By frequency modulating the CW signal,By frequency modulating the CW signal,
differences between the transmitted anddifferences between the transmitted and
received frequencies can be used to estimatereceived frequencies can be used to estimate
Pulse Vs. Continuous WavePulse Vs. Continuous Wave
Pulse EchoPulse Echo
Single AntennaSingle Antenna
Gives Range,Gives Range,
usually Alt. as wellusually Alt. as well
Susceptible ToSusceptible To
Physical RangePhysical Range
Determined By PWDetermined By PW
and PRF.and PRF.
Continuous WaveContinuous Wave
Requires 2 AntennaeRequires 2 Antennae
Range or Alt. InfoRange or Alt. Info
High SNRHigh SNR
More Difficult to JamMore Difficult to Jam
But Easily DeceivedBut Easily Deceived
Amp can be tuned toAmp can be tuned to
look for expectedlook for expected
Specific Types of RadarSpecific Types of Radar
Frequency Modulated CW RadarFrequency Modulated CW Radar
Use for radar altimeters and missile guidance.
Pulse DopplerPulse Doppler
Carrier wave frequency within pulse is compared with a
reference signal to detect moving targets.
Moving Target Indicator (MTI) SystemMoving Target Indicator (MTI) System
Signals compared with previous return to enhance moving
targets. (search radars)
Frequency Agile SystemsFrequency Agile Systems
Difficult to jam.