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137 Cards in this Set
- Front
- Back
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What does stealth Technology use to defeat enemy radars (3)?
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1. Platform shape
2. Coating of platform to create destructive interference 3. Radar absorbent materials |
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What is important concerning the use of chaff?
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1. The length of filament should be half of the wavelength of the victim radar
2. Chaff is prone to rejection by velocity discrimination 3. Chaff works best if the launching platform maneuvers during use. |
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What type of jamming concentrates all the power output of the jammer in a very narrow bandwidth?
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Spot Jamming
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True or False: Self screening or escort jamming has the advantage over stand-off jamming because the escort jammer is closer to the threat radar, and is able to maintain the optimum bearing to the victim radar target.
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TRUE
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What is the burn-through range?
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Burn-through range is the range at which the strength of the radar echo becomes equal to or greater than the Electronic Attack (EA) signal.
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What are 4 important characteristics of ES receivers?
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1. the ability to scan a wide spectrum of frequencies
2. the ability to receive and process both weak and strong signals 3. the ability to distinguish between two radars signals operating at close to the same frequency 4. the ability to measure bearing to a radar to calculate its position |
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What is ES?
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Electronic Support is passive surveillance of the EM spectrum being used by the enemy.
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What is EP?
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Electronic Protection is the preserving of the EM spectrum for use by friendly forces.
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What is EC?
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Electronic Combat is any military action involving the use of the EM spectrum.
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What is the principal characteristic of radiated energy measured in Hz?
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frequency
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What is the distance between two corresponding parts of a wave?
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wavelength
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the angle of reflection=the angle of ______
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the angle of incidence
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When a wave passes through a small opening it is said to be what?
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diffraction
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What are 3 types of attenuation of electromagnetic energy?
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1. Speading
2. Scattering 3. Absorption This is true for electromagnetic waves (RADAR), waves in optical spectrum (visual, IR, UV), and sound waves (SONAR). |
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In what mode of wave propagation is a lower frequency wave front connected to the surface of the earth at one end and the ionosphere at the other?
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Ground waves
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Tropospheric ducting is caused by what?
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temperature inversion
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What wave propagation properly allows longer wavelengths to "bend" over the horizon?
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Refraction
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Maxwell's Theory indicates that a changing electric current creates an electric field that will generate a corresponding:
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time-varying magnetic field
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What is the simplest type of electromagnetic antenna?
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dipole antenna
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The polarization of an electromagnetic wave is define as:
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orientation of the electric field
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What type of polarization is used in satellite communications antennae?
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Circular
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The 2 main ways of concentrating RADAR energy into a beam are through the use of:
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1. Reflectors
2. Arrays |
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The process of creating and controlling directionality in antennae is called what?
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Beamforming
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How is the width of a beam measured?
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It is the angle between the half power points
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In a Broadside Array antennas are excited in phase and constructive interference occurs on what axis?
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the z-axis
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A linear antenna array consisting of several half-wave dipole antennas, spaced 1/2 wavelength apart and excited by the same signal 180 deg out of phase, would form what type of beam pattern?
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Endfire
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What type of element serves as a reflector and will modify the radiation pattern of the radar in a specific direction?
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A parasitic element
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What type of wave propagation occurs when an electromagnetic wave encounters a change in its medium or a totally diff medium and some or all of the wave may propagate into the new medium and some may be reflected from it?
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Reflection
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What type of wave propagation occurs when waves bend around obstacles and when a plane wave passes thru an opening (aperture) and spreads out from the opening in a fan shape?
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diffraction
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What type of wave propagation occurs when two or more waves are received at a single point?
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Interference
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What type of interference occurs with a phase shift 120-240 deg?
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Destructive interference
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What type of interference occurs with a phase shift 240-120 deg?
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Constructive interference
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What is spreading?
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EM signal loss due to energy distributed over an increasingly larger area.
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What is absorption?
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EM signal loss due to energy dissipated into a medium.
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What is scattering?
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EM signal loss due to energy bouncing off suspended particles within a medium.
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As the frequency of EM signal increases, effects due to what types of loss increase?
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effects of scattering and absorption
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The higher in the atmosphere EM signals travel, the lower the _____?
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attenuation
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What are ground waves and what is their frequency range?
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Vertically polarized ground waves travel between the earth and ionosphere by using the earth as a conductive surface.
Optimize communications at low frequencies. ELF, VLF, LF |
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What are sky waves and what is their frequency range?
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Sky waves propagate from the earth to the ionosphere and frequencies below the "critical" frequency are completely refracted from the ionosphere back to the earth. Their range of transmission depends upon the height of the ionosphere (fn of time of day/night).
LF, MF, HF, VHF |
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What are space waves and what is their frequency range?
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Space waves propagate from the earth, through the ionosphere, and into space due to higher frequency that do not refract off ionosphere.
Space waves are essentially LOS waves since their propagation tends to "straighten out." Have reduced ranges due to ionospheric scatter (scatter in all directions by E-region of ionosphere) and tropospheric scatter (scatter by the air). VHF, UHF, SHF, EHF |
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What does the transmitter of a pulsed radar do?
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generates high-power pulses under control of the timer
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What does the duplexer of a pulsed radar do?
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directs transmit/return signal to appropriate path allowing one antenna to handle both tasks
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What does the antenna of a pulsed radar do
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radiates and receives signal
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What does the receiver of a pulsed radar do?
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converts incoming electromagnetic energy into an electrical signal
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What does the synchronizer of a pulsed radar do?
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provides timing source and coordination for radar
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What does the display of a pulsed radar do?
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produces a visual presentation of return signal
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Define duty cycle
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the duty cycle is the ratio comparing the time that the transmitter is on, to one complete cycle of operation
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What is pulse width?
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PW is the active transmit time or the duration of the EM pulse.
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What is the pulse repletion frequency?
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PRF is the number of times a pulse is transmitted per second.
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What are 2 advantages of pulsed radar?
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1. Enables you to see farther through conditions that impair visual detection such as nighttime, fog, and clouds
2. gives an accurate measurement of range and can indicate something about target motion |
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What are disadvantages of pulsed radar?
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1. Poor target resolution
2. Poor target identification when compared to abilities of eye. |
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What is noise?
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Noise is unwanted voltage inputs to the receiver
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What is the threshold level?
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the threshold level is the level a signal must exceed to be displayed on an indicator.
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What is Smin?
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Smin is the minimum signal required for detection.
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What is the Scan Rate?
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the scan rate is how fast the antenna is rotated
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What is the radar cross section?
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the radar cross section is the effective area that reflects radar energy
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What determines Rres and Rmin?
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the pulse shape is what determines range resolution and min range
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What is the antenna aperture?
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The antenna aperture is the radar's collector of energy
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What is beam width?
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Beam width is the narrowness of antenna beam.
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What is antenna gain?
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Antenna gain is the antenna's ability to concentrate energy
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What determines max Runamb?
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PRF determines max Runamb.
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What are 4 major factors that determine the radar cross section?
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1. target size
2. shape 3. skin material 4. aspect angle |
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The weaker the signal that the receiver is capable of detecting (Smin), the ______ the effective range of the radar set.
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The weaker the signal that the receiver is capable of detecting (Smin), the greater the effective range of the radar set.
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Power Gain (G) is always _______ than Directive Gain (Gdir).
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Power Gain (G) is always smaller than Directive Gain (Gdir).
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When compared to long-range search radars, tracking or fire-control radars would have __________ beam width, __________ frequencies and __________ wavelengths.
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When compared to long-range search radars, tracking or fire-control radars would have narrow beam width, higher frequencies and shorter wavelengths.
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Evaluating the associated formulas in the RADAR section of your Appendix, altering the Pulse Repetition Frequency (PRF) directly affects:
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Evaluating the associated formulas in the RADAR section of your Appendix, altering the Pulse Repetition Frequency (PRF) directly affects max Runamb
Runamb = c / (2*PRF) |
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What RADAR parameter directly affects minimum detection range?
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Pulse Width directly affects minimum detection range.
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Explain how a wave propagates from a center-fed, dipole antenna.
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The change E field creates a time varying B field that is perpendicular to the E field. The changing B field then creates an E field in a constant energy transfer.
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Is RADAR LOS horizon longer or shorter than the geometric LOS horizon and why?
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RADAR LOS is longer than geometric (visual) LOS. EM wave propagation is subject to refraction due to changes in the earth's atmosphere. Lower frequency radio waves bend around the curvature of the earth. High frequency visual wavelengths do not refract as much.
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The maximum dist that any RADAR can effectively "see" a target is limited by one of the following 3 radar range equations:
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Runamb
Rlos Simplified Rmax equation |
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A target will be detected by the radar when the power returned to the receiver is _________ than or equal to the minimum signal for detection (Smin).
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A target will be detected by the radar when the power returned to the receiver is greater than or equal to the minimum signal for detection (Smin).
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Why, in the simplified max radar equation, is the range the 4th root dependent?
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Range is the 4th root dependent because of losses due to spherical spreading leaving AND returning.
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What is effective antenna aperture (Ae)?
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area of the antenna multipled by the antenna efficiency
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What frequency should you choose to improve the angular resolution (with a smaller beam width) of a radar with a rectangular antenna of fixed dimensions?
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the highest frequency possible
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Doubling the area of a radar antenna will increase the range by about _____.
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Doubling the area of a radar antenna will increase the range by about 40%.
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What are the main determinants to radar range resolution?
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The PW and PCR are the main determinants to radar range resolution.
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What are the main determinants to radar angular resolution?
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BW, wavelength, L, and k
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Why does higher frequency provide better angular resolution?
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Higher frequency results in lower wavelength and consequently lower bandwidth and smaller Rcross --> better angular resolution
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How does increasing the radio frequency affect the power density inside of the main lobe of an antenna?
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As frequency increases, wavelength decreases and consequently power gain increases--> bigger power density
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What type of RADAR signal attenuation is accounted for in the simplified radar equation?
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Only losses due to spherical spreading are accounted for in the simplified radar equation.
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How often does a Continuous wave (CW) radar send out pulses of energy?
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CW radars only send out one long pulse of energy continuously.
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Range can be determined for CW radars thru the use of _________.
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Range can be determined for CW radars thru the use of frequency modulation.
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Relative bearing =
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Relative bearing = True bearing - Heading
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Velocity (line-of-sight) is the_______________.
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Velocity (line-of-sight) is the rate of change in range along the line from the target to the receiver.
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How many antennas does a CW radar have?
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A CW radar has 2 antennas.
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The DC of a CW radar =
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The DC of a CW radar = 1
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What is target angle?
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Target angle is the angle relative bearing from the target's heading to you
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What is a disadvantage of CW radar?
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A disadvantage of a CW radar is that is can't detect targets that have no radial relative motion.
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The main difference between a pulse doppler radar and a basic pulse radar is that the pulse doppler radar has a component that measures _______________.
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The main difference between a pulse doppler radar and a basic pulse radar is that the pulse doppler radar has a component that measures change in frequency.
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Pulse Doppler Radars use a __________ to selectively identify different velocity characteristics of targets.
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Pulse Doppler Radars use a bandpass filter to selectively identify different velocity characteristics of targets.
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What are two advantages pulse doppler radars have over CW radars?
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Pulse doppler radars can measure range as well as velocity.
Pulse doppler systems require only one antenna. |
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CW Radar exploits what property?
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change in frequency
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MIT pulse radar exploits what property?
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change in phase
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Basic pulse radar exploits what property?
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change in time
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_____ speed is a limitation of MTI and pulse Doppler radar systems.
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Blind speed is a limitation of MTI and pulse Doppler radar systems.
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Most Doppler systems avoid the problem of blind speeds by __________.
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Most Doppler systems aviod the problem of blind speeds by using a higher PRF.
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_____ is a signal processing technique in which the receiver is able to separate two or more targets within the same returning pulse on the basis of the frequency.
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Pulse compression is a signal processing technique in which the receiver is able to separate two or more targets within the same returning pulse on the basis of the frequency.
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Pulse compression improves what?
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Pulse compression improves Rres.
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Why is the basic CW radar unable to determine range?
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The basic CW radar is unable to determine range since there is no basis for measurement of a time delay between pulses.
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What is the min detection range of a CW radar equal to and why?
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The min detection range of a CW radar is zero since the receiver is never blanked.
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Which type of synthetic aperture radar system is often space-based ?
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SAR radar systems are often space-based.
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An Inverse Synthetic Aperture Radar (ISAR) detects the frequency shift (Doppler Shift) created by:
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An ISAR detects the frequency shift (Doppler Shift) created by the target's yaw, pitch, or roll.
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A SAR achieves a large effective aperture by:
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A SAR achieves a large effective aperture by motion of the transmitter/receiver.
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True or False: The cross range resolution of an ISAR is affected by the range.
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False. The cross range resolution is independent of range.
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In order to create an image with a radar, the radar must have a very _______ Rcross
created by a _________ beam width. In SAR, this beamwidth is created by a _______ synthetic antenna length. |
In order to create an image with a radar, the radar must have a very small Rcross
created by a narrow beam width. In SAR, this beamwidth is created by a long synthetic antenna length. |
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Lower frequency radars known as over the horizon radar :
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Lower frequency radars known as over the horizon radar travel relatively long distances.
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What are the 4 advantages of electronic scanning?
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1. High Data rates
2. Instantaneous beam positioning 3. No mechanical failures associated with rotating radar antenna systems 4. increased flexibility (multi-mode operation, transmission of guidance and control orders, simultaneous weapons control and search fns, etc) |
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If looking at a phased array from behind the radar panel, which corner would the reference element (0,0) be located?
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(0,0) on a phased array is located at the upper right-hand corner if looking at panel from behind.
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What is the phase shift applied to the reference point of a phased array radar?
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There is no phase shift applied to (0,0) of a phased array radar.
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What is one of the simpler, most practical and economical types of beam steering?
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Frequency scanning
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Antenna arrays may use one of these 3 methods to steer the beam:
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1. time delays
2. changing frequency 3. changing the phase of each element. |
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In phase scanning, the phase is changed by feeding the signal from the radar transmitter through:
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phase-shifting networks
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Which elements are in phase when a phased array radar radiates along the antenna boresight?
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All elements are in phase when a phased array radar radiates along the antenna boresight.
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What is the purpose of a tracking system?
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The purpose of a tracking system is to determine the location of a target with sufficient accuracy for weapons delivery.
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What are the two categories of tracking systems?
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1. Servo-trackers
2. Track-while-scan |
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Servo-controlled tracking systems require an error signal as feedback to adjust the radar position. Because of this, servo trackers will always:
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Because Servo-controlled tracking systems require an error signal as feedback to adjust the radar position, they will always lag the target.
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What is the order of gates used by a TWS radar during a normal tracking sequence?
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The order of gates used by TWS radar during a normal tracking sequence is:
1. acquisition gate 2. tracking gate 3. turn gate |
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Name the TWS gates in order from largest to smallest:
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Largest to smallest:
1. acquisition gate 2. turn gate 3. tracking gate |
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How does a TWS system respond when a target fails to appear in an established tracking gate ?
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If a target fails to appear in an established tracking gate, a turn gate is established around the last known position of the target.
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Mono pulse radars use:
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Mono pulse radars use a single pulse (with multiple beams) to determine target position.
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What is NUATING as it applies to a weapon system?
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to NUATE is to rotate the beam in a small circle around a focal point
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How does a servo tracking mechanism drive the antenna to a new position?
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A servo tracking mechanism uses the error between the antenna position and the target postition to drive the antenna to a new position.
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What are principles of TWS radar (4)?
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TWS:
1. Computer predicts future target position based on history 2. can track multiple targets at once (unlike a servo-tracker) 3. uses a search radar 4. supplies fire-control quality data |
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What are the unique components of a CW radar and what are the purposes of the components?
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Components of CW radar:
1. mixer- measures change in frequency 2. discriminator- eliminates targets when there is no change in frequency 3. second antenna- receives incoming signal |
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How do you know if you are opening or closing the target when using a pulsed-doppler radar?
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If the doppler shift (change in frequency) is greater than or equal to 0, then you are closing the target.
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What does a CW radar measure than does not allow it to also measure range?
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A CW radar measures doppler shift from velocity LOS and consequently can not measure range.
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How does a FMCW radar derive range info?
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The FMCW radar derive radar info from transmitted frequency at the time the reflected signal is received.
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What types of signals do MTI radars reject?
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MTI radars reject signals from fixed or slow-moving targets such as buildings, hills, trees, sea and rain and retain for detection signals from moving targets such as aircraft.
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Is an MTI function used with a basic pulse radar or CW radar?
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A pulse radar uses MTI.
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What info about the target does the MTI measure?
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MTI measures the following about a target: range (change in time), and movement (phase shift).
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How does the info measured by an MTI used to identify a moving target?
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A changing phase shift indicates a moving target. The same phase shift from return to return indicates a stationary target.
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What must a target do in order for an ISAR to provide improved target resolution?
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ISAR needs a target moving periodically in roll, pitch, and yaw.
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What can an ISAR operator do to improve the ISAR return image?
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An ISAR operator can increase integration time (T) to improve ISAR Rcross. (Look at target longer)
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What is another name for a phased-array RADAR?
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Electronically-scanned array
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What is another name for an electronically-scanned array?
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Phased array RADAR
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What elemental principle of how waves interact allows for phased arrays to steer a beam of electromagnetic energy?
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The principle of constructive/destructive interference allows for phased arrays to steer a beam of EM energy.
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If a beam must be pointed up, wave energy from the lower antenna elements must travel a greater ______ than the energy radiating from the upper elements to arrive in phase at the same point in space. The phasers of the lower elements must therefore _______ the phase of their outgoing signal in order to create ________ interference and an upward-pointing beam.
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If a beam must be pointed up, wave energy from the lower antenna elements must travel a greater DISTANCE than the energy radiating from the upper elements to arrive in phase at the same point in space. The phasers of the lower elements must therefore ADVANCE the phase of their outgoing signal in order to create CONSTRUCTIVE interference and an upward-pointing beam.
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