410A Radar Flashcards
Define wavelength
Physical distance of one complete wave
Define frequency
Number of cycles that the RF energy completes per second.
Hz
f=c/wavelength
Define phase
A 360 degree cycle of an Em wave.
The degree to which individual cycles of a wave coincide with those of a reference wave of the same frequency
Define polarization
Orientation of the electrical field in an Em wave.
Vertical, Horizontal, Spiral
Define BW, Beamwidth
BW is angular measurement of the size of the mainbeam, normally expressed in degrees.
Define PW, Pulse Width
PW is the time radar is transmitting each pulse.
Units Time
Define PL, Pulse Length
PL is the distance between the leading and trailing edges of a pulse.
Units Distance
Define PRF, Pulse Repetition Frequency
Rate at which pulses are transmitted.
Measured in pulses per second.
Lower Frequency Radar Characteristics
Long Wavelength; large antenna; components able to generate high transmit power; low atmospheric attenuation; good long range detection capability, but poor precision; best long range for EW radars
Medium Frequency Radar Characteristics
Smaller wavelengths allow smaller antennas; components cannot handle as much power, therefore less detection range; some atmospheric attenuation; used by ASR, HF, GCI, SAM, and ADA acquisition radars.
Higher Frequency Radar Characteristics
Higher frequencies have shorter wavelengths and smaller antennas; smaller components allow less power with shorter detection range; high precision; atmospheric attenuation more of a problem; fire control radars for fighters, SAM and ADA.
Linear Polarization
Horizontal, vertical, slant
Circular Polarization
Right hand, left hand waves whose polarization rotates through 360 degrees in every wavelength
Radar Power
Strength of radar signal hitting a target = 1/R^2
Strength of radar signal sent/received by a radar = 1/R^4
Radar Reflection
The process of reradiating an incident radio wave.
Radar Refraction
The bending of Em energy
through different materials.
Radar Ducting
The bending of radar rays due to atmospheric conditions
Subrefraction decreases radar LOS by bending the radar beam upward.
Superrefraction increases radar LOS detection by bending the radar beam downward.
Radar Diffraction
Phenomenon observed when a radio wave spreads around objects whose size is comparable to its wavelength and bend around the edges of larger objects.
Diffraction increases with wavelength.
Characteristics of Parabolic Antenna
Large Sidelobes (disadvantage)
Easiest and cheapest to manufacture (advantage)
Transmitter in front of dish
Basic satellite dish
Characteristics of mechanically scanned planar array antennas
MSA
Flat-faced antenna mechanically scanned in AZ and EL
Consists of an array of many individual radiators of equal phase distributed over a flat surface
Slots cut in the walls of a complex of waveguides behind the antennas face
Designed to distribute the radiated power across the array so as to minimize sidelobes
ESA Characteristics
Mounted in fixed position
Beam is steered by individually controlling the phase of the radio waves transmitted and received by each radiating element
Passive vs Active ESA
Passive-beam is steered by an electronically controlled phase shifter placed behind radiating element, controlled by BSA or central processor
Active- Instead of a phase shifter, a small T/R module is placed behind radiating element
Limitations of ESA
FOR-Apparent size of antenna decreases with off boresight; results in increased beam width and decrease in efficiency; limits maximum look angle to +-60 degrees (BW doubles, power half)
Electronically complex
Advantages/Disadvantages of CW Radar
Continuous Wave
Continuously Transmits- separate transmitter vs receiver (disadvantage), High AOP (advantage)
Greater detection range (Hight AOP)- unable to determine range (disadvantage)
Range Resolution (Pulsed Radar)
Radar’s ability to resolve multiple targets in a range
Range resolution is a factor of Pulse Width (PW)
Resolution Cell (Pulsed Radar)
The smallest amount of 3D space in which a radar cannot distinguish between multiple targets
AZ/EL is based on BW and Range
Range Resolution based on PW
Average Output Power (AOP)
AOP is peak power averaged over the PRI (Pulse Repetition Interval)
Pulse Ranging
Radar transmits RF, RF reflects off target, RF echoes received
Time between transmit and receive is converted to range using this equation…Range=((T)xc)/(2)
Range Rate
Is computed on the basis of change in the measured range with time - not the best method to determine target speed
Advan/Disadvan of Pulsed Radar
A- only one antenna req, good ground mapping and weather detection, good range accuracy, simple electronics
D- Lower AOP limits detection range (low PRF), cannot filter out ground clutter without incresed processing, not very accurate in velocity measurments
Impact of antenna on Radiation Pattern
Antenna determines beam shape and size
Large Antenna = Large Sidelobes
Impact of Antenna on BW
Larger antenna = narrower BW
Impact of antenna on angular resolution
Larger antenna = narrower BW = higher angular resolution in AZ/EL
Define polarization
The orientation of the electric field
CW/Doppler Characteristics
Accurate measuring velocity
Susceptible to ground clutter
Good at ground mapping
Can be defeated by a Doppler notch
How CW/Doppler measure velocity
Measures the shift in frequency of an Em wave radiated, reflected or received by an object in motion.
How CW/Doppler cancels ground clutter
By filtering out using Doppler processing
Impact of mainbeam and Sidelobe clutter on tgt detection
Mainbeam clutter is the Doppler notch and normally a factor during look down.
Advan/Disadvan of CW/Doppler radar
A- can filter ground clutter, high AOP increases detection range, very accurate velocity measurement, good agains high aspect targets, simple
D- two antennas more weight/space, no range information, susceptible to Doppler notch, degraded capabilities against beam/stern targets
Pulsed Doppler radar
Combines advantages of pulsed and Doppler radars
MPRF vs HPRF in pulsed Doppler radars
M- good all aspect tgt detection, decreased detection range compared to H
H- good long range detection for high aspect targets, difficult to determine range to target
Advan/Disadvan of pulse Doppler radars
A- one antenna req, filters ground clutter, high AOP, good all aspect capability
D- complicated, susceptible to Doppler notch, range measurement is difficult
Radar ground mapping
Using radar waves to create a fine resolution of topographical features an objects on the ground.
Radar acq techniques
Circular scanning
Sector scan
Spiral scan
Raster scan
Radar tracking techniques
Lobing
Mono pulse
Conical
