Radar Fundamentals Flashcards
Define wavelength
Physical distance of one complete wave
Define frequency
Number of cycles that the RF energy completes per second
Lower freq radar characteristics
Long wavelength Large antenna Components generate high transmit power Low atmospheric attenuation Good long range detection capability Best long range for EW Radars
Med freq radar characteristics
Smaller wavelength allow smaller antenna
Components cannot handle much power—less detection range
Some atmospheric attenuation
Used by ASR, HF,GCI,SAM and ADA Radars acquisition radars
High freq radars characteristics
Shorter wavelength and smaller antennas
Smaller components allow less power—shorter detection range
High precision
Atmospheric attenuation more of a problem
Fire control radars- SAM and ADA
Define amplitude
The electromagnetic signal strength
Define Phase
A compete 360 degree cycle of an EM wave
Define polarization
Orientation of the electrical field in an EM wave
Define Beam Width
An angular size of the mainbeam, normally expressed in degrees
Technically the width in degrees between the points where the mainbeam decreases to half power
Define Pulse Width (Obj 9)
The time radar is transmitting each pulse (Tao)
Define Pulse Length (Obj 9)
The distance between the leading and trailing edge of a Pulse
Define PRF (Obj 9)
The rate at which pulses are transmitted
Discuss polarization
Linear: horizontal, vertical and slant
Circular: right hand, left hand waves whose polarization rotates thru 360 degrees in every wavelength
Discuss power
Strength of a radar signal:
Signal hitting a tgt: 1/r^2
Signal received by a radar: 1/r^4
Discuss reflection
The process of reradiating an incident radio wave
Discuss refraction
The bending of an Em wave
Discuss diffraction
Phenomenon observed when a radio wave spreads around objects whose size is comparable to its wavelength and bends around the edges of larger objects; it increases with wavelength
Discuss subrefraction ducting
Decreases radar LOS by bending the radar beam upward, decreasing its detection range
Discuss superrefraction ducting
Increase radar detection range by bending the beam downward, increasing the radar horizon and overcoming the masking caused by the earth’s curvature
Discuss the impact of the antenna on the radar pattern, BW, and angular resolution
Antenna determines beam shape and size
Larger antenna = narrower BW
Narrower BW = higher angular resolution in AZ/EL
Characteristics of parabolic antennas
Large sidelobes
Easiest and cheapest to manufacture
Transmitter/receiver at focus of parabola
Discuss the characteristics of mechanically scanned planar array antennas (Obj 6)
Flat faced antenna mechanically scanned in AZ/EL
Consists of an array of many Indiv 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
Discuss ESA characteristics. (Obj 7)
Mounted in a fixed position
Beam is steered by individually controlling the Phase of the radio waves transmitted and received by each radiating segment
Discuss passive ESA (Obj 7)
Operates in conjunction With the same type of central transmitter as an MSA
Beam is steered by an electronically controlled Phase shifter placed behind each radiating element
Discuss active ESA (Obj 7)
Instead of a Phase shifter, a small T/R module is placed behind each radiating element
Discuss ESA limitations (Obj 7)
Field of Regard (FOR)
- Apparent size of the antenna decreases when
viewed from angles off-bore sight
Electronically complex
Advantages of Continuous Wave (CW) Radar (Obj 8)
High Average Output Power (AOP) with greater detection range
Disadvantages of CW Radar (Obj 8)
Separate transmitter and receiver
Unable to determine range
Discuss Average Output Power (Obj 9)
Peak power averaged over the PRI
Discuss pulse ranging (Obj 9)
Range accuracy is good
Radar transmits an RF signal, RF reflects off a tgt, RF echoes are received
Discuss range resolution (Obj 9)
Is determined by tau
Pulse length/2
Discuss range rate (Obj 9)
Computed on the basis of change in the measured range with time
*Not the best method to determine tgt speed
Discuss resolution cell (Obj 9)
The smallest amount of 3D airspace in which radar cannot distinguish between multiple tgts
- AZ/EL resolution is based on BW and range to the tgts
- range resolution is based on tau
Advantages of pulsed radar (Obj 10)
Only one antenna required
Good range accuracy
Simple electronics
Good Ground mapping and weather detection
Disadvantages of pulses radar (Obj 10)
Lower AOP limits range detection
Cannot filter out ground clutter without increased processing
Not very accurate in velocity measurements
How does CW/Doppler measure tgt velocity (Objective 11)
Measures the shift in freq of an EM wave radiated reflected or received by an object in motion
How CW/Doppler cancels ground clutter (Objective 11)
Filtered out using Doppler processing
Discuss the impact of mainbeam and sidelobe clutter on tgt detection (Objective 11)
Mainbeam clutter is good for ground mapping, bad when searching for aircraft
- normally a factor only during lookdown
- Sidelobes are lower in amplitude due to power output
- Sidelobes have less Doppler shift due to angular difference between velocity of the radar and LOS to the ground
Characteristics of PD radars (Objective 13)
Pulsed radar allows for one antenna; simpler, lighter system
Accurate range measurement (*time Domain)
Doppler allows for accurate measurement of tgt velocity, all aspect detection and tracking and ground clutter rejection (*frequency Domain)
Combine the capabilities of pulsed and Doppler radars
*Advantages of CW/Doppler radar (Objective 12)
Can filter out ground clutter
High AOP increase detection range
Very accurate velocity measurement
Simple
Good against high aspect angle tgts
*Disadvantages of CW/Doppler radar (Objective 12)
Susceptible to Doppler notch
Two antenna required; more weight and space
No range information
Degraded capabilities against beam and stern targets
Impact operating PRF has on MPRF and HPRF PD radars (Objective 13)
MPRF
- good all aspect detection
- decreased detection range compared to HPRF (lower AOP)
HPRF
- good long range (higher AOP) detection for high-aspect tgts
- difficult to determine range to tgt
- Time to listen for return target echoes limited
- PD allows look-down, shoot-down capability
- Complicated electronics
*Advantages of PD radars (Objective 14)
Only one antenna required
Filters out ground clutter
High output power
Good all aspect capability (MPRF)
*Disadvantage of PD radars (Objective 14)
Complicated electronics
Susceptible of Doppler notch
Range measurement difficult with an HPRF radar
Discuss radar ground mapping considerations (Objective 15)
Objective is to make radar maps of sufficiently fine resolution so that topographical features and objects on the ground can be recognized
Resolution distance (dr) = range resolution; function of tao.
Real beam antenna: Based on BW and range to target
To improve (dr), PW must be decreased
- Decreasing PW decreased AOP - negatively impacts radar detection range
- How much PW can be decreased is a trade-off in desired detection range
da is limited to the physical size of the antenna
- larger antenna improves da - physical limit to aircraft's radar size
Discuss radar acquisition techniques (Objective 16)
Circular
Sector
Spiral
Raster
*Discuss SAR considerations (Objective 15)
- Side looking radar system which utilizes the forward motion of the platform to simulate an extremely large antenna or aperture electronically
- Requires time and compute power.
- Radar must be pointed to the side.
- The beam of the antenna must be wide enough for the area/target to fall within the beam for every position of the antenna in the entire length of an array
da min = 1/2 length of real antenna (for a radar position at a fixed angle relative to the flight path
Discuss tracking techniques (Objective 16)
Monopulse - Angular traking information derived from one reflected pulse
Conical
Angular resolution
Based off BW and range to the targets
