Wavelength
physical distance of one complete wave
Frequency
number of cycles that the RF energy completes per second, measured in HZ, inversely proportional to wavelength
Amplitude
the electromagnetic signal strength, sometimes expressed in dB or watts
Phase
the degree to which individual cycles of a wave coincide with those of a reference wave of the same frequency, usually expressed in degrees with 360 being a complete cycle.
Polarization
determined by the radar antenna and refers to the orientation of the RF wave as it travels through space. Two types Linear (horizontal, vertical, slant) and circular (RH, LH)
Beamwidth (BW)
an angular size of the mainbeam, normally expressed in degrees
Pulse width (PW)
is the time radar is transmitting each pulse
Pulse Length (PL)
is the distance between the leading and trailing edges of a pulse
Pulse repetition frequency (PRF)
rate at which pulses are transmitted, measured in pulses per second
Power
as signal travels down range, the signal spreads out, dramatically reducing the power of the signal. Power loss is equal to the inverse of the range squared.
Reflection
process of reradiating an incident radio wave
Refraction
the bending of Em energy
Diffraction
occurs when a wave spreads around an object whose size is comparable to a wavelength and bends around the edges of larger obstructions and is caused by energy at each point in a wave being passed on as if a radiator existed at that point.
Ducting
a form of refraction caused by slight differences of c in two different media.
Superrefraction (ducting)
increases a radar detection range by bending the radar beam downward, increasing the radar horizon and overcoming the masking caused by the Earth's curvature.
Subrefraction (ducting)
decreases radar LOS by bending the radar beam upward, decreasing its range.
Characteristics of Lower Frequency Radar
Large antenna, 100MHz to 4 GHz, suffer from poor angular resolution and will experience range inaccuracies due to man-made and natural background electrical noise
Characteristics of Medium Frequency Radar
4 GHz to 9 GHz, have better precision than lower-freq radars, some atmospheric attenuation
Characteristics of Higher Frequency Radar
9 GHz to 10 GHz, shorter wavelengths and smaller antennas, less power and shorter detection range, high precision, atmospheric attenuation more of a problem, FC radars for fighters, SAM and ADA
Impact of the antenna on the radiation pattern
Radiation pattern is affected by antenna gain (ability to concentrate energy in the desired direction), a large antenna and higher operating frequency increases the systems' power gain. More main lobe is best.
Impact of the antenna on the Beamwidth (BW)
smaller antennas have wider BW, larger antennas have narrower BW, lower frequency have wider BW, higher frequency have narrower BW
Impact of the antenna on Angular Resolution
beamwidth affects the angular resolution (the ability to distinguish between two targets that are close to each other in both AZ and EL at the same range from the radar)
Characteristics of a mechanically scanned planar array antenna
flat-faced antenna mechanically scanned in AZ and EL. Replaces the parabolic reflector with a flat plate that is a series of small and precisely positioned EM energy emitters. Provide relatively high-aperture efficiency (Gain) and low back radiation (side
Principle disadvantage of the mechanically scanned planar array antenna is
the higher cost, also difficult to engineer for a circular polarization.
ESA radar characteristics
mounted in a fixed position, beam is steered by individually controlling the phase of the radio waves transmitted and received by each radiating element.
Passive ESA
operates in conjunction with the same type of central transmitter and receiver as an MSA, beam is steered by an electronically controlled phase shifter placed immediately behind each radiating element.
Active ESA
uses a small T/R module is placed behind each radiating element instead of a phase shifter.
ESA Limitations
FOR and electronically complex
Polarization
the orientation of the electric field
Frequency
the number of cycles that the RF energy completes per second
Electromagnetic radiation
sync'd oscillations or electric and magnetic fields that propagate through a vacuum at the speed of light
Radar Beamwidth
the angular size of the mainbeam, normally expressed in degrees
Phase differences
the degree to which individual cycles of a wave coincide with those of a reference wave of the same frequency
Order the following in order of decreasing wavelength: Extremely low frequency waves, IR radiation, microwaves, radio, UV radiation, Visible light, x-rays/gamma rays
Extremely low-frequency waves, radio, microwaves, IR radiation, visible light, UV radiation, x-rays/gamma
A radar's operating frequency impacts what performance parameters of the radars performance?
All of the Above: Atmospheric attenuation, physical size, transmitted power, Doppler considerations
The four characteristics of the lower-frequency radars include:
Long wavelengths, transmits high power, low atmospheric attenuation, best for long-range EW radars
The three characteristics of the medium-frequency radars include:
Shorter wavelengths, less detection range, some atmospheric attenuation.
The five characteristics of the higher-frequency radars include:
small antennas, shorter wavelengths, some atmospheric attenuation, high precision, used for FC radars for fighters, SAMS and ADA
The strength of a radar signal hitting a target is approximately proportional to what parameter?
1/(range to the target)2 (squared)
For a fixed antenna size or radar operating at a higher frequency than another will the beamwidth be narrower or wider?
Narrower
For a fixed operating frequency which radar will have a narrower beamwidth: large antenna or a small antenna?
Large antenna
Angular (azimuth and elevation) resolution is a function of beamwidth and range to the targets?
TRUE
An ESA antenna differs in what two fundamental ways from a mechanically scanned array radar?
The antenna is mounted in a fixed position, the radar beam is steered by individually controlling the phase of the radio waves transmitted and received by each radiating element.
Select the advantage(s) common to passive and active electronically scanned array radar
Extreme beam agility, facilitate radar cross-section reduction, high reliability
List the advantages of CW radar
Continuously transmits, high AOP, greater detection range
List the disadvantages of CW radar
Separate transmitter and receiver, unable to determine range
Range Resolution
radar's ability to resolve multiple targets in range
Resolution Cell
the smallest amount of 3D airspace in which a radar cannot distinguish between multiple targets
Average ouput power (AOP)
is peak power averaged over the PRI
Pulse Ranging
radar transmits an RF signal, RF reflects off a target, RF echoes are received. Time between transmit and receive is converted to range using
Range = (Measured Time (T) x c)/ 2
Range Rate
computed on the basis of change in the measured range with time, not the best method to determine target speed.
List the advantages of pulsed radar
Only one antenna is required
Good Ground mapping and weather detection
Good range accuracy
Simple electronics
List the disadvantages of pulsed radar
Lower AOP limits detection range (LPRF)
Cannon filter out ground clutter without increasing processing
Not very accurate in velocity measurements
List the advantages of a CW/Doppler radar
Can filter out ground clutter
High AOP increases detection range
Very accurate velocity measurement
Good against high aspect angle targets
Simple
List the disadvantages of a CW/Doppler radar
Two antennas required: more weight and space
No range information
Susceptible to Doppler notch
Degraded capabilities against beam and stern targets
How Does CW/Doppler cancel out ground clutter?
CW/Doppler cancels ground clutter by using Doppler processing (the highest amplitude radar return (the ground) within the main beam of the radar with a Doppler shift matching the aircraft's groundspeed can be filtered out.
How does a CW/Doppler radar measure target velocity?
A CW Doppler radar transmits a continuous signal at the radar's operating frequency. The signal is reflected by a moving target and travels back to the receiving antenna. The frequency of the reflected signal is the frequency change due to the Doppler Eff
How does Mainbeam Clutter of a CW/Doppler Radar impact target detection?
Mainbeam clutter (Doppler notch) is great for ground mapping, bad for searching for aircraft.
CW/Doppler cancels ground clutter by using Doppler processing (the highest amplitude radar return (the ground) within the main beam of the radar with a Doppler s
How does the Sidelobe Clutter of a CW/Doppler Radar impact target detection?
Sidelobe clutter is only a problem below 5k feet AGL ( less Doppler shift due to angular difference between velocity of the radar and LOS to the ground). The amplitude is much less than Mainbeam clutter.
List the characteristics of PD radars:
Combines the capes of PR and Doppler radars
accurate range measurement
one radar (simpler)
accurate measurement of target velocity
all aspect target detection and tracking
ground clutter rejection.
What is the impact of MPRF on a PD radar?
good all aspect target detection, decreased detection range compared to HPRF (lower AOP)
What is the impact of HPRF on a PD radar?
good long range detection for high-aspect targets, difficult to determine range to target
List the advantages of PD Radars
Only one antenna required
Filters out ground clutter
High output power
Good all aspect capability (MPRF)
List the disadvantages of PD Radars
Complicated electronics
Susceptible to Doppler notch
Range measurement difficult with an HPRF PD radar
List the common radar acquisition techniques:
circular scanning, sector scanning, spiral scanning, raster scanning
List the common radar tracking techniques
Monopulse and conical scan
Circular Scanning
most common mechanical scanning, antenna continuously rotates 360 deg around a vertical axis in either CW or CCW direction, long scan times are associated.
Sector Scanning
two types, bidirectional scans back and forth through a desired sector, a unidirectional sector scans in only one direction.
Spiral Scanning
a special case of conical scan
Raster Scanning
covers a rectangular shaped sector by scanning back and forth while changing level (EL) after each sweep.
Monopulse Tracking
amplitude output is divided into two channels so that the differences can be measured across the antenna can be measured
Conical Scan Tracking
radar beam is nutated around the target
Range resolution is based on what parameter?
Pulse width (PW)
Resolution cell
the smallest amount of 3d air space in which a radar cannot distinguish between multiple targets.