Week 2 Flashcards
Constructive interference
Adding two waves with a shift of 0 degrees
2 times the amplitude
Destructive interference
Adding two waves with a shift of 180 degrees
An isotropic point source generates
Spherical waves
Validity of plane wave assumption
For a plane wave, intensity and phase are constant across planar surfaces that are oriented perpendicular to the direction of propagation
At great enough distances from the source, the spherical wavefronts will appear approximately planar across an aperture of fixed dimension D
Phase variation across D of less than 1/16 cycle (22.5 deg) is small enough to treat the waves as planar
Valid in the far field region which is R > 2D^2/λ m
Antenna
Transducer that converts bound waves in a waveguide to unbound waves in free space and vice versa
Characterized by gain and beamwidth
Directional antennas
Transmit and receive EM waves over a small angular region (most radars use these)
Antenna gain
The increase in power density at a given point in space when the test antenna is used in place of an isotropic antenna
Varies with angle
If quotes as just “gain”
The maximum (on axis, bore sight) value is inferred
Antenna pattern
The variation of antenna gain with angle
Have a main lobe and sidelobes
Usually presented as gain vs angle in a plane
Beamwidth
Angular extent between the 3dB points of the main beam
Sidelobes
All directional antennas have sidelobes in their patterns that are defined by nulls
The forward direction produces a main beam because of constructive interference
Radar targets can appear in the sidelobes of the antenna - gives an erroneous direction
Jammers can radiate noise and false signals into antenna sidelobes
Sidelobes can be reduced but not eliminated
Nulls
Directions in which no energy is radiated
Due to destructive interference of transmissions from across the antenna aperture
Why do we have lobes
Interference
Because we have a physical dimension antenna, we are going to have constructive interference
Radar targets
Scatter EM waves with patterns that are similar to antenna patterns
Pattern depends on size and shape of target
Scattering of EM waves described by the RCS of the target
RCS will vary with direction
Rayleigh scatterer
If target has linear dimension d «_space;λ then it is more ‘point-like’ and scatters the wave in all directions (more isotropic)
Ex: raindrop with d = 6 mm seen by 10 cm radar
In the optical domain, Rayleigh scattering accounts for the blue color of the sky
Optical scatterer
If target has linear dimension d»_space; λ then it is more ‘large’ and scatters the wave in preferred directions (more directive, unless it is a sphere)
Ex: automobile seen with λ=3 cm (x-band radar)
Larger targets
Have specific scatter patterns
Ex: flat plate a,b»_space; λ (behaves like an antenna with aperture a,b; energy reflected in direction of ray reflection)
Corner reflector
Has a large backscatter RCS over a wide angular range (RCS will be large)
Most aircraft are seen as several corner reflectors
Stealth technology
Targets such as aircraft can be designed to have low RCS in certain directions
Key elements:
- avoid corner reflectors
- use rounded surfaces
- use flat plates to direct energy away
RCS
Based on an equivalence to the reflection of radio signals by a metal sphere
To declare that a target is detected
Pr >= Smin
Smin
Set by the noise in the receiver
Typical value of (S/N)min
15 dB = 31.6
How high should SNR be to confidently detect target
10-20 dB
Want to avoid false targets
Receiver noise
Rx noise is thermal in origin and is assumed to be spread evenly in frequency (constant PSD)
White noise
Dependence of noise power on Bn creates an incentive to make the receiver BW smalle
Radar sensitivity
Being able to detect a target at greater range amounts to greater sensitivity
Sensitivity increases with the energy in a pulse
AN nomenclature for US systems
Developed by US military in WWII
AN/xxx-nn
-A = army, N = navy, nn = identifier of version of radar
x-letters designate
- type of installation (S: ship, T: ground, transportable, F: fixed ground)
- type of equipment (P: radar)
- purpose (S: search)
Svalbard ISR radar
498-502 MHz
Pt = 1 MW
32 m and 42 m dishes
44 dBi gain
Over the horizon radar system
Utilizes ionospheric refraction (bending) on HF frequencies (3-30 MHz) to see out to enormous ranges (thousands of km)
Multiple pulses
Integrating the returns from the multiple pulses improves the ability to detect the target
- helps because signal tends to add up systematically while noise adds up randomly
- integration can be coherent or noncoherent
Receiver performs the function of adding up the returned pulses
Coherent integration
Adding in both amplitude and phase
Gives more improvement but requires more complicated hardware
Integration gain
A boost to the S/N ratio over that expected from a single pulse
Improves the S/N in the radar receiver
Losses
Anything that detracts from ideal S/N
System losses
Microwave plumbing losses Between tx and antenna, rx and antenna Waveguide run Rotating joint T-R joint (duplexes) Waveguide components
Beam shape loss / scanning loss
Surveillance radar must rotate antenna
As the antenna beam sweeps over a target the target is hit with multiple pulses
The pulses strike the target at different points of the antenna pattern
The hits that occur away from the axis have an antenna gain that is less than Gmax
-will incur a loss as compared to having the pulses all hit at the peak of the antenna pattern
Usually only count pulses that strike between the 3dB points of the antenna pattern
For N>10, loss is typically 1.6 to 2 dB
Radome loss
If antenna has an enclosure (radome), there will be RF loss (1.2 dB two-way)
Rx filter mismatch loss
Radar pulse has sinX/X spectrum
Rx bandwidth filter is ~ rectangular (causes loss since sinc function hard to implement in hardware)
Typically 0.9 dB
Reason for B ~ 1/τ
Larger filter to get more power through - get more noise
Shrinking filter - reduces noise but choking off signal
