Week 1 Flashcards
Surveillance radar
Scans the horizon looking for targets
Air traffic control, air defense, ships
Tracking radar
Locks onto target and follows it
Law enforcement radar
Measures vehicle velocity
Radar acronym
RAdio Detection And Ranging
A basic type of radar can
Detect the presence of a target
Measure the range
Find the azimuth and elevation
Range
The distance between the radar and the target
Azimuth and Elevation
The direction to the target using a directional antenna
Synthetic aperture radar
Maps the ground with high resolution for earth resources and military purposes
Weather radar
Ground based - maps precipitation over wide area
Aircraft based - warns of dangerous rain areas
Radar Applications
Ground probing - looking for land mines
Movement sensing - security, door openers
Terrain avoidance - low flying military aircraft
Radar altimeter - accurate measurement of aircraft height (essential for blind landing)
Space radar - detect satellites, debris, space weather
Autonomous vehicles - key sensor technology (lidar - light radar)
Radar definition
An electrical system that transmits RF EM waves towards a region of interest and receives and detects these EM waves when reflected from objects in that region
Pulse radar
Sends out a short burst of RF energy
For a point target, the received echo is spread in time by the width of the pulse.
The echo from the point target has the appearance of the transmitted pulse but is greatly reduced in magnitude
Series of pulses, typically 10 to 50
Standard radar processing integrates over all the pulses and echoes in the series to produce a single pulse-echo combination
Mono static radar
TX and RX are co-located and often share an antenna
Bistatic radar
Tx and Rx are in separate locations
Tx power
kW to MW
Rx power
nW levels and lower
Measuring range
Measure time for radar pulse to go to target and back
Radar measures time from transmitted pulse to received echo
Pulses repetition interval
Tp
Time between pulses
Range Ambiguity
Pulsed radars process the echoes (or returns) from many pulses collectively
When a successive pulse is transmitted before the echo from the previous pulse is received, the range measurement for the echo will be wrong
Measured range is correct only for
R < Rua
Rua - maximum unambiguous range
Range resolution
If two targets are close together, the echoes will overlap in time —> the radar will report a single target
Minimum spacing at which two targets can be separated by radar
Point targets can be separated provided their echoes are received with time delay greater than one pulse width
If we want to resolve targets that are closer
We must use a shorter pulse
CW radar
Sends out a continuous wave of RF energy
Tx and Rx operate continually
Because of the continual transmission, a mono static CW radar must be low power (to protect the receiver) —> limits CW radars to near range operations
Police radars
Can we measure the range to a target with a CW radar
No, not if we use a simple waveform
Radar transmitters are rated by
Pulse power
Pulse power
Pt - rms output power of the transmitter while it transmits a pulse
Time-range radar correspondences
1 ms - 150 km
1 us - 150 m
1 ns - .150 m = 15 cm
Early radars operated in the bands:
HF: 3 to 30 MHz
VHF: 30 to 300 MHz
UHF: 300 to 1000 MHz
Most radars today operate at
Microwave frequencies
- 1000 to 100,000 MHz = 1 GHz to 100 GHz
HF Band
3 to 30 MHz
VHF band
30 to 300 MHz
UHF band
300 to 1000 MHz
L-band
1-2 GHz
S-band
2 to 4 GHz
C-band
4 to 8 GHz
X-band
8 to 12 GHz
Ku-band
12 to 18 GHz
K-band
18 to 27 GHz
Ka-band
27 to 40 GHz
W-band
75 to 110 GHz
MHz / m radar systems
f[MHz]λ[m] = 300
GHz / cm radar systems
f[GHz]λ[cm] = 30
Motion of a wave
In the direction of the wave vector k, k = 2π/λ [rad/m]
Coherent radar
Detect the amplitude of returned signal
Provides info on the phase of the returned signal, measured relative to the transmitted signal
LO, STALO - listening time
Stable local oscillator
LO, STALO
Used to generate the transmit signal and to process the returned signal
Pulse coherent radar
Phase ca be measured on the signal returned from successive pulses
Can measure the difference in phase between the echo and the LO reference
If the phase changes between pulses, gives rise to Doppler shift
If the phase is changing between pulses
The distance to the target is changing (target is approaching or receding)
Doppler shift
ωd = dφm/dt fd = ωd/2π = 2vr/λ
Sign of the Doppler shift
Positive for motion toward radar
Negative for away
Resolving the Doppler frequency
Ambiguity can arise
If phase changes by more than π rad between pulses, can’t tell where phase is increasing or decreasing
If phase change is 2π between pulses, then dφ/dt and the apparent speed is zero
Must sample at least twice as fast as fd
-sample the phase once per pulse (sampling rate = PRF)
Doppler shift / velocity measurement only correct for
|fd| < fd_max
|vr| < vr_max
Ambiguity and PRF regimes
Resolving an ambiguity in range - lower PRF
Resolving an ambiguity in Doppler shift or velocity - higher PRF
Low PRF
Unambiguous in range
Ambiguous in Doppler shift / velocity
High PRF
Ambiguous in range
Unambiguous in Doppler shift / velocity
Medium PRF
Ambiguous in both range and Doppler shift/velocity
Radar range equation
Calculates the power received from a target at a given range
Calculates the max range at which a target of known radar reflectivity can be detected
Gain generally refers to
The gain in the direction of peak antenna performance
Radar cross section
RCS, expresses the relationship between the power density incident on the target and the power density received at the antenna from the target as a result of reflection (scattering)
Power density at the radar receiver
Target has become its own transmitter, an isotropic radiator with Prefl = Pint
Effective collective area
Receiving antenna scoops radio wave power out of the air over this area
Smin
Minimum power needed in the receiver to reliably detect the target
Usually determined by noise in the receiver
Max range of detection of a given target is a measure of
Radar sensitivity
Losses in a radar
Propagation effects, losses in waveguides and components
May be as high as 20 dB
Max range of the radar will be (10^-20/10)^1/4 = 31.6% of the range predicted by the idealized equation
