03-01-2013, 03:15 PM
Radar Fundamentals
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Radio Detection and Ranging
• Bistatic: the transmit and receive antennas are at different locations as
viewed from the target (e.g., ground transmitter and airborne receiver).
• Monostatic: the transmitter and receiver are colocated as viewed from
the target (i.e., the same antenna is used to transmit and receive).
• Quasi-monostatic: the transmit
and receive antennas are slightly
separated but still appear to
be at the same location as
viewed from the target
(e.g., separate transmit
and receive antennas on
the same aircraft).
Radar Functions
• Normal radar functions:
1. range (from pulse delay)
2. velocity (from Doppler frequency shift)
3. angular direction (from antenna pointing)
• Signature analysis and inverse scattering:
4. target size (from magnitude of return)
5. target shape and components (return as a function of
direction)
6. moving parts (modulation of the return)
7. material composition
• The complexity (cost & size) of the radar increases with the extent
of the functions that the radar performs.
Superposition of Waves
• If multiple signal sources of the same frequency are present, or multiple
paths exist between a radar and target, then the total signal at a location
is the sum (superposition principle).
• The result is interference: constructive interference occurs if the waves
add; destructive interference occurs if the waves cancel.
• Example: ground bounce multi-path can be misinterpreted as multiple
targets.
Wave Polarization
• Polarization refers to the shape of the curve traced by the tip of the
electric field vector as a function of time at a point in space.
• Microwave systems are generally designed for linear or circular
polarization.
• Two orthogonal linearly polarized antennas can be used to generate
circular polarization.
Radar Antenna Tradeoffs
• Airborne applications:
Size, weight, power consumption
Power handling
Location on platform and required field of view
Many systems operating over a wide frequency spectrum
Isolation and interference
Reliability and maintainability
Radomes (antenna enclosures or covers)
• Accommodate as many systems as possible to avoid operational
restrictions (multi-mission, multi-band, etc.)
• Signatures must be controlled: radar cross section (RCS), infrared
(IR), acoustic, and visible (camouflage)
• New antenna architectures and technologies
Conformal, integrated
Digital “smart” antennas with multiple beams
Broadband
Doppler Filter Banks
The radar’s operating band is divided into narrow sub-bands. Ideally there
should be no overlap in sub-band frequency characteristics.
• The noise bandwidth of the Doppler filters is small compared to that of the
radar’s total bandwidth, which improves the SNR.
• Velocity estimates can be made by monitoring the power out of each filter.
• If a signal is present in a filter, the target's velocity range is known.