Microwave Spectrum and Basics of RADARs

Vocabulary flashcards covering key terms, concepts and components from microwave spectrum fundamentals to advanced radar techniques such as SAR, Doppler, FMCW, antenna theory and radar imaging distortions.

Microwave Region

Portion of the EM spectrum with wavelengths from 1 mm to 1 m.

Atmospheric Window (Microwaves)

Frequency-dependent transparency of the atmosphere; attenuation decreases with increasing wavelength.

W/V Bands

Millimetre-wave radar bands offering sub-centimetre resolution but severe atmospheric attenuation; used in short-range automotive radars.

Ka Band

Microwave band (~26–40 GHz) with high resolution and moderate atmospheric loss; suitable for airborne but challenging for spaceborne radars.

K Band

Radar band around 18–27 GHz, good for airborne imaging; sensitive to weather.

Ku Band

Microwave band (~12–18 GHz) giving good resolution, moderate attenuation.

X Band

8–12 GHz radar band, best resolution practical from space; widely used for scientific SAR missions.

S Band

2–4 GHz band usually reserved for telecom/telemetry; sometimes used for planetary radars.

L Band

1–2 GHz band famous in radar imaging and oceanography; penetrates foliage and senses sea-surface roughness.

P Band

300–1000 MHz band enabling ground-penetrating radar; impractical from space due to huge antenna size.

Antenna Length–Wavelength Rule

Required physical antenna size grows proportionally with wavelength to keep beamwidth constant.

Wavelength–Resolution Relation

Shorter wavelengths (higher frequencies) produce finer spatial resolution.

Microwave Penetration

Longer wavelengths can penetrate vegetation or soil; shorter wavelengths interact mainly with surface features.

RADAR Acronym

Radio Detection And Ranging – active sensor measuring presence and distance of objects using microwaves.

Plane-Position Indicator (PPI)

Classical rotating-beam radar display showing range and azimuth as bright dots.

Imaging Radar

Radar able to generate a 2-D matrix (image) rather than isolated blips.

Synthetic Aperture Radar (SAR)

Imaging radar that synthesises a long aperture from platform motion to achieve high azimuth resolution with a compact antenna.

Bistatic Radar

Configuration with separate transmitting and receiving antennas; can operate in continuous-wave mode and provides geometric diversity.

Monostatic Radar

Radar using the same antenna for transmission and reception (not simultaneously); typically pulse-based.

Duplexer (Circulator)

Switch allowing a monostatic antenna to change rapidly between transmit and receive modes.

Diplexer

Device that separates or combines signals of different frequencies.

Continuous-Wave (CW) Radar

Radar transmitting continuously; used in Doppler and bistatic systems.

Pulse Radar

Radar transmitting discrete pulses; measures distance via two-way travel time.

ULTRASTALO

Ultra-stable local oscillator providing highly stable frequency/time reference inside a radar.

Low Noise Amplifier (LNA)

Receiver or transmitter stage that amplifies signals while adding minimal noise.

Left/Right Ambiguity

In nadir-pointing radar, echoes from symmetric left and right points arrive simultaneously and cannot be distinguished.

Side-Looking Radar

Radar pointing off-nadir to remove left/right ambiguity and enable unique slant-range mapping.

Slant Range

Instantaneous line-of-sight distance from radar to ground target; directly related to echo time delay.

Ground Range

Horizontal distance on the ground corresponding to a given slant range.

Beamwidth

Angular width of an antenna’s main lobe (usually defined at −3 dB points).

Swath Width

Ground width covered by the radar beam in the cross-track direction.

Near Range

Edge of swath closest to nadir; shortest slant range.

Far Range

Edge of swath furthest from nadir; longest slant range.

Look Angle

Angle between radar line of sight and nadir (for Earth curvature).

Incident Angle

Angle between radar beam and local surface normal.

Azimuth (Radar)

Along-track coordinate parallel to platform motion.

Backscattering

Portion of incident microwave energy reflected back toward the radar.

Specular Reflection

Mirror-like reflection causing low backscatter toward radar; common on calm water or smooth surfaces.

Lambertian Reflection

Diffuse reflection radiating equally in all directions; often yields higher backscatter.

Slant-Range Scale Distortion

Compression of near-range features in radar images because range is measured in slant space, not ground space.

Foreshortening

Radar distortion where slopes facing the radar appear compressed and bright.

Layover

Extreme foreshortening where mountain tops (closer) appear displaced toward the radar, overtaking their bases.

Shadowing (Radar)

Areas with no echo because steep terrain blocks illumination; appear dark in images.

Clinometry

Estimating object height from radar shadow length given geometry.

Polarization (Radar)

Orientation of electric field in transmitted/received wave (H or V); used for additional discrimination.

Parallel Polarization

Same transmit and receive polarization (HH or VV); usually stronger echoes.

Cross Polarization

Orthogonal transmit/receive polarization (HV or VH); highlights depolarizing targets like vegetation.

Fully Polarimetric Radar

System transmitting and receiving both H and V to obtain HH, HV, VH, VV channels.

Corner Reflector (CR)

Trihedral metallic reflector returning incident waves back to source, acting as point target for calibration.

Linear Time-Invariant (LTI) System

System whose response to inputs is linear and time-invariant; radar analysis often assumes this model.

Ground-Penetrating Radar (GPR)

Radar (usually P-band or lower) designed to detect subsurface features.

Multiple Reflections (Radar)

Successive scattering events; random in vegetation, deterministic in urban scenes.

Range Resolution (Pulse Radar)

Minimum separable distance along slant range: ΔR = c Τ / 2.

Ground Range Resolution

Projection of range resolution onto ground: ΔRg = ΔR / sin θ.

Pulse Repetition Frequency (PRF)

Number of pulses transmitted per second; inverse of PRI.

Pulse Repetition Interval (PRI)

Time between successive transmitted pulses.

Scene Duration (SD)

Time span between first near-range echo and last far-range echo for a pulse.

Range Ambiguity

Misassociation of echoes to wrong pulses when SD exceeds PRI.

Blind Range

Distance zone unobservable because radar is transmitting (monostatic systems).

Coverage–Resolution Trade-off

Constraint where PRF must satisfy Coverage/Resolution < PRF < 1/SD to avoid ambiguities.

Isotropic Antenna

Ideal antenna radiating equally in all directions; used as reference model.

Directional Antenna

Real antenna concentrating energy into a preferred direction (main lobe).

Radiation Pattern

Angular distribution of radiated (or received) power from an antenna.

Main Lobe

Region of highest gain in radiation pattern.

Side Lobes

Secondary lobes with lower gain occurring away from main lobe.

Nulls (Antenna)

Directions where antenna gain drops to zero.

Antenna Boresight

Direction of maximum gain (θ = 0°).

Antenna Directivity

Ratio of radiation intensity in a given direction to average intensity; measures beam concentration.

Antenna Gain

Directivity multiplied by efficiency η; represents actual radiated power in a direction.

Antenna Efficiency

Fraction of input power that is actually radiated (0–1).

Half-Power Beamwidth (3 dB)

Angular width where pattern falls to −3 dB (half power) of peak.

Null-to-Null Beamwidth

Angular span between first zeros of main lobe.

Effective Area (Ae)

Antenna’s equivalent collecting area: Ae = (λ²/4π) G(θ,φ).

Uniform Rectangular Aperture

Ideal flat antenna with uniform current distribution; produces sinc radiation pattern.

Phased Array Antenna

Antenna composed of many elements with controllable amplitude/phase enabling beam steering and weighting.

Antenna Weighting

Non-uniform excitation of array elements to suppress side lobes at expense of beamwidth.

Beamforming

Signal processing to shape or steer antenna pattern electronically.

Null Steering

Adjusting array phases to place pattern nulls toward interference sources.

Radar Equation (Bistatic)

Formula linking received power to transmit power, gains, ranges, target properties and wavelength.

Radar Cross-Section (σ)

Effective area representing target’s ability to reflect radar energy back toward receiver.

Backscattering Coefficient (σ⁰)

Normalized radar cross-section per unit surface area; used for distributed targets (pixels).

Dynamic Range (Radar)

Range between minimum detectable power (noise level) and maximum non-saturating power; measured in bits.

Doppler Effect (Radar)

Shift in received frequency proportional to relative radial velocity: fD = −(2/λ) (dR/dt).

Doppler Radar

CW radar measuring target velocity via Doppler frequency shift.

Quadrature Demodulation

Process mixing received signal with cosine and sine references to obtain complex baseband (I/Q).

Beat Signal

Low-frequency difference between transmitted and received FMCW signals; encodes range and velocity.

Frequency-Modulated Continuous-Wave (FMCW) Radar

Radar using periodic linear frequency sweeps to measure range via beat frequency.

Chirp Radar

Pulse radar transmitting linearly frequency-modulated pulses; enables pulse compression.

Chirp Rate (α)

Slope of frequency sweep: α = B/Τ (Hz / s).

Pulse Compression

Matched filtering of received chirp to shorten effective pulse and improve range resolution.

Matched Filter

Filter matched to transmitted signal (time-reversed conjugate) maximising SNR.

Synthetic Aperture

Effective long antenna created by coherent processing of echoes received at sequential positions.

Coherent Radar

Radar preserving phase information; requires stable oscillators and precise timing.

Real Aperture Radar (RAR)

Imaging radar whose azimuth resolution is limited by physical antenna length: Δa = R λ / l.

Synthetic Aperture Radar (SAR) Azimuth Resolution

Independent of range and antenna length; determined by half physical antenna length (~l/2).

Start-and-Stop Approximation

Assumption that platform motion during each pulse is negligible; simplifies SAR processing.

Fast Time

Time axis within a single pulse related to range.

Slow Time

Time axis across successive pulses related to along-track (azimuth) position.

Moving Target Indicator (MTI) Radar

Coherent pulse radar cancelling stationary clutter to detect moving objects using Doppler shifts.