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Radar Fundamentals

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Radar Fundamentals Radar Fundamentals Prof. David Jenn Department of Electrical & Computer Engineering 833 Dyer Road, Room 437 Monterey, CA 93943 (831) 656-2254 [email protected], [email protected] http://www.nps.navy.mil/faculty/jenn Overview • Introduction • Radar functions • Antennas basics • Radar range equation • System parameters • Electromagnetic waves • Scattering mechanisms • Radar cross section and stealth • Sample radar systems 2 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 SCATTERED WAVE FRONTS be at the same location as RECEIVER viewed from the target (RX) (e.g., separate transmit Rr θ TARGET and receive antennas on TRANSMITTER Rt the same aircraft). (TX) INCIDENT WAVE FRONTS 3 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. 4 Electromagnetic Spectrum Wavelength (λ, in a vacuum and approximately in air) Microns Meters 10-3 10-2 10-1 110-5 10-4 10-3 10-2 10-1 1101 102 103 104 105 EHF SHF UHF VHF HF MF LF Radio Microwave Millimeter Ultraviolet Infrared Typical radar Visible frequencies Optical 300 GHz 300 MHz 109 108 107 106 105 104 103 102 10 1 100 10 1 100 10 1 Giga Mega Kilo Frequency (f, cps, Hz) 5 Radar Bands and Usage 8 (Similar to Table 1.1 and Section 1.5 in Skolnik) 6 Time Delay Ranging • Target range is the fundamental quantity measured by most radars. It is obtained by recording the round trip travel time of a pulse, TR , and computing range from: Bistatic: RRtr+ = cT R cT Monostatic: R = R (RRR== ) 2 tr where c = 3x108 m/s is the velocity of light in free space. TRANSMITTED PULSE RECEIVED PULSE AMPLITUDE T TIME R 7 Classification by Function Radars Civilian Military Weather Avoidance Navagation & Tracking Search & Surveillance High Resolution Imaging & Mapping Space Flight Proximity Fuzes Sounding Countermeasures 8 Classification by Waveform Radars CW Pulsed FMCW Noncoherent Coherent Low PRF Medium High PRF PRF Note: MTI ("Pulse Pulse Dopplerdoppler") CW = continuous wave FMCW = frequency modulated continuous wave PRF = pulse repetition frequency MTI = moving target indicator 9 Plane Waves • Wave propagates in the z direction E • Wavelength, λ x λ Eo DIRECTION OF • Radian frequency ω = 2π f PROPAGATION (rad/sec) t t • Frequency, f (Hz) 1 2 • Phase velocity in free space z is c (m/s) • x-polarized (direction of the electric field vector) − Eo • Eo, maximum amplitude of the wave Electric field vector 10 Wavefronts and Rays • In the antenna far-field the waves are spherical (2/)RD> 2 λ • Wavefronts at large distances are locally plane • Wave propagation can be accurately modeled with a locally plane wave approximation RADIATION Local region in the far field of PLANE WAVE FRONTS PATTERN the source can be approximated R by a plane wave D ANTENNA RAYS 11 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. Airborne Radar Target ht Grazing Angle,ψ hr dt dr 12 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. LINEAR VERTICAL, V ELECTRIC FIELD POLARIZATION VECTOR AT AN 1 INSTANT IN TIME ELECTRIC 2 FIELDS ORTHOGANAL TRANSMITTING 3 ANTENNAS CIRCULAR 4 POLARIZATION 5 HORIZONTAL, H 1 HORIZONTAL ANTENNA RECEIVES ONLY 2 6 HORIZONTALLY POLARIZED RADIATION 3 4 13 Antenna Parameters • Gain is the radiation intensity relative to a lossless isotropic reference. Low gain High gain (Small in wavelengths) (Large in wavelengths) • Fundamental equation for gain: Aperture area 2 GA= 4/πλe AAe = ε, effective area A = aperture area εε=≤≤efficiency (0 1) ANTENNA DIRECTIONAL λ = cf/, wavelength RADIATION PATTERN • In general, an increase in gain is accompanied by a decrease in beamwidth, and is achieved by increasing the antenna size relative to the wavelength. • With regard to radar, high gain and narrow beams are desirable for long detection and tracking ranges and accurate direction measurement. 14 Antenna Parameters • Half power beamwidth, HPBW (θB) • Polarization • Sidelobe level SCAN ANGLE PEAK GAIN • Antenna noise temperature (TA) 3 dB • Operating bandwidth HPBW MAXIMUM • Radar cross section and other signatures SIDELOBE LEVEL GAIN (dB)GAIN (dB) G 0.5G 0 θ θ PATTERN ANGLE s Rectangular dB pattern plot Polar voltage pattern plot 15 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 16 Radar Range Equation G • Quasi-monostatic TX t Pt R RX G σ P = transmit power (W) r t Pr Pr = received power (W) Gt = transmit antenna gain Gr = receive antenna gain σ = radar cross section (RCS, m2 ) Aer = effective aperture area of receive antenna PG σA PG G σλ2 P = t t er = t t r r (4πR2 )2 (4π)3 R4 17 Minimum Detection Range • The minimum received power that the radar receiver can "sense" is referred to a the minimum detectable signal (MDS) and is denoted S min . • Given the MDS, the maximum detection range can be obtained: 2 ⎛ 2 ⎞ 1 / 4 PtGtGrσλ PtGtGrσλ Pr = Smin = 3 4 ⇒ Rmax = ⎜ 3 ⎟ (4π) R ⎝ (4π) Smin ⎠ Pr 4 Pr ∝1/ R Smin R Rmax 18 Radar Block Diagram • This receiver is a superheterodyne receiver because of the intermediate frequency (IF) amplifier. (Similar to Figure 1.4 in Skolnik.) • Coherent radar uses the same local oscillator reference for transmit and receive. 19 Coordinate Systems • Radar coordinate systems spherical polar: (r,θ,φ) azimuth/elevation: (Az,El) Constant Az cut or (α,γ ) ZENITH Constant El cut • The radar is located at the origin of z the coordinate system; the Earth's surface lies in the x-y plane. CONSTANT TargetELEVATION • Azimuth (α) is generally measured clockwise from a reference (like a P compass) but the spherical system θ r azimuth angle (φ ) is measured Radar γ y counterclockwise from the x axis. φ Therefore α γ = 90 −θ x HORIZON α = 360 −φ 20 Radar Display Types "A" DISPLAY "B" DISPLAY TARGET RETURN TARGET BLIP RANGE RECEIVED POWER -1800 180 RANGE (TIME) AZIMUTH PLAN POSITION INDICATOR (PPI) "C" DISPLAY AZIMUTH RANGE UNITS 90 TARGET BLIP TARGET BLIP ELEVATION RADAR AT 0 CENTER -1800 180 AZIMUTH 21 Pulsed Waveform • In practice multiple pulses are transmitted to: 1. cover search patterns 2. track moving targets 3. integrate (sum) several target returns to improve detection •The pulse trainis a common waveform Po = peak instantaneous power (W) τ = pulse width (sec) fTpp=1/ , pulse repetition frequency (PRF, Hz) Tp = interpulse period (sec) N = number of pulses T p Po TIME 22 τ Range Ambiguities • For convenience we omit the sinusoidal carrier when drawing the pulse train T p Po TIME τ • When multiple pulses are transmitted there is the possibility of a range ambiguity. TRANSMITTED TRANSMITTED TARGET PULSE 1 PULSE 2 RETURN TIME TR T 2 R1 2R • To determine the range unambiguously requires that T p ≥ . The unambiguous range is c cTp c Ru = = 2 2 fp 23 Range Resolution • Two targets are resolved if their returns do not overlap. The range resolution corresponding to a pulse width τ is ∆ R = R 2 − R 1 = c τ /2 . TIME STEP 1 TIME STEP 2 cτ /2 to to +τ /2 R1 R1 R2 R2 cτ /2 TARGET cτ R1 R1 R2 R2 TIME STEP 3 TIME STEP 4 to + τ to +3τ /2 24 Range Gates • Typical pulse train and range gates DWELL TIME = N / PRF 123 M 123 M 123 M 123 M t L L L L L M RANGE GATES TRANSMIT PULSES • Analog implementation of range gates . OUTPUTS ARE CALLED . "RANGE BINS" TO SIGNAL • Gates are opened and closed sequentially M M PROCESSOR • The time each gate is closed corresponds to . a range increment RECEIVER . • Gates must cover the entire interpulse period . or the ranges of interest . • For tracking a target a single gate can remain closed until the target leaves the bin M M . 25 Clutter and Interference INTERFERENCE TARGET TH PA ECT DIR TX TH A P LTI U RX M RANGE GATE CLU TTE R SPHERICAL WAVEFRONT (IN ANTENNA FAR FIELD) GROUND TARGET The point target ANTENNA MAIN LOBE approximation is good when the target extent RAIN (MAINBEAM CLUTTER) << ∆R GROUND SIDELOBE CLUTTER IN RANGE GATE GROUND (SIDELOBE CLUTTER) 26 Thermal Noise • In practice the received signal is "corrupted" (distorted from the ideal shape and amplitude) by thermal noise, interference and clutter.
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