EE 529 Remote Sensing Techniques. Radar

Transcription

1 EE 59 Remote Sensing Techniques Radar

2 Outline Radar Resolution Radar Range Equation Signal-to-Noise Ratio Doppler Frequency

3 Basic function of an active radar Radar RADAR: Radio Detection and Ranging Detection of targets Estimation of their positions Advanced functions Measure reflectivity (identification, geo-science) Estimate target motion (MTI) Active (Tx-Rx) radar Cartography (SAR imaging) Estimate 3-D information (interferometry, tomography)

4 Basic function of an active radar Radar RADAR: Radio Detection and Ranging Detection of targets Estimation of their positions Carried out by: Measurement of delay between transmission and return Calculation of frequency difference between transmitted and echoed signal Active (Tx-Rx) radar

5 Radar Frequency Bands

6 Radar Frequency Bands Better Propagation Better Resolution

7 Radar Principle PRI T R cτ PRF 1/T c( T p + t r ) Rmin ct R max Pulse Repetition Frequency

8 Radar Principle T PRI T p + T p r Duty Cycle T Receiver listening time: Time between the end of one transmission and the start of the next, divided into two parts: Recovery Time: Represents time immediately following the transmission Listening Time: Time during which radar can receive and process echoes

9 Radar Principle

10 Range estimation using a pulsed radar One possibility among many (SFCW, FMCW, Digital waveforms, ) Emitted pulsed signal: Radar Principle Radar Radar-object distance: d Object Estimated distance

11 Range Ambiguity ct R max Echo of 1 shows after every PRI Echo of from pulse in the first PRI appears after the first PRI Depend on PRI/PRF

12 Resolution t T p Resolution is the ability of a radar system to distinguish between two or more targets at different positions.

13 Range Resolution Minimum distance between distinguishable targets Radar Object 1 Object Object 1 Object Resolution

14 Basic Configuration of Radar

15 Basic Configuration of Radar Waveform generator: Produces AM, LFM, SFM, etc. Transmitter: Generates Pulses or waveforms to be transmitted Antenna Systems: Directs the energy from the transmitter in a beam Picks up echoes and passes them to the receiver Monostatic or Bistatic Receiver: Performs preprocessing Signal Processor: Processes data and extracts information Timing and control: Synchronizes and controls various functional blocks

16 Antenna Parameters Power density: Average radiated power P d D PT 4πr 4πr P P T d,max Radiating transmitting power Directivity: Ratio of maximum power density to average power density in all directions from an ideal source Gain G G U max U ref 4π φ θ 3dB 3dB Maximum radiation intensity of a reference antenna φ db λ W θ db λ L A e LW G 4 πa e λ

17 Antenna Parameters V SAR φ db H θ db R 0 W y x x W x y r

18 Radar Range Equation Power density from an isotropic antenna 4πR P T Transmitter antenna gain GT Power density at object S I PT G 4πR T σ Power density at receiver input S R P G π σ π T T 4 R 4 R

19 Radar Range Equation Power at receiver S P G σa T T e 4πR 4πR Receiver antenna gain G 4 R πa e λ Power at receiver S PT GTσ G (4πR ) 4π PT G (4π ) T 3 G R Rλ Rσλ 4

20 Radar Range Equation Bistatic Radar

21 Radar Range Equation Monostatic Radar

22 Radar Range Equation Minimum detectable Power at receiver S min Maximum radar range R max 1/ 4 P 3 ( 4 ) TG λ σ π Smin x R 16 x P or 4 x max T Ae Input noise to a lossless antenna N i kt e B Boltzman s constant Bandwidth Effective temperature

23 Signal-to to-noise Ratio Noise Figure F ( SNR) ( SNR) i o Input of the receiver Output of the receiver S kt BF( SNR) i e o Minimum detectable signal power S kt min ebf( SNR) omin Maximum radar range R max 1/ 4 P 3 ( 4 ) ( ) TG λ σ π kte BF SNR omin ( SNR) o PT G λ σ 3 (4π ) kte BFR 4

24 Signal-to to-noise Ratio Received Power Transmitter Transmitter peak power Transmitter antenna gain Radar-target distance Receiver Receiver antenna gain Scattering Radar Cross Section (R.C.S) Noise Scattering coefficient (extended target) Boltzmann constant ( J/K) Temperature of Receiver (K) Noise figure Receiver noise equivalent bandwidth ( ) o

25 Radar Losses Radar losses cause a drop in SNR Types of radar losses: Transmit and receive losses S PT GT σae L, < 1 4πR 4πR L Antenna Pattern losses Atmospheric losses ( SNR) o PT G λ σl 3 (4π ) kte BFR 4 Processing losses Other losses (4π ) 4 kte FR ( SNR) P G λ σl τ T 3 o

26 Doppler Frequency

27 Doppler Frequency

28 Doppler Frequency

29 Doppler Doppler Frequency Frequency and and Ambiguity Ambiguity ] ) ( Re[ ), ( t j e r E t r E ω For an object at distance R: ] ) ( Re[ ) /, ( ] ) / ( [ φ ω + c R t j c e R E c R t R E ( ) dt dr f dt c R t d f c c r λ φ ω π ) / ( 1 + dt dr f d λ λ π φ R d 4 Doppler ambiguity in case of >π φ d

30 Radar Waveform Amplitude Modulation Continuous Wave Radar Pulse Modulation Radar B 1 T p R ct p c B Short pulse provides better resolution but requires higher transmitted power

31 Radar Waveform Forward propagation Reflection Backward propagation Focusing filter? : Loss term : Additive Gaussian noise : Focusing filter Detection principle Optimal solution: matched filter

32 Radar Waveform Resolution: should be as SMALL as possible Detection: ( SNR) o PT G λ σ 3 (4π ) kte BFR PT G (4π ) 3 λ T pσ 4 kt FR e 4 should be as LARGE as possible

33 $y 0 $x Linear Polarization ( z t) E x, $y ( z t) E, 0 $x z 0 $z δ δ y δ x E + ( z, t) E0x E0 y α tan cos( ω t kz + δ ) 1 E E 0 y 0x E0 y 0 α 0 E 0 0x α π / x-polarized y-polarized

34 Elliptical Polarization $y $y 0 E ( z, t 0) 0 $x 0 τ A φ 0 φ $x A : WAVE AMPLITUDE φ : ORIENTATION ANGLE π π φ τ : ELLIPTICITY ANGLE π 0 τ 4

35 Rotation Sense ROTATION SENSE: LOOKING INTO THE DIRECTION OF THE WAVE PROPAGATION $y $x 0 +τ τ $z COUNTER-CLOCKWISE ROTATION CLOCKWISE ROTATION LEFT HANDED POLARISATION RIGHT HANDED POLARISATION ELLIPTICITY ANGLE : τ > 0 π π τ 4 4 ELLIPTICITY ANGLE : τ < 0

36 Geometrical Parameters GEOMETRICAL PARAMETERS ORIENTATION ANGLE ELLIPTICITY ANGLE E0 x E0 y E0 x E0 y tanφ cosδ sinτ sinδ E0 x E0 y E0 x + E0 y δ δ y δ x POLARISATION HANDENESS: Sign(τ)