DEP. OF SIGNAL THEORY AND COMMUNICATIONS
HD2-4: Introduction to LIDAR (laser radar) Remote Sensing Systems Francesc Rocadenbosch Remote Sensing Lab. (RSLAB) Universitat Politècnica de Catalunya
http://www.tsc.upc.edu Campus Nord, D4-016, E08034, Barcelona (SPAIN)
[email protected]
(C) 2007
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
Introduction to LIDAR Remote Sensing Systems Chap.1 Optical and Technological Considerations Francesc Rocadenbosch Remote Sensing Lab. (RSLAB) Universitat Politècnica de Catalunya Campus Nord, D4-016
[email protected]
INTRODUCTION
LIDAR (LIgth Detection And Ranging)
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
LIDAR REMOTE SENSING
Strong optical interaction between laser/atmospheric species of interest • λ ≈ r particles, λ >> r airborne molecules Interacting mechanisms: • scattering by gases ( α g , sca ) and particles ( α • absorption ( α g , abs )
p , sca)
KEYS: • Highly collimated → • ΔR(spatial resolution) ≈ meters • Δt = [secondsminutes] Fig. SOURCE: Measures (1992); R.M. Measures, "Laser Remote Sensing. Fundamentals and Applications". John Wiley & Sons, 1984. (Reprint de 1992, Krieger Publishing Company).
IGARSS 07, (C) F. Rocadenbosch (RSLAB)
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INTRODUCTION
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
LIDAR REMOTE SENSING
MOTIVATION OF LASER PROBING: Features Associated To Optical Wavelengths • Strong optical interaction • High directivity of radiation
Δθ ≈
⎧ λ = 532 nm ⎫ λ ⇒ ⎨ ⎬ ⇒ Δ θ ≈ 50 µrad ⎩ D = 1 cm ⎭ D
– (Comparison with RADAR) to achieve the same angular resolution at 3 GHz, f = 3 GHz ⇒ λ = 10 cm ⇒ D ≈ 1800 m !
• Larger (optical) Doppler shifts than at RF wavelengths
fd = −
2vr λ
f dlidar λ radar ≈ ≈ 105 radar λ fd lidar
→
IGARSS 07, (C) F. Rocadenbosch (RSLAB)
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INTRODUCTION
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
LIDAR REMOTE SENSING
HISTORICAL BACKGROUND • (1930) Searchligths • (1960) Laser invention – Offers: High collimation, purity and spectral coherence (Δλ≈ 0.01 nm)
• (1962) Fiocco & Smullin – bounce a laser beam off the Moon. Study atmospheric turbid layers
• (1963) Ligda – Q-switching: Enables short width (τl), high-energy laser pulses – (Ep ≈ 1J, τl ≈ 10ns, PRF ≈ 10Hz)
• (1973) Semiconductor laser (GaAs) – Laser diode arrays. Trade-off between peak energy (Ep) ↓ and PRF ↑
E = Ep
τl = E p τl PRF T
• (2002) TLD-technologies and ps-lidar – Spectroscopic Lidar (detection of chemical species), 3D mapping IGARSS 07, (C) F. Rocadenbosch (RSLAB)
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OPTICAL AND TECHNOLOGICAL CONSIDERATIONS
BEER’S (or BOUGUER’S) LAW DEP. OF SIGNAL THEORY AND COMMUNICATIONS
LIDAR REMOTE SENSING
Describes intensity of a laser beam propagating in an inhomog. medium
[
I (λ ) R = T (λ, R ) = exp − ∫0 α(r, λ )dr I0
]
• where: I0 is the intensity at r=0, I is the intensity at r=R, α is the atmospheric extinction coef., T(λ,R) is the transmissivity in (0,R) and,
α = α g , sca + α p , sca + α g ,abs
[km −1 ]
SPECTRAL BANDS Lidars operate in atmospheric transmission windows • 0.4-0.7 μm (VIS), 0.7-1.5 μm (NIR), 3-5 μm y 9-13 μm (IR) • “eye-safe”: λ >1.4 μm (100 mW/cm2, 1J/cm2) • Trade-off: Laser and detector availability! – Ej. Ruby (0.69 μm), Nd:YAG (1.064 μm), CO2 (9-10 μm), “eye-safe” 1.55μm 5
IGARSS 07, (C) F. Rocadenbosch (RSLAB)
OPTICAL AND TECHNOLOGICAL CONSIDERATIONS RAYLEIGH SCATTERING (i.e., molecular/gas scattering, r
