DEP. OF SIGNAL THEORY AND COMMUNICATIONS
Chap.1. Introduction to Optical
Remote Sensing
ORS active: LIDAR
Francesc Rocadenbosch
ETSETB, Dep. TSC, EEF Group
Campus Nord, D4-016
roca@tsc.upc.edu
INTRODUCTION
LIDAR (LIgth Detection And Ranging)
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL 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 →
• ∆R(spatial resolution)
≈ meters
• ∆t = [secondsminutes]
Fig. SOURCE: Measures (1992); R.M. Measures,
"Laser Remote Sensing. Fundamentals and
Applications". John Wiley & Sons, 1984. (Reprint
de 1992, Krieger Publishing Company).
(C) F. Rocadenbosch 2005-2006
2
INTRODUCTION
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
MOTIVATION OF LASER PROBING: Features
Associated To Optical Wavelengths
• Strong optical interaction
• High directivity of radiation
⎧ λ = 532 nm ⎫
λ
⇒ ⎨
∆θ ≈
⎬ ⇒ ∆ 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
2v
fd = − r
λ
→
f dlidar λ radar
5
≈
≈
10
f dradar λ lidar
(C) F. Rocadenbosch 2005-2006
3
INTRODUCTION
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
HISTORICAL BACKGROUND
• (1930) Searchligths
• (1960) Laser invention
– Offers: High collimation, purity and spectral coherence (∆λ≈ 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
(C) F. Rocadenbosch 2005-2006
4
OPTICAL AND TECHNOLOGICAL CONSIDERATIONS
BEER’S (or BOUGUER’S) LAW
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
Describes intensity of a laser beam propagating in a 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
(C) F. Rocadenbosch 2005-2006
5
OPTICAL AND TECHNOLOGICAL CONSIDERATIONS
A) Based on their APPLICATION
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
ELASTIC-BACKSCATTER LIDAR (or “backscatter lidar”) measures...
• the average content of particulate and molecular matter (be them
contaminating or not) in the atmosphere
• winds (cross-correlation techniques) and others (range-finders, CMM, ...)
WIND LIDAR (Doppler lidar)
SPECTROSCOPIC LIDAR → measurement of chemical species
B) Based on their CONFIGURATION
MONO-STATIC LIDAR
• Types: 1) Backscatter, 2) DIAL, 3) Raman, 4) Doppler, 5) Fluorescence,
6) Others
BI-STATIC LIDAR
• Types: 1) Long-path absorption
Airborne (helicopter, plane, satellite), mobile (van, truck), or ground-based.
(C) F. Rocadenbosch 2005-2006
6
BACKSCATTER LIDAR
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
OPERATIONAL PRINCIPLE
• Same emission and reception wavelengths (λ0=λR)
• Uses elastic Mie scattering (λ ≈ r, aerosols) and Rayleigh scattering
(λ >> r, molecules) to interrogate the intervining atmosphere
ENVIRONMENTAL APPLICATIONS
• Pollution monitoring (source strength and location)
• Aerosol monitoring: Air Quality regulations, Fires
• Feedback to/from Transport models
– to forecast movement of pollutants and related photochemical effects
METEOROLOGICAL APPLIC.
• Rain, snow, clouds, ...
• Atmospheric attenuation
estimation (dB/km)
(C) F. Rocadenbosch 2005-2006
7
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
UPC BACKSCATTER LIDAR
LASER
Gain medium
Nd:YAG
Energy
0.5 J/532 nm
Divergence
0.1mrad
Pulse length
10 ns
PRF
10 Hz
RECEIVER
Focal length
2m
20 cm
Aperture ∅
Detector
APD (EGG C30954)
Net Responsivity 6×101-3×106 V/W
Bandwidth
10 MHz
SYSTEM SPECS
Configuration
Vertical biaxial
System NEP
70 fW·Hz-1/2
Min. Det. Power < 5 nW
Acquisition
20 Msps/12bit
Spatial resolution 7.5 m
∆R = 7.5 m, ∆t = 1 min
(C) F. Rocadenbosch 2005-2006
8
DIAL
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
OPERACIONAL PRINCIPLE
• DIAL (Differential Absorption Lidar)
• Uses two (or more) tuning wavelengths, one of which is absorbed by the
atmospheric species of interest, and another one that is not.
1
Pλ (R )
Na ≈
ln
2(σ′a − σ a )R Pλ′ (R )
where:
Na is the molecule concentration, σ a , σ ′a are
the molecule absorption cross-sections at λ , λ ′
and, P λ , P λ ′ are the backscattered return
powers at λ , λ ′, normalised to the transmitted
ones.
Fig. Contours of NO2 concentration (ppm) in the vicinity of a chemical plant, as
measured by differential absorption lidar. (SOURCE: K. W. ROTHE et al. 1974. Appl.
Phys. 4, 181 (1974)).
(C) F. Rocadenbosch 2005-2006
9
DIAL
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
APLICATIONS
1) Concentration of chemical
species in the atmosphere, car
exhausts, refineries,...
Measurement types:
• range-resolved (RR), and
• column-content (CC)
• e.g., SO2, NH3, O3, CO, CO2,
HCl, vapor H2O, NO, N2O, SF6
Typ. Resolutions: ppb to ppm. Typ.
Ranges: a few kms.
2) Temperature and humidity
Fig. SOURCE: Whiteman, D. N.; Melfi, S. H. Cloud liquid water,
mean droplet radius and number density measurements using a
Raman lidar. J. Geophys. Res. 1999, 104 (D24), 31411-31419
(C) F. Rocadenbosch 2005-2006
10
RAMAN LIDAR
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
OPERATIONAL PRINCIPLE
1) In contrast to elastic systems,
the return wavelength, λR, is
shifted from the incident one, λ0.
2) Wavelength shift, κ, depends
on each molecular species.
λ0
λR =
1 − κλ 0
3) Very faint returns.
• requires photon counting
• very often, night-time
operation
Fig. ADAPTED FROM: Inaba, H. Detection of Atoms and
Molecules by Raman Scattering and Resonance Fluorescence.
In Laser Monitoring of the Atmosphere, Hinkley, E. D., Ed.;
Springer-Verlag: New York, 1976; Chap. 5, 153-236.
(C) F. Rocadenbosch 2005-2006
11
RAMAN LIDAR
APLICATIONS
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
1) Self-calibrated lidar (N2 shift)
• Absolute concentration of any
atmospheric species can be
determined by comparison to the
N2-atmospheric return
2) Temperature profiler (±2K)
Fig. SOURCE: Measures (1992).
3) Spectroscopic sensing (COMPARISON WITH DIAL)
• Low detection sensitivity at long ranges due to the low Raman cross
sections that ...
• limit the method to the detection of species present in high concentrations
(e.g. smoke stacks in industrial plants, 100-1000 ppm, 30-100 m).
• In contrast, measurements are always range resolved (RR) and there is no
need to tune the laser in absorption bands.
(C) F. Rocadenbosch 2005-2006
12
DOPPLER LIDAR
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
Uses airborne particles&molecules as “tracers” along with the
Doppler principle to invert the wind radial component
• (1992) First commercial system. Specs.: 30-3000 m range, 1-m/s
resolution, 150-m spatial resolution and 5-min integration time.
• (Today) Wind sensors: LAWS (NASA) and ALADIN (ESA).
(C) F. Rocadenbosch 2005-2006
13
DOPPLER LIDAR
TECHNIQUES
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
fd = −
2v r
λ
• Coherent Detection: Optical heterodyne detection
• Incoherent Detection: E.g. Uses high-resolution filters (FabryPérot) as frequency (fd)-amplitude transducers (edge-technique).
(C) F. Rocadenbosch 2005-2006
14
Wind measurement
example using a Doppler
lidar at Eldorado Canyon
during a mesofront
invasion.
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
DOPPLER LIDAR
SOURCE: Courtesy of NOAA (National
Oceanics and Atmospherics Administration).
(C) F. Rocadenbosch 2005-2006
15
ABSORPTION LIDAR
DEP. OF SIGNAL THEORY AND COMMUNICATIONS
OPTICAL REMOTE SENSING
OPERATIONAL PRINC.: “Long-path absorption”. See also TDLAS.
APPLICATIONS
Column-content (CC) gas detection
• Sensitivity defined by [ppm·m]
(C) F. Rocadenbosch 2005-2006
16