lecture 39. lidar class review (1)
TRANSCRIPT
Lecture 39. Lidar Class Review (1)
Concept of Lidar Remote Sensing
Picture of Lidar Remote Sensing
General Lidar Equation and Basic Assumptions
Physical Processes Involved in Lidar
Lidar Classification
Lidar Equation in Different Forms
Lidar Architecture
Altitude and Range Determination
Concept of Remote Sensing Remote Sensing is the science and technology of obtaining
information about an object without having the sensor in directphysical-contact with the object.
The nature of remote sensing is one kind of measurements, i.e.,to obtain or acquire information of an object using experimentalmethods.
There must be some interaction between the object and theinstruments in order to acquire the information of the object. Theinteraction can be direct (local) or remote.
For remote sensing, remote interaction must be introduced tocarry away the object information so that the information can beacquired by the sensor remotely.
The interaction between radiation and the object is the mostcommon interaction used in modern remote sensing. The radiationincludes electromagnetic radiation and acoustic waves.
Concept of Lidar Remote Sensing
Remote sensing has passive and active remote sensing, in optical,radio, and acoustic frequency ranges.
LIDAR, RADAR, and SODAR are three main active remotesensing technologies.
LIDAR stands for Light Detection And Ranging.
- a laser radar in optical frequency range.
Lidar started in the pre-laser times in 1930s with searchlight,and then quickly evolved to modern lidars using ns laser pulses.
Due to its unique feature and advanced laser spectroscopy andtechnology, lidar provides much higher accuracy, precision, andresolution in measurements of atmosphere and environmentalparameters as well as targets and objects, than other remotesensing.
Physical Picture in LIDAR Remote Sensing
General Lidar Equation Lidar equation is the fundamental equation in lidar remote
sensing field to relate the received photon counts (or lightpower) with the transmitted laser photon numbers (or laserpower), light transmission in atmosphere or medium, physicalinteraction between light and objects, and lidar systemefficiency and geometry, etc.
Basic Assumptions for Lidar EquationIndependent scattering & Single scattering
Independent scattering: particles are separated adequatelyand undergo random motion so that the contribution to the totalscattered energy by many particles have no phase relation.Thus, the total intensity is simply a sum of the intensityscattered from each particle.
Single scattering: a photon is scattered only once. Multiplescatter is excluded in our consideration.
General Form of Lidar EquationNS ( ,R) = NL ( L ) ( , L , ,R) R[ ]
A
R2T ( L ,R)T ( ,R)[ ] ( , L )G(R)[ ] +NB
PS ( ,R) = PL ( L ) ( , L , ,R) R[ ]A
R2T ( L ,R)T ( ,R)[ ] ( , L )G(R)[ ] +PB
General Lidar Equation in and
NS ( ,R) =PL ( L ) t
hc L
( , L ,R) R[ ]
A
R2
exp ( L , r )d r 0R
exp ( , r )d r 0
R
( , L )G(R)[ ] + NB
( , L ,R) =d i ( L )
dni (R) pi ( )
i
T ( L ,R)T ( ,R) = exp ( L ,r)dr0
R+ ( ,r)dr
0
R
Volume scatter coefficient
Transmission
Physical Picture in Lidar Equation
T ( L ,R)
NL ( L )
T ( ,R)
( , L , ,R)
A
R2
( , L )G(R)
( , L ,R)
Illustration for LIDAR Equation
Physical Processes in LIDAR Interaction between light and objects
(1) Scattering (instantaneous elastic & inelastic):
Mie, Rayleigh, Raman, Brillouin scattering
(2) Absorption and differential absorption
(3) Laser induced fluorescence
(4) Resonance fluorescence
(5) Doppler shift and Doppler broadening
(6) Boltzmann distribution
(7) Reflection from target or surface
Light propagation in atmosphere or medium: transmission vs extinction
Extinction = Scattering + Absorption
( ,R) = i,ext ( )ni (R)[ ]i
T ( ,R) = exp ( ,r)dr0
R
Physical ProcessElastic Scattering
by Aerosols and Clouds
Elastic ScatteringBy Air Molecules
Absorption by Atoms and Molecules
Inelastic Scattering
Resonance Scattering/Fluorescence By Atoms
Doppler Shift
DeviceMie Lidar
Rayleigh Lidar
DIAL
Raman Lidar
Resonance Fluorescence
Lidar
Wind Lidar
ObjectiveAerosols, Clouds:
Geometry, Thickness
Gaseous Pollutants
Ozone
Humidity (H2O)
Aerosols, Clouds:Optical DensityTemperature in
Lower Atmosphere
Stratos & MesosDensity & Temp
Temperature, WindDensity, Clouds
in Mid-Upper Atmos
Wind, Turbulence
Reflection from Surfaces Target LidarLaser Altimeter
Topography, Target
Laser Induced Fluorescence Fluorescence Lidar Marine, Vegetation
Backscatter Cross-Section Comparison
Two-photon process
Inelastic scattering, instantaneous
10-30 cm2sr-1Raman Scattering
(Wavelength Dependent)
Two-photon process
Elastic scattering, instantaneous
10-27 cm2sr-1Rayleigh Scattering
(Wavelength Dependent)
Two single-photon process
Inelastic scattering, delayed (lifetime)
10-19 cm2sr-1Fluorescence FromMolecule, Liquid, Solid
Single-photon process10-19 cm2sr-1Molecular Absorption
Two single-photon process (absorptionand spontaneous emission)
Delayed (radiative lifetime)
10-13 cm2sr-1Atomic Absorption andResonance Fluorescence
Two-photon process
Elastic scattering, instantaneous
10-8 - 10-10 cm2sr-1Mie (Aerosol) Scattering
MechanismBackscatterCross-Section
Physical Process
Scattering Form of Lidar Equation Rayleigh, Mie, and Raman scattering processes are instantaneous
scattering processes, so there are no finite relaxation effects involved,but infinitely short duration.
For Rayleigh and Mie scattering, there is no frequency shift whenthe atmospheric particles are at rest, so
NS ( ,R) =PL ( ) t
hc
( ,R) R( )
A
R2
T2( ,R) ( )G(R)( )+NB
NS ( ,R) =PL ( L ) t
hc L
( , L ,R) z( )
A
R2
T ( L ,R)T ( ,R)( ) ( , L )G(R)( )+NB
L , pi ( ) 1, pi ( ) <1
For Raman scattering, there is large frequency shift, so
T ( L ,R)T ( ,R) = exp ( L ,r) + ( ,r)[ ]dr0R{ }
where
Fluorescence Form of Lidar Equation
NS ( ,R) =PL ( ) t
hc
eff ( ,R)nc(z)RB ( ) R( )
A
4 R2
Ta2( ,R)Tc
2( ,R)( ) ( )G(R)( )+NB
Tc(R) = exp eff ( , r )nc( r )d r Rbottom
R
= exp c( , r )d r Rbottom
R
Here, ( ,R) is the extinction coefficient mainly caused by theconstituent absorption.
c ( ,R) = eff ( ,R)nc (R)
Here, TC(R) is the transmission caused by the constituent extinction.
Resonance fluorescence and laser-induced-fluorescence are NOTinstantaneous processes, but have delays due to the radiativelifetime of the excited states.
Differential Absorption/Scattering Form
NS ( on ,R) = NL ( on ) scatter ( on ,R) R[ ]A
R2
exp 2 ( on , r )d r 0
z
exp 2 abs( on , r )nc( r )d r 0z
( on )G(R)[ ] + NB
For the laser with wavelength on on the molecular absorption line
NS ( off ,R) = NL ( off ) scatter ( off ,R) R[ ]A
R2
exp 2 ( off , r )d r 0
z
exp 2 abs( off , r )nc( r )d r 0z
( off )G(R)[ ] + NB
For the laser with wavelength off off the molecular absorption line
abs(R) = abs( ON ,R) abs( OFF ,R)
Differential absorption cross-section
Basic Architecture of LIDAR
Transmitter(Light Source)
Receiver(Light Collection
& Detection)
Data Acquisition & Control System
Transceiver(Light Source
Light CollectionLidar Detection)
Data Acquisition & Control System
Basic Configurations of LIDARBistatic and Monostatic
Bistatic Configuration Monostatic Configuration
APulsedLaser
z
z
R = c t 2
ReceiverTransmitter
Biaxial Arrangement
Coaxial Arrangement
Altitude Determination from Geometry
Bistatic configuration involves a considerableseparation of the transmitter and receiver to achievespatial resolution in optical probing study.
It originated from CW searchlight, and modulationwas used to improve SNR.
h =d tan( T ) tan( R ) +HT tan( R ) +HR tan( T )
tan( T ) + tan( R ) TR
HTHR
d
h
The range information isdetermined from geometryconfiguration, rather than thetime-of-flight.
Range Determination from TOF For nanosecond pulsed-laser lidar, the range is determined by the time
of flight of the photons propagating from lidar transmitter to the objectsand returning to the lidar receiver.
For atmospheric (scattering) lidar, the ultimate resolution is limited bythe pulse duration time, as atmospheric scattering has no distinct peak.
Altitude = Platform Base Altitude - Range ± Interference of aerosols and clouds
R= c t /2 R= c t /2 For target lidar (e.g., laser altimeter), the distinct peak due to the
strong reflection of light from surface or target, the range resolution canbe significantly improved by digitizing the return pulse and compare shape.
t is pulse width