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