Lecture 03. Lidar RemoteSensing Overview (1)

Introduction

History from searchlight to modern lidar

Various modern lidars

Altitude/Range determination

Basic lidar architecture

Summary

Introduction: Lidar LIDAR is the acronym of Light Detection And

Ranging - a laser radar in light frequency range.

Although lidar and radar share similar detectionprinciples, large differences exist in the physicalprocesses, the treatment approaches, and the systemhardware, due to huge frequency difference of theradiation used in lidar and radar.

Lidar uses the concept of photons, while radar usesthe concept of electromagnetic waves.

Lidar started in the pre-laser times in 1930s withsearchlight beams, and then quickly evolved to modernlidars using nano-second laser pulses.

History: Searchlight Modern Lidar

Bistatic Configuration

ReceiverCW

Light

Monostatic Configuration

APulsedLaser

z

z

R = c t 2

CW searchlight ns laser pulse

History: Searchlight Lidar Hulburt [1937] pioneered the aerosol

measurements using the searchlight technique, whophotographed the searchlight beam to 10 km.

Johnson [1939] followed a proposal of Tuve et al.[1935] and modulated the searchlight beam with amechanical shutter rotating at 10 cycles persecond. Scattering to a height of 34 km wasmeasured with good agreement between theory andexperiment above 8 km.

Elterman [1951, 1954, 1966] pushed theatmospheric study using searchlight to a high leveland made practical devices.

Lidar Started with Searchlight

ReceiverCWLight

Light Detection and Ranging (LIDAR) actuallystarted with using the CW searchlights to measurestratospheric aerosols and molecular density in1930s, well before the first (ruby) laser wasinvented in 1960.

Atmospheric aerosol anddensity measurementsusing searchlight tech.

Scattering light intensityis proportional to theatmosphere density inthe aerosol free region

Iscatter natmos

Searchlight

Transmitter(Projector)Angle fixed

Receiver(Collector)Anglescanning

h =d tan( T ) tan( R ) +HT tan( R ) +HR tan( T )

tan( T ) + tan( R )

h

d

T = 75o

R = 0o 57o

HT =1.39km

HR = 2.76km

d = 30.2km

h = 2.76km 35.3km

Photographing vs. Modulation-- DC detection vs. AC detection

B

B1

B2

S1

S2

IB1

IS1

IB2IS2

SNR=IS2IB2

SNR=AS2AB2

>>1

Although night-sky may still have quite strong background (DC), its AC component atthe modulation frequency is very small, while the searchlight is much stronger at themodulation frequency. Therefore, the AC detection of modulated searchlightdramatically improves the SNR, resulting in higher detection range.

Density measured by searchlight

CCD-Imaging Lidar with Similar Idea

Modern CCD-imaging lidarutilizes an similar idea asthe searchlight lidar.

The bistatic lidar seemsto have better near-fielddetection.

History: Modern Lidar The first laser - a ruby laser was invented in

1960 by Schawlow and Townes [1958] (fundamentalwork) and Maiman [1960] (construction).

The first giant-pulse technique (Q-Switch) wasinvented by McClung and Hellwarth [1962].

The first laser studies of the atmosphere wereundertaken by Fiocco and Smullin [1963] for upperregion and by Ligda [1963] for troposphere.

Following this, great strides were made both inthe development of lidar technologies/systems, andin the sophistication of their applications.

Modern Lidar: Atmosphere Lidar

The first application of lidar was the detection ofatmospheric aerosols and density. Basically, it is toknow whether there are aerosols/density in the regionsand how much. However, the composition of atmospherecannot be told, because only the scattering intensitywas detected but nothing about the spectra.

An important advance in lidar was the recognitionthat the spectra of the detected radiation containedhighly specific information related to the species,which could be used to determine the composition ofthe object region.

Modern Lidar Advancement

The broad selection of laser wavelengths becameavailable and some lasers could be precisely tuned tospecific frequencies. All these advancements enhancedthe effective spectral analysis of the returnedradiation from objects.

This ability added a new dimension to remote sensingand made possible an extraordinary variety ofapplications, ranging from groundbased probing of thetrace-constituent distribution in the tenuous outerreaches of the atmosphere (upper atmosphere lidar), tolower atmosphere constituents (differential absorptionlidar), to airborne chlorophyll mapping of the oceans toestablish rich fishing areas (fluorescence lidar).

Atmosphere Lidar

Mie Scattering From Aerosols

Rayleigh Scattering From Air Molecules

ResonantFluorescence

From Metal Atoms

RayleighScattering

Na Fluorescence

Range Determined From Time-of-Flight: R = c · t / 2

150 200 250 300 3500

20

40

60

80

100

120

MSIS90 Temperature

Temperature (K)

Alt

itude

(km

)

Troposphere

Stratosphere

Mesosphere

Thermosphere

Mesopause

Stratopause

Tropopause

Typical Atmosphere Lidar Profile

Modern Lidar: Target Lidar Besides atmosphere, our environment includes many

other things, like the solid earth, cryosphere,hydrosphere, and non-gas-phase objects on the earth, inthe ocean, and in the air (e.g., plants, oil, buildings) etc.Study of our environment demands good measurementtechnology and approach for measurements in all sorts ofoccasions. Therefore, lidar technology for target(anything other than gas phase objects) detection isessential and highly demanded.

Two main categories for target lidars: (1) lidars forranging (laser altimeter) and (2) lidars for speciesidentification (fluorescence lidar).

Laser Altimeter

The time-of-flight information from a lidar systemcan be used for laser ranging and laser altimetry fromairborne or spaceborne platforms to measure theheights of surfaces with high resolution and accuracy.

Downward-pointing laser systems were operated in amode where surface scattering and reflectionrepresented the dominant form of interaction.

The reflected pulses from the solid surface (earthground, ice sheet, etc) dominant the return signals,which allow a determination of the time-of-flight withmuch higher resolution than the pulse duration time.

Laser Altimeter

Lidar for Hydrosphere

Laser Altimeter and Ranging

The resolution is now determined by theresolution of the timer for recording pulses,instead of the pulse duration width.

By computing the centroid, the resolution canbe further improved.

Laser Altimeter vs Radar Altimeter

Much better resolution and precision

Fluorescence Lidar

A notable advance was made with the realizationthat use of a short-wavelength laser could broadenthe spectrum of applications, as a result of laser-induced fluorescence, and led to the development ofa new form of remote sensor “laser fluorosensor” or“fluorescence lidar”.

The fluorescence signal could indicate thepresence of high organic contamination and enablethe dispersion of various kinds of effluent plumes tobe remotely mapped.

Scenarios for Fluorescence Lidar

Aquatic monitoringVia folding mirror

VegetationMonitoring

AirborneFluorescence

Detection of Historic Monument

Altitude and Range Determination

Searchlight lidar and CCD-imaging lidar: determinealtitude through thegeometry calculation.

h =d tan( T ) tan( R ) +HT tan( R ) +HR tan( T )

tan( T ) + tan( R )

h

d

Altitude and Range Determination

Modern atmosphere lidars: Due to the use ofnanosecond pulse lasers, the range can bedetermined by the time-of-flight through equationR = C·t/2, where C is the light speed in the medium,t is the time-of-flight, and 2 for the round-trip ofthe photons traveled.

The ultimate resolution of range determination islimited by the pulse duration time. For example, a5-ns pulse gives 75 cm as the highest resolution foran atmospheric lidar where signals are continuous.

Ultimate resolution: R=C t/2

Altitude and Range Determination

Target lidar - laser altimeter: Distinct peakcoming from the reflection of surfaces allows amore precise measurement of the time-of-flightthrough rising edge or peak comparison, thusenabling higher resolution than the pulse durationlimitation.

For example, a laser altimeter using 5-ns pulseduration can have better than 5 cm resolution andaccuracy.

Basic Architecture of LIDAR

Transmitter(Light Source)

Receiver(Light Collection

& Detection)

Data Acquisition & Control System

Function of Transmitter

A transmitter is to provide laser pulses thatmeet certain requirements depending onapplication needs (e.g., wavelength, frequencyaccuracy, bandwidth, pulse duration time, pulseenergy, repetition rate, divergence angle, etc).

Usually, transmitter consists of lasers,collimating optics, diagnostic equipment, andwavelength control system.

Function of Receiver

A receiver is to collect and detect returnedphoton signals while compressing background noise.

Usually, it consists of telescopes, filters,collimating optics, photon detectors, discriminators,etc.

The bandwidth of the filters determineswhether the receiver can spectrally distinguishthe returned photons.

Function of Data Acquisition

Data acquisition and control system are torecord returned data and corresponding time-of-flight, provide system control and coordination totransmitter and receiver.

Usually, it consists of multi-channel scalerwhich has very precise clock so can record timeprecisely, discriminator, computer and software.

This part has become more and more importantto modern lidars. Recording every single pulsereturn has been done by several groups, enablingvarious data acquisition modes.

LIDAR Configurations:Bistatic vs. Monostatic

Bistatic configuration involves a considerableseparation of the transmitter and receiver toachieve spatial resolution in optical probing study.

Monostatic configuration has the transmitterand receiver locating at the same location, so thatin effect one has a single-ended system. Theprecise determination of range is enabled by thenanosecond pulsed lasers.

A monostatic lidar can have either coaxial orbiaxial arrangement.

Basic Configurations of LIDARBistatic and Monostatic

Bistatic Configuration Monostatic Configuration

APulsedLaser

z

z

R = c t 2

ReceiverTransmitter

Coaxial vs. Biaxial Arrangements

In a coaxial system, the axis of the laser beamis coincident with the axis of the receiver optics.

In the biaxial arrangement, the laser beam onlyenters the field of view of the receiver opticsbeyond some predetermined range.

Biaxial arrangement helps avoiding near-fieldbackscattered radiation saturating photo-detector.

The near-field backscattering problem in acoaxial system can be overcome by either gatingof the photo-detector or use of a fast shutter orchopper.

Biaxial Arrangement

Coaxial Arrangement

Summary

LIDAR actually started with CW searchlight usinggeometry to determine altitude. The invention of laserspushed lidar to a whole new level - modern laser remotesensing. The time-of-flight of a short pulse is used toprecisely determine range and altitude.

Modern lidars have various formats and utilize differentways to determine altitude and range precisely.

Basic lidar architecture includes transmitter, receiverand data acquisition/control system. Each has specialfunctions. There are bistatic and monostatic configurations,and coaxial and biaxial arrangements.