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CONTENTS
1) INTRODUCTION
2) PRINCIPLE OF LIDAR
3) LIDAR INSTRUMENTATION
4) LIDAR DESIGN
5) TYPES OF LIDAR
a) DIAL
b) VIL
c) GALE
d) RAMAN LIDAR
e) HSRL
6) LIDAR APPLICATIONS
7) LIDAR/RADAR COMPARISONS
8) CONCLUSION
9) REFERENCES
ABSTRACT
‘LIDAR’ is the acronym for Light Detection & Ranging. Since the instrument
detects and radiates we can simply locate it to RADAR ie, radio detection and ranging.
Both have the same working principles, but the only difference between these two is that
in the medium used for ranging and detecting. In LIDAR laser is used in the same way as
radio waves in RADAR.
LIDAR popularly known as the Laser-Radar uses lasers as the transmitting
materials. The laser is back scattered by the target object and the received signal gives
not only the range of the object but also gives us an idea about the composition of the
object, which is got from its light scattering properties.
The LIDAR technique for detecting and ranging was widely recognized only after
the discovery of Ruby Laser by Miaman in 1960. Rapid developments since then in the
laser technology, opto electronics and fast signal processing devices have resulted in
establishing the potentials of LIDAR’S for a wide variety of atmosphere studies.
Different types of LIDARS are being used depending on the type of scattering processes
and for type of signal.
INTRODUCTION
LIDAR is Light detection and ranging, and is a laser remote sensing
technique used in both science and industry. It is the optical equivalent of the microwave
radar, and so is often reffered to as Laser Rader. Lidars are used to precisely measure
distances and properties of far away objects.
A LIDAR basically consist of transmitter, receiver and detector. The
LIDAR’s transmitter is a laser, while its receiver is an optical telescope. When a short
monochromatic light pulse is transmitted into the atmosphere, it will be back scattered by
both air molecules and suspended particles. This back-scattered light is collected by a
telescope close to the transmitter and detected by a suitable opto electronic detector. The
detected signal can then be processed suitably or stored for later processing.
Modern LIDAR systems combine the capabilities of radar and optical
systems to allow simultaneous measurement of range, velocity, temperature, reflexivity,
azimuth and elevation angle. These six dimensions of target information can be utilized
in fire control and weapon system applications to allow target acquisition, tracking,
classification and imaging. The modulation capabilities of microwave radar systems can
be applied to laser transmitters to allow accurate target measurement and time/frequency
gating of atmospheric terrain background clutter. The optical resolution associated with
laser systems results in a very small angular beam width to allow imaging, aim point
assessment precise target tracking and autonomous operation.
BASIC PRINCIPLE OF LIDAR
In RADAR, radio waves are transmitted into the atmosphere, which scatters
some of the power back to the Rader’s receiver. A LIDAR also transmits & receives
electro-magnetic radiation; but at a higher frequency, and operate in the ultra violet,
visible and infrared region of the electro magnetic spectrum. The LIDAR is also
popularly known as LASER RADAR.
A LIDAR basically consists of a transmitter, receiver & detector. The LIDAR’S
transmitter is an optical telescope. When a short monochromatic light pulse is transmitted
into the atmosphere, it will be back scattered by both air molecules and suspended
particles. This back-scattered light is collected by a telescope close to the transmitter and
detected by a suitable opto electronic detector. The detected signal can then be processed
suitably or stored for later processing.
THE SCATTERING PROCESS
In a typical LIDAR experiment laser light is transmitted into the atmosphere.
The beam can interact with the atmosphere in a number of ways.


Laser light can be scattered elastically (ie; no change in wavelength or colours)
from molecules in the atmosphere (Rayteigh scattering) and from particles (Mic
scattering).
Laser light can be scattered elastically from molecules in the atmosphere. In this
instance the wavelength of scattered light is shifted and the change in wavelength
depend on the molecule that scattered the light. This is known as RAMAN
Scattering. LIDAR in this case is RAMAN LIDAR

A specific molecule in the atmosphere can absorb laser light. In small molecules,
the molecule at the same or at other wavelength can then radiate this absorbed
energy. This process is known as Fluorescence. Under certain circumstances this
can be particularly sensitive and specific.
LIDAR INSTRUMENTATION
The main components of the LIDAR are,
 A solid state ‘LASER’ which produce brief pulses of high intensity
light.
 An ‘Optical Telescope’ which directs the laser emission into the sky,
and collect the light back scattered by atmosphere.
 A ‘Detector’, which analyses the back, scattered light.
LASER
Laser is the abbreviated form for ‘Light Amplification by Stimulated Emission of
Radiation’. As the name indicates process of stimulated emission is used for the
amplification of light. Different kinds of lasers are used depending on the power and
wavelength required. The laser may be either continuous wave or pulsed. Some important
lasers are given below.

Solidstate laser
: Ruby laser, Nd: YAG

Gas laser
: He-Ne laser

Liquid laser
: Dyes laser

Semi conductor laser
: Diode laser
High voltage electricity causes the quartz flash tube to emit an intense
burst of light, exiting some of the atoms in the Ruby crystal to high level.
At a specific energy level, some atoms emit particles of light called
Photons. At first the photons are emitted in all directions. Photon from one atom
stimulates emission of photon from other atom and the light intensity is rapidly
amplified. Mirrors at each end reflect the photons back and forth, continuing this
process of stimulated emission and amplification. The photons leave through the
partially silvered mirror. This is laser light. For some LIDAR applications more than
one laser is used. The final output of both channels of the transmitter is pulsed with
repetition rate of 20 times/second and the pulse width is about 7ns.
OPTICAL TELESCOPE
Usually the LIDAR uses the same optical telescope as transmitter and
receiver. It uses an optical switch, to switch the LIDAR between transmit and receive
mode, by a high speed (2400 rpm) mechanical shutter. The rotation of the shutter is
phase locked with the firing of laser so that the out going light passes through one of
five symmetrically placed apertures. While light backscattered from ranges in excess
of 10 km is directed into the direction system by the reflective skyward surface of the
shutter when it rotates into the received beam. To prevent the saturation of the
detection system by the strong signal returned from the lower atmosphere, the shutter
apertures are shaped to allow the portion of the received light reaching the detection
system to increase with the range over the interval of 10 km to 18 km.
The receiving system records the scattered light received by the receiver
at fixed time intervals. LIDARS typically use extremely sensitive detectors called
photo multiplier tubes to detect back scattered light. Photo multiplier tubes convert
the individual quanta of light, photons, first into electric currents and then into digital
photo counts that can be stored and processed on a computer.
DETECTOR : SPECTROMETER
The detectors used are usually spectrometers, which also function as
optical filters. They rejecting unwanted optical signal so that only the required wave
length of light is analyzed.
A typically used spectrometer is F.P.S [FABRY PEROT
SPECTROMETER]. The F P S consists of two etalons 0.3 nm band pass interference
filter and a cooled photo multiplier detector. Each etalon consists of pair of extremely
flat glass plate placed parallel to each other. The facing surface is coated with a
partially reflecting layer. Each etalon act as a filter with a pass-band that is periodic
in wavelength. In order to maintain the stability of etalons a high level of thermal and
vibration isolation has been incorporated in the design.
The fixed and scanning wavelength modes of the F.P.S will allow the
measurement of the total power of the backscattered signal. The response function of
the F.P.S will be obtained by the injection of light from the out going laser pulses to
the receiver.
Nd : YAG
LASER SYSTEM
It stands for “ Neodymium: Yttrium Aluminium Garnet ”.
It is a solid-state laser in which energy levels of Neodymium ion take part in laser
emission.
Four Level System
It is a four level laser system. Level 1[E1] is the ground level while E2,
E3, E4 are excited levels of system. Atoms in level1 are excited to level4 from which
they make a non-radiative transition to the level 3. Level 3 corresponds to upper laser
level and is a metastable state having longer lifetimes. The transition from level3 to
level 2 forms the laser transmission. The level 2 has very little lifetime, so that the
atoms in level 2 are quickly moved to level1. This maintains population inversion
between level3 and level 2.
The main pump lines for the excitation of the ion have 8100A and
7500A wavelength and the pumping is done using krypton arc pumps. The laser has
the wavelength of 10600A. The difference between lower laser level and ground
laser level is 0.26 eV, the ratio of its population to that of ground state at room
temperature is very small. So the lower laser level is unpopulated.
It finds applications in range finders, resistor trimming micro matching
operation, welding and hole drilling.
TYPICAL LIDAR BLOCK DIAGRAM
In this configuration the laser is modulated to provide information to the transmitted
signal, which is coupled through the interfero meter, optics and scanner to illuminate the
scan field of interest. The received signal is coupled, via reciprocity through the interfero
meter, to the receiver detector where it is mixed with a sample of the laser signal in the
form of a local oscillator.
The receiver output is processed by the signal processor to extract target
information and then processed by the data processor where all information is compiled
to provide target position, range, velocity and an image. The back scattered Dopplershifter target signal was then processed in a surface acoustic wave signal processor,
recorded on tape and subsequently played back on the ground through a CRT display.
The receiver assembly is used to collect back-scattered light and detector, which
converts the incoming light stream into electrical impulses. Photodiodes based on silicon
do not respond at all in the eye-safe region (1.55m). It is not possible to use InGaAs
photodiodes but they generally do not respond to the low levels of light that would be
anticipated when the laser beam scatters from a very distant point. We require a photo
detector that both responds and has sufficient gain so that the electrical signals are
measurable.
InGaAs avalanche photodiodes (APDs) fit this bill. These semi conductors are
engineered so that when a 1.55m photon hits them, an electron is efficiently created. In
an APD this electron is then accelerated across a barrier by means of an applied voltage
where it induces other electrons to follow. One photon therefore, results in a cascade of
electrons and hence a measurable current.
One unfortunate draw back of InGaAs avalanche diodes is due to the device’s
ability to store electrical charge; its high capacitance. This capacitance induces noise in
the first stage of electrical amplification. This is a fundamental issue, which affects the
ultimate range of the LIDAR system.
We use a 22 diameter InGaAs APD which minimizes the effect of devices
capacitance but which requires innovative optical design to effectively couple 10 inches
of collected light onto a very small-area detector.
For the MTI radar aspects of the system, Doppler filters are used and the surface
acoustic wave delay line processor is programmed to give an MTI cue, in real time, when
a Doppler return greater than 5knots is indicated in adjacent pixels. Additionally, it may
be noted that several target clutter intensity spots cause false alarm indications, which are
distributed through the scene. Altering the target detection algorithms to require an “N
detection out of M trials” detection approach results in significant false alarm reduction.
LIDAR DESIGN
The design of basic LIDAR system is illustrated above. To begin a powerful laser
transmits a short and intense pulse of light. The pulse is expanded to minimize its
divergence, and is directed by a tilted mirror into the atmosphere. As the pulse travels
upwards it is scattered by atmospheric constituents and aerosol particles. Light that is
back scattered and in the field or view of a telescope receiver is collected and channeled
towards the detector by a fibre or other optic. Filters are used to eliminate light away
from the laser’s wavelength, and a mechanical shutter blocks the intense low level
returns. The amount of light received is measured as a function of time (or distance)
using sensitive photo detectors, and the signals are digitized for storage on a computer. A
timing unit performs co-ordination of the experiment. When each laser pulse exits the
atmosphere, another pulse is transmitted and the process is repeated.
As the speed of light ‘c’ is well known, the time of flight ‘t’ from the laser to
the scattering volume at altitude ‘z’ and back to the detector is given as t=2z/C. Thus, as
the detected lights recorded in sample bins the detector records the time since the laser
fired.
Echos that are detected soon after the laser fired are from low altitudes, while
echos that are detected later are from higher altitudes.
All photons arrive in the range
are stored in one range bin, where Dt is
the temporal resolution of the measurement. A single profile is recorded each time the
laser fires. The single shot profiles are added together at each altitude to build up the
signal at each altitude. If a laser fire at a repetition rate of 100 pulses/sec then if we add
upto 1000 shots to record a single statistically significant profile the temporal resolution
of the measurement ‘Dt’ is 10sec.
The power incident on a LIDAR receiver as a result of scattering from an
atmosphere containing particular matter is given by:
Where the optical depth, is defined as
T(r) =
Where,
e (r )dr
Eo
C
Ar
r
a
m
e
-
Laser pulse energy in joules
Speed of light in m/sec.
Collecting area of the Receiver in m²
Range of the scattering volume in mts.
Aerosol scattering cross section /unit volume in m
Scattering cross section/unit volume for air molecule
- The extinction cross-section per unit volume
= Back scatter phase function in str
LIDAR SYSTEM FOR ATMOSPHERIC MEASURES
DIAL – Differential Absorption LIDAR
Different types of physical processes in the atmosphere are related to
different type of light scattering. Choosing different types of scattering processes
allows atmospheric composition temperature and wind to be measured. DIAL
systems are used for the study of the atmospheric composition. DIAL systems are
based on the fact that the absorption of light by the atmosphere is different at
different wavelength.
In this measurements are made by two different wavelengths. One
wavelength (I on) is chosen in region of high absorption crossection of the gaseous
constituent under study, where as at the second wavelength, (I off) the gaseous
absorption should be minimum.
DIAL transmits short pulses of laser light into the atmosphere. The laser
beam loses light to scattering and it travels. At each range some of the light is
backscattered into the detector because the light takes longer time to return from the
more distant ranges, the time delay of the return pulses can be converted into the
corresponding distance between the atmospheric scatterer and lidar.
The end result is a profile of atmospheric scattering Vs distance. Analysis of
this signal can yield information about the distribution of aerosols in the atmosphere.
The amount of backscatter indicates the density of scatterer. This can be used to
measure cloud base height or track plumes of pollution.
Other properties of atmosphere can also be deduced from LIDAR return
signals. A frequency shift in the light because of Doppler effect permits the
measurement of wind speed. By detecting the depolarization, one can discriminate
between liquid droplets and non-spherical ice particles. DIAL is also be used to
measures the concentration of atmospheric gases.
Ozone Measurement Using DIAL
In this technique two different laser beams are transmitted vertically into
the atmosphere. The laser is tuned between spectral regions of high and low
absorption. One has the wavelength 308nm absorbed by ozone and other 351nm,
which is not. These two beams scattered elastically by molecules and particles, and a
30” telescope collects the back-scattered light. The 308nm signal falls of much more
quickly than the other due to ozone absorption. The difference in absorption of light
at different wavelength can be used to determine the amount of ozone. Ozone
concentration as a function of altitude can be extracted from a ratio of the two
backscattered signals.
VIL – Volume Imaging LIDAR
VIL is an elastic aerosol backscatter LIDAR designed to image the
four-dimensional structure of the atmosphere. It can measures formation of clouds, it
also gives the aerosol concentration.
Schematic Diagram of VIL
The transmitter of a VIL employs a pulsed Nd: YAG laser. The
receiver consists of a telescope, interference filter and avalanche photo diode.
Scanning is performed using a best computer controlled beam steering unit consist of
two flat rotating mirrors mounted at 45 degree on the on the optical axis of the
transmitter receiver system.
The VIL transmits a small diameter (0.3cm) beam of light at a
wavelength of 1.06 microns out into the atmosphere. As the beam travels through the
atmosphere its light is scattered by aerosol particles that fall in the path of the beam.
A portion of the beam is scattered back towards the point of origin where a telescope
is located that focuses the return light on to a photo diode. The signal is then
digitized with a high-speed digitizer and stored along with timing information. The
amount of light that comes back to he telescope is proportional to the number of
aerosol particles at a particular location in space. The scanning of Volume Imaging
LIDAR can provide information about the atmosphere’s optical properties. It can be
used to make 3D maps of aerosol structure, it can measure wind speed and direction
and can be used to study atmospheric flows.
GALE: An Advanced Wind LIDAR
GALE stands for Giant Aperture Lidar Experiment. GALE measures
wind and temperature using resonance flourescence scattering. Resonance
fluorescence scattering means that when sodium atoms are illuminated at a precise
wavelength (589nm) they become excited and radiate light. The LIDAR’S receiver
measures a fraction of this light.
By adjusting the wavelength of the transmitted signal by a tiny
amount, the shift of the spectral time from its central wavelength can be measured.
The shift in the central wavelength is called Doppler shift and the wind is determined
from the Doppler effect. Wind measurement by GALE shows the dynamic structure
of the upper atmosphere due to wave activity.
Temperature is measured by using sodium resonance- fluorescence
scattering and by using Reyleigh scattering from air molecule. This Reyleigh
scattering is responsible for the blue sky.
The LIDAR equation relates the number of received photo counts to
the atmospheric density by equation
P(z) =
scattering and the transmission of atmosphere.
‘z’ is the height and N(z) is the number of photo counts at each height. The number
of photo counts received by a lidar depends inversely on the square of the altitude.
From the given altitude the solid angle is equal to the area of the receiving telescope
divided by the square of the altitude. The transmitted beam is not perfectly parallel,
so the area illuminated grows with height. Since the number of molecules illuminated
by a laser at a given height is both large (due to relatively large size of laser spot,
about 2000m2 at 100km altitude) and relatively uniform. The z² factor in the
conversion from photo counts to density makes the illuminated area constant with
height.
The temperature can be found by using the two methods.
1)

g = Acceleration due to gravity
A = Area of transmitting beam
Z = Height
2) Using the ideal gas law
This law is the empirical relation between 
Ie ; P = RT
Where ‘R’ is the universal gas constant.
Ie; 287 J/Kg for air
Combining the above equations we get the relation to the temperature
measurement.
RAMAN LIDAR
Raman LIDAR is based on the process of Raman scattering. This LIDAR
is in elastic scattering processes. This means that there is an exchange of energy
between scattered photon and the scattering molecules. Raman scattered light is
shifted a different amount by different types of molecules. This allows the type of
scattering molecules to be identified from the wavelength of the scattered light. This
allows us to measure the photons from specific molecule in the atmosphere. Raman
scattering is a very weak process and the signal can be two or four orders of
magnitude, weaker than elastic backscattered signal. Also the weak scattering crosssection typically limits Raman LIDAR to nighttime measurement at ranges of less
than 10Km. To increase the Raman signal and make daytime measurement, high
power LIDAR systems have been developed to operate wavelength from 248.5nm to
268.5nm.
High Spectral Resolution LIDAR (HSRL)
The HSRL measures optical properties of the atmosphere by
separating the Doppler broadened molecular backscatter return from the unbroadened
aerosol return. The molecular signal is used as a calibration target, which is available
at each point in the LIDAR profile. This calibration allows unambiguous
measurement of aerosol scattering cross-section, optical depth, and backscatter phase
function also measurements of depolarization and multiple scattering can be
performed. The HSRL has a significant signal strength advantage over the Raman
technique. Another advantage of the HSRL is that it can provide daytime
measurements while sky noise background limits the measurement of the weak
Raman signal to nighttime.
The fully developed HSRL employs an iodine absorption filter
instead of a high-resolution etalon. The spectrum of the electronic transition in
molecular iodine has more than 22000 absorption lines in the visible lengths, and 8
of them are easily reached by thermally turning a frequency doubled Nd: YAG laser
output. Compared to barium, the advantage of iodine is that instead of requiring a
dye laser, a narrow bandwidth, frequency doubled Nd: YAG laser can be used. The
received backscatter signal is divided into two channels. One channel detecting the
sample from the total backscatter spectrum and the other channel the spectrum
filtered by the iodine absorption filter. This signal contains the information about the
wings of the molecular spectrum and a small aerosol cross-talk signal.
The measurement shows that, the use of an iodine absorption filter
enables accurate measurements of cloud optical parameters. Because of cross talk
between channels can be accurately corrected because the 160 rad field of view of
HSRL effectively suppress multiple scattering phase function possible.
LIDAR APPLICATIONS
Weather Forecasting
Clouds
Clouds are formed by the ascent of moist air in the lower atmosphere.
Different types of clouds form at different altitude and are an indicator of atmosphere
stability. The arrival of different types of clouds helps us to protect the next day’s
weather.
Stratospheric Aerosols
The atmosphere contains a small proportion by volume of microscopic
suspended particles generally refered to as aerosols. These particles play a major role
in atmospheric chemistry. They help in the cooling of lower atmosphere and assist
the destruction of ozone. A portion of the stratospheric aerosols is natural in origin,
coming from sources such as meteor ablation and volcanic events. The remainder of
the anthropogenic origin, coming from sources such as CFC emission and from high
altitude aircraft exhausts.
There are many applications of LIDAR technology that are used in both science
and industry.
 Atmospheric science

Dynamic measurements: temperature, winds and waves.

Climate measurements: clouds, aerosols & water vapour.

Ozone measurements: depletions and polar stratospheric clouds.

High altitude trace metal measurements: sodium and potassium.
 Astronomy

Sodium layer guide stars for adaptic optics.

Planetary surface relief mapping.
 Topographic mapping

Erossion monitoring.
 Bathymetry (underwater mapping)

Harbour profiling for marine safety.
 Forest ground and canopy measurement

Used to access forest growth and health
 Building and factory construction

Measurement
allow
for
precise
prefabrication,
improving
efficiency and reducing cost.
 Mine shaft mapping

Allows cavern monitoring for worker safety.
 Aircraft docking

For safe aircraft maneuvering near airport terminals.
 Automobile speed monitoring

A replacement for hand held radar guns.
OTHER LIDAR APPLICATIONS
LIDAR GUN
LIDAR GUN is also known as police lidar. This is used to point out the
vehicles and detect their speed. It is a great use to traffic regulation. It is used for
checking the speedy vehicles from distances. The basic principle is that the laser is
used to aim the moving object and the reflected light is used to calculate the speed.
AIR BORNE LIDAR
LIDARS have been used extensively in air borne systems, mainly for
getting depth of a water body and for terrain mapping.
Air borne LIDAR is an aircraft mounted laser system designed to
measure the 3D co-ordinates of a passive target. The LIDAR operating principle uses
pulsed light to measure the variations in surface features. The distance from the
aircraft to the ground features is determined by measuring the elapsed time between
the generation and return of each laser pulse. These laser pulses are stored on a hard
drive for a post mission processing. From the processed data a 3D model of the earth
surface is created.
LIDAR/RADAR COMPARISONS
LIDAR system synthesis, performance evaluation and analysis established that
optical radar techniques had many wavelength related advantages and some
disadvantages. Large information bandwidth and extremely high angular resolution
possibilities are some of the advantages of LIDARs over Radars, while low efficiencies
and atmospheric propagation limitations are some of the disadvantages.
EMERGING TRENDS AND TECHNOLOGY
LASER technology is a vast field involving optical communication
and remote sensing, behind the drive to put more lidar in space lies the continuing
development of solid state lasers. With the advancement in laser system, improved
and effective lasers have been developed which improvise on the exciting system.
New semi conductor solid state lasers have led to improved performance of Lidar
systems employed in remote sensing.
The MARS POLAR LANDER has a lidar based system using,
which it will be possible to measure and completely analyses the Mar’s atmosphere
for the first time. This is an important landmark in space exploration.
CONCLUSION
It has been shown that LIDAR can be used for atmospheric
research in quite different application. Mainly this is related to detailed process
studies, where advantage is taken of the capability to observe the vertical distribution
of key constituents with rather high resolution. Although most measurements are
quite demanding, the technique has been developed to a stage where high accuracy
can be achieved. In the case of ozone measurements this can be done in a kind of
routine operation, already in the case of water vapour or temperature. Some further
development is required to optimize the performance and to achieve the routine
operation.
The use of LIDAR has not been limited to atmospheric studies but
with many space applications being planned to put into use. This will help us in
exploring the unexplored and unexplained phenomenon in our solar system.
REFERENCES
1) “Laser Radar Systems and Techniques” by Christian G. Bachman.
2) “ Laser Radar Systems” by Albert V. Jalalian.
3) “ LIDAR- Principle and Operation”.
4) “ Exploring the atmosphere with LIDAR” by R. Sica.
5) http:// www.noaa.com
6) http:// www.lidar.com