Operating principle, capabilities,
completed projects and results
Principle of operation
The acronym LIDAR is composed of the words Light Detection And Ranging. As a
complete analogy to a radar, where the original wavelength has been replaced by the
shorter one, from microwaves - wavelength meters to centimetres down to optical
region of hundreds to thousands of nanometer wavelength. Although the first Lidar
experiments have been carried out using the electrical spark as a light source , the
real development of Lidar techniques took place after the laser has been invented in the
The principle of the Lidar is as follows: the short powerful light pulse is generated
and through transmitting optics is aimed to the desired direction. The optical signal
propagates in the media, which is to be examined. The backscattered light is collected
by the receiving optics, detected by the detector, converted to the electrical signal and
adequately processed. The block scheme of the single frequency Lidar is on Figure 1.
Single frequency Lidar
Figure 1 Block scheme of the Lidar
Considering the knowledge of the temporal profile of the received signal and the
speed of light, one can evaluate the backscattering characteristics of the medium, in
which the signal was propagating. In the simplest case: if the signal is propagating
through non-scattering medium and is reflected by the solid, non-transparent target, the
distance D of this target from the transmitter may be evaluated by the simple formula:
where T is the measured propagation time and v is the (group) velocity of light in the
propagating medium, the factor 1/2 expresses the fact, that the signal is propagating
back and forth. In the first approximation, one can estimate the distance of 15
centimeters for each one nanosecond of propagation time.
1. Single frequency Lidar capabilities
The use of the short pulse lasers for rangefinding is known since early sixties.
The compact laser rangefinders have been developed for military applications soon after
Q-switched ruby laser discovery. The single frequency Lidar rangefinder has been used
in the project Apollo as an altimeter measuring the altitude of the spaceship above the
lunar surface. The first laser application for atmospheric remote sensing was the use of a
ruby laser to sound the mesosphere . Since that time, numerous projects of Lidar
remote sensing of the atmosphere appeared . In principle, Lidar is capable to
determine the profile of the backscattering / extinction coefficients of the atmosphere.
The height, propagation, relative density of the clouds and various atmospheric effect
may be determined.. Thus the single frequency Lidar is a good tool to monitor the
atmospheric transparency, horizontal visibility conditions, lower clouds boundary
heights, vertical profile of thin clouds. Single frequency Lidar is capable to monitor the
solid scattering particles in the atmosphere: the dust, air polluting solid particles etc. All
these potential capabilities have been widely used in numerous applications, some of
them will be described bellow.
The scattering of the laser light in the atmosphere is an elastic interaction process
and, in principle, any laser wavelength from ultraviolet to the infrared ranges. However,
the scattering efficiency and extinction properties of clouds, atmosphere molecules,
pollutants and aerosol particles are wavelength dependent. The choice of the laser
emission wavelength and of the laser source thus requires a trade-off which includes
such parameters as “atmospheric transparency windows”, laser and detector efficiencies
and eye safety requirements. Despite the laser wavelength selection, the single
frequency Lidar is unable to resolve between different scattering substances of close
scattering cross-sections. Certain a-priory knowledge about the examined medium must
exist before the single frequency Lidar data are interpreted.
2. Lidar energy budget link
Let us assume the case, in which the light signal is propagating in a non scattering
medium and hitting a solid target. Then, the echo signal strength may be evaluated on
the basis of the radar equation . Let us consider the simplest case: Lidar echo from a
solid target, the medium between the Lidar and a solid target is not absorbing and not
scattering the light. In such a case the echo signal strength may be expressed using the
simplified formulas for two different cases:
for the case, when the entire laser beam transmitted hits the target :
N E r
and for the case, when the target is small, only the part of the laser is hitting the target.
N E r
where N … number of photoelectrons
E … transmitted energy
… overall efficiency of the instrument
… divergence of the transmitted laser beam
R … distance of the target
r … target reflectivity
In practice the formula (2) describes the Lidar echo signal strength in most remote
sensing cases, namely in atmosphere, clouds, ocean, ice, soil and vegetation remote
sensing and in rangefinding of large targets. The formula (3) is applied mostly in
rangefinding of very distant objects.
Concluding: the echo signal strength is proportional to transmitted energy, reflectivity of
the target, overall efficiency of the system. For the case (2) the signal strength decreases
with the square of the range, for the case (3) the signal is decreasing with the power of
four of the range, fortunately, the echo signal strength is increasing with inverse value of
the square of the laser divergence in this case. The estimates listed above have been
made for the cases, when the extinction coefficient of the propagating medium is
negligible in comparison to the backscattered signal from the target of interest.
For the general case, when the signal is passing the medium characterized with the
scattering and extinction the Lidar echo signal strength profile N(R) may be expressed
in the form
N ( R) E 2 ( R) exp 2 ( x )dx
where N(R) … number of detected photoelectrons
(R) … scattering parameter of the medium
(R) … extinction parameter of the medium
where (R) denotes the range R dependence. The exponential term on the right hand side
expresses the fact of extinction - signal attenuation when propagating in the medium.
The factor two is included because of the signal passes the medium back and forth.
3. Single and multi-photon approach
In general, the echo signal intensities in Lidar are extremely low. This leads to high
requirements on the laser output power, to the size of the echo signal collecting
telescope and on the sensitivity of the detector. Two different approaches to echo signal
detection may be applied: multi-photon detection and a photon counting approach. In a
multiphoton approach, the optical signal is detected by the photodetector ( photodiode
or photomultiplier), its output is a electrical signal, its amplitude temporal profile is
corresponding to the detected optical signal temporal profile. This electrical signal is
consequently processed: amplified, filtered, digitized, etc. From each measurement,
each Lidar laser fire, the complete temporal profile of the echo signal is obtained.
However, the amount of information available from one measurement is limited by the
entire signal strength, sensitivity of the detector, gain and noise of the signal processing
circuits and last but not least by the quantum nature of the light. Attempting to analyze
the Lidar signal from distant targets, the echo signal will split to the contributions of
individual photons. In such a case a different approach must be applied - the photon
counting approach, in which the information is acquired on the basis of repetitive
measurements and applying the statistical treatment of the data set.
Each the approach has its own advantages and limitations.
The multi-photon approach is providing a large amount of information from each
measurement-laser shot. In principle, all the information expected from the Lidar
measurement may be obtained in s single shot measurement. The availability of low
noise FET amplifiers, fast analog to digital convertors with high dynamical range, high
digitizing speed and fast data processing computers and algorithms permit to acquire
large volumes of data in short times. On the other hand, multi-photon systems require
high power lasers and large optical telescopes to accomplish the multi-photon echo
signal strength; the operation of such systems might represents usually serious optical
hazard due to high energy densities transmitted.
The photon counting approach is providing lower amount of information in each
measurement. In most set-ups, maximum one photoelectron may be detected in one
laser fire. The Lidar information, the temporal echo signal profile, may be obtained by
repeating the measurement and accumulating the data: the times of registration of
individual photoelectrons. The most important difference to the previous approach is the
fact, that the signal strength may be lower by several orders of magnitude. Applying the
high quantum efficiency photon counters with low dark count rate recently available,
one can detect the useful signals with the average signal strength much lower than 1
photoelectron per measurement. This is not a violation of the quantum theory. The
expression “average echo signal strength 0.01 photoelectron” has to be understood in
such a sense, for example, that in one thousand measurements ten photons of interest
have been registered. The main advantage of the photon counting approach is the
capability to detect extremely low signals, thus enabling the use of low power laser
transmitters and small size optical telescopes. Thus, the photon counting systems may
be small, lightweight, low power consuming. Thanks to the low power laser
requirements, the diode or diode pumped lasers may be used. The main disadvantage is
the necessity to repeat the measurement many times, what means, the average value
over the whole measuring time interval is detected. This fact represents a serious
limitation for applications in which the target characteristics is changing very fast in
comparison to the entire measurement interval. The typical example of application not
suitable for photon counting Lidar is the sounding of clouds from the space orbiting
As a conclusion: most of the Lidars developed and operated up to now are operating
in the multi-photon operating mode. Thanks to the fast development in the solid state
photon counting devices, diode lasers, fast electronics and data processing, the photon
counting Lidar is a promising option for the future. [4,5,6]
4. Photon counting Lidar for the USSR / NASA space mission to MARS
The Czech Technical University has been involved in research and development of the
Lidar for the space applications in the missions to the planet Mars since 1989.
Following our contribution to the Soviet mission PHOBOS, our group has been invited
to participate in Mars projects, as well. The ultimate goals of the projects were:
* to establish an altimeter for the aerostat (balloon) probe on Mars,
* to monitor the atmospheric dust and clouds from the aerostat, nadir looking
* to monitor the atmosphere dust and clouds from the landing apparatus, zenith
The main limitations put on the instrumentation were : minimal mass, minimal
average power consumption, operating temperature range as wide as -100oC up to
+20oC, operation at the Mars atmosphere with a pressure in the range 5 to 10 mBars.
The mass and power restrictions on the proposed balloon probe were strict, the total
mass of the device must be kept bellow 900 grams, the average power consumption
bellow 30 mW ! It was planned, that the Lidar will operate mainly during the Mars
night, the night temperatures drop down to -100oC, the temperature variations and
vertical gradients during the day are large. Due to the mass and power restrictions, no
active thermal stabilization is feasible
The candidates for the project were :
flashlamp pumped compact laser Lidar / rangefinder (multi-photon),
photon counting Lidar
The microwave radar was not accepted due to mass and power requirements. The
additional limitation is, that it is not providing the atmospheric dust and clouds
The flashlamp pumped compact laser Lidar / rangefinder was considered to be the
most promising candidate. The device is existing, all the technology is well elaborated
for military applications. The existing devices fulfill the mass and power requirements
together with the altimetry operation up to 5 km. However, the high voltage needed for
laser flashlamp would cause serious problems in the Mars atmosphere. Additionally, the
device is not providing the information about the atmosphere.
The photon counting single frequency Lidar developed on the Czech Technical
University is fulfilling all the requirements: low mass, low average power, low
operating voltage, altimetry and atmospheric monitoring function. The device was
manufactured at the Space Research Institute, Moscow, Russia in 1992. Its photograph
is in Figure 2 and Figure 3.
Figure 2 Breadboard sample of the photon counting Lidar, right to left: receiving optics,
transmitting optics, control electronics board, battery power supply, PC for data
Figure 3 Flying module of the photon counting Lidar for planet Mars altimetry 0-5 km
and Mars atmospheric studies at 0 – 100 km altitude. Design CTU Prague, final
construction IKI Moscow.
The photon counting single frequency Lidar for the space mission to Mars was
constructed and deployed. Unfortunately, the problems at the rocket launch phase did
not permit the probe to reach the desired space orbit and the entire mission has been
The NASA Mars Surveyor Program has been developed to explore the planet Mars over
the decade 1997 to 2006. The main objective of the Mars „98 Lander Surveyor was to
observe the global distribution and time variation of temperature, pressure, dust, water
vapor and condensates in the Mars Atmosphere. The single frequency Lidar based on
solid state photon counting technology was one of the sensors onboard the Lander. The
aim of the Lidar experiment was to characterize optical properties of the atmospheric
particulates. It is fixed position, upward viewing instrument mounted on the upper deck
of the Lander. The scheme of the Lander configuration are plotted on Figure 4.
For this mission, the device developed for altimetry onboard USSR Mars probe was
used with only minor changes to adopt it for atmospheric studies – the signal processing
algorithms were modified.
The transmitter was based on the GaAlAs laser diode emitting 0.4 uJoule in 100
nanoseconds long pulses at the wavelength 880 nanomenters with the repetition rate of
2.5 kHz. The device was able to operate in two modes. In an active mode, laser pulses
are emitted and their echoes are registered in order to characterise ice and dust hazes in
the lowest layers of the atmosphere (<3km). During the passive sounding, sunlight is
used to characterize the optical properties of atmospheric particulates. The total amount
of the light scattered on the particulates above the Lidar will be monitored. During the
sunrise, the sun is illuminating the atmospheric layers from the top to the bottom
consequently. During the sunset, the situation is repeating in a reverse order. Thus,
recording the amount of the backscattered light in a vertical column above the device as
time series in the sunrise and sunset, assuming the experiment geometry, one can
evaluate the vertical profile of concentration of the scattering particles up to the highest
The application of the flying module of our photon counting Lidar demonstrated the
capabilities of photon counting approach to both planetary altimetry (low altitudes) and
planetary atmosphere studies.
5. Photon counting Lidar application in atmospheric studies
The Compact Lidar for meteorology and ecology has been developed under the roof of
the Czech Technical University and Grant Agency of Czech Republic grants in the years
The operating principle, main philosophy and block scheme of the device is the same
as for the space projects. The compact form, suitable for field application has been
developed, the device has been completed using commercially available components
whenever possible. The photograph of the prototype in the field tests is on Figures 2 and
4. The Lidar consists of the optical head containing laser transmitter, receiver with
filter and detector and the transmit and receive optics. The control unit consists of the
housing, power supplies, time-of-flight counter, gate circuits and the interface to the
computer. For the experiment control, data acquisition and processing, the personal
computer (PC) is used.
Figure 4 Photon counting Lidar for ground atmospheric studies, monitoring the air
pollution emitter from the Dvur Kralove cement plant.
Lidar has been used to monitor the presence and propagation of air pollutants in the
air atmosphere. The propagation of the smoke particles from the factories smoke
stacks and from the big Diesel engines mounted on the ship has been investigated. On
the Figure 5. the air pollution from the smoke stack monitoring is demonstrated, the
steel factory at Kraluv Dvur, Central Bohemia has been monitored from the distance
greater than 200 meters. The relative concentration of dust particles and their
propagation has been monitored by a series of consequent measurements. On Figure 6.
there is a plot of Lidar data acquired when ranging over the exhaustion pipe of the
passenger ship on the Volga river and the exhaustion from the factory pipe at Ribinsk..
The peaks in the data distribution corresponding to the solid particles content - the dust -
may be seen.
Figure 5 Air pollution monitoring at cement plant factory, the rangefinding of the pipe
(upper) and remote sensing of the polluted air (bottom) traces.
Figure 6 Air pollution monitoring near machinery factory, Rybinsk, Russia.
The photon counting Lidar has been used to measure the heights of the lower cloud
boundary. This parameter is of crucial importance for the air traffic control. The most
important is the measurement of the heights in the range 50-800 meters. On Fig. 7. there
is a plot of the Lidar results. The Lidar was pointed at the elevation angle 50 o, the
multiple layer density profile of the cloud may bee seen. The experiment demonstrates
the photon counting Lidar capability to monitor the lower clouds boundary for air traffic
control. The device has been tested at the Prague International Airport and compared to
the commercial meteo sensor made by Vaisala, Finland, the results coincided within
their resolution limits.
Figure 7 Lower cloud boundary measurement using photon counting Lidar, Prague
international airport, multiple cloud layer structure may be seen.
6. Photon counting Lidar - conclusion
The single frequency Lidar operating on a photon counting principle has been
demonstrated to be a powerful tool for applications in
ranging to solid objects / altimetry in the range 0 - 5 km,
lower clouds boundary heights monitoring in the range 0-1 km,
atmospheric backscatter - visibility - transparency monitoring in all the range,
air pollution monitoring,
space applications in the planet Mars atmosphere investigation.
In all the applications the device relative simplicity, low mass, low power
consumption, laser radiation eye-safety was of crucial importance.
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