SIST ISO 28902-4:2026

• Abstract:

  This document specifies the determination of height-resolved profiles of atmospheric backscattering by means of active optical sounding. The measurements allow the following properties of the atmosphere up to several kilometres above ground to be derived: ; cloud bases; ; upper boundaries of optically thin clouds; ; upper and lower boundaries and internal structures of particle layers: ; height of structures, e.g. inversions, boundary layer height, mixing layer height (under suitable conditions); ; attenuated backscatter of the particles; ; particle backscatter and extinction coefficients (requires further assumptions). ; The document also addresses the depolarisation lidar and the use of multi-wavelength systems. This allows further parameters to be determined: ; particle size classification (Ångström exponent, colour ratio); ; shape classification (linear depolarisation degree). ; The following fields of application are particularly important: ; air quality monitoring (vertical structure of the boundary layer); ; aviation safety (cloud base and visual range) (see ISO 28902-1[8]); ; particle content and transport (e.g. volcanic dust); ; weather forecasting and climate modelling (e.g. atmospheric boundary layer, cloud base, cloud microphysics); ; satellite remote sensing (validation). ; Examples that illustrate these applications are discussed in Annex A. ; The benefits of scanning systems for parameters mentioned above are also discussed in Clause A.7. ; In addition, particle backscatter lidars that measure at least two carefully selected wavelengths can be used to determine atmospheric gas concentrations. This is known as the differential absorption lidar “DIAL” technique. This technique is not part of this document and has been described in VDI 4210-1[12]. ; This document does not specify extended lidar techniques that monitor the following parameters quantitatively: inelastic scattering effects such as, Raman scattering, Doppler broadening, Doppler shift, multiple scattering, modulation techniques, and spectral separation of molecular and particle backscattering [high spectral resolution lidar (HSRL)]. Some of these extended techniques are or will be described in other parts of the ISO 28902 series. ; This document does not address special features of airborne or satellite-borne systems.

• Status: Published
• Public Enquiry End Date: 19-Jul-2026
• Publication Date: 13-Sep-2026
• ICS: 07.060 - Geology. Meteorology. Hydrology
• ICS: 13.040.20 - Ambient atmospheres
• Technical Committee: KAZ - Air quality
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 20-Aug-2026
• Due Date: 25-Oct-2026
• Completion Date: 14-Sep-2026
• Ref Project: ISO 28902-4:2026 - Air quality — Environmental meteorology — Part 4: Ground-based remote sensing of meteorological parameters by particle backscatter lidar

Overview

SIST ISO 28902-4:2026 is an international standard developed by SIST under the ISO framework, focusing on air quality and environmental meteorology. This part of the ISO 28902 series details the use of ground-based remote sensing of meteorological parameters using particle backscatter lidar systems. The document outlines methods to obtain height-resolved atmospheric profiles, leveraging the capability of lidar technology to monitor and analyze atmospheric conditions several kilometers above the ground.

This standard is crucial in supporting consistent, high-quality atmospheric measurement practices, enhancing data quality for scientific, meteorological, and regulatory applications.

Key Topics

• Height-Resolved Atmospheric Profiles:
  Describes procedures for using particle backscatter lidar to determine vertical profiles of atmospheric backscattering, which can reveal crucial features such as:

  • Cloud bases and upper boundaries of optically thin clouds
  • Internal structures and boundaries of particle layers (e.g., pollutant layers, dust)
  • Key layer heights, like atmospheric inversions, boundary layer, and mixing layer

• Lidar-Related Atmospheric Parameters:
  The standard covers the measurement and inference of parameters, including:

  • Attenuated backscatter of particles
  • Particle backscatter and extinction coefficients (with specific assumptions)
  • Lidar ratio, backscatter ratio, and Ångström exponent for particle characterization

• Advanced Techniques:
  Addresses the use of:

  • Depolarisation lidar for particle shape classification (linear depolarisation degree)
  • Multi-wavelength systems to classify particle size (e.g., Ångström exponent, color ratio)

• Excluded Techniques:
  Does not include detailed methods for extended lidar techniques such as:

  • Differential absorption lidar (DIAL) - for gas concentration (referenced in VDI 4210-1)
  • Inelastic scattering methods (e.g., Raman, Doppler, HSRL)
  • Airborne or spaceborne lidar systems

Applications

• Air Quality Monitoring:
  Particle backscatter lidar systems are central to evaluating the vertical structure of the atmospheric boundary layer, critical for understanding and managing air pollution and the dispersion of pollutants.

• Aviation Safety:
  Real-time information about cloud base height and visual range supports safer flight operations and improves air traffic management, connecting with related standards such as ISO 28902-1.

• Atmospheric Research and Climate Science:
  The ability to detail cloud positions, vertical particle distributions, and boundary layer dynamics enhances weather forecasting and climate modeling efforts by providing reliable data on cloud microphysics and atmospheric structure.

• Dust and Particle Transport Tracking:
  Lidar systems can detect, identify, and track the movement of aerosols and particulates, including events like volcanic ash transport or desert dust intrusion, aiding in both public safety and scientific research.

• Satellite Remote Sensing Validation:
  Ground-based lidar measurements are essential for cross-verification and calibration of satellite-based remote sensing data, ensuring improved reliability and accuracy for large-scale earth observation systems.

Related Standards

• ISO 28902-1:
  Ground-based remote sensing of visual range by lidar systems; significant for aviation safety applications.

• ISO 28902-2 & ISO 28902-3:
  Methods for wind measurements using heterodyne pulsed and continuous-wave Doppler lidar.

• ISO 19926-1:
  System performance requirements for ground-based weather radar atmospheric measurements.

• VDI 4210-1:
  Addresses the DIAL technique for gas concentration measurements with lidar.

The comprehensive approach of SIST ISO 28902-4:2026 supports enhanced consistency in atmospheric lidar measurements, advancing air quality assessment, meteorological research, and operational safety in critical sectors. Its alignment with related international standards ensures integrated, high-confidence environmental monitoring across scientific, regulatory, and industrial domains.

SLOVENSKI STANDARD
01-oktober-2026
Kakovost zraka - Okoljska meteorologija - 4. del: Daljinsko zaznavanje
meteoroloških parametrov s tal z lidarjem na osnovi povratnega sipanja na delcih
Air quality - Environmental meteorology - Part 4: Ground-based remote sensing of
meteorological parameters by particle backscatter lidar
Qualité de l'air - Météorologie de l'environnement - Partie 4: Télédétection basée sur le
sol des paramètres météorologiques par lidar à rétrodiffusion des particules
Ta slovenski standard je istoveten z: ISO 28902-4:2026
ICS:
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
13.040.20 Kakovost okoljskega zraka Ambient atmospheres
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

International
Standard
ISO 28902-4
First edition
Air quality — Environmental
2026-08
meteorology —
Part 4:
Ground-based remote sensing of
meteorological parameters by
particle backscatter lidar
Qualité de l'air — Météorologie de l'environnement —
Partie 4: Télédétection basée sur le sol des paramètres
météorologiques par lidar à rétrodiffusion des particules
Reference number
© ISO 2026
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
4 Symbols and abbreviated terms. 4
4.1 Symbols .4
4.2 Abbreviated terms .6
5 Fundamental principles of the backscatter lidar . 7
5.1 General .7
5.2 Choice of suitable wavelengths .9
5.2.1 General .9
5.2.2 Overlap function .10
5.3 Attenuated backscatter coefficient .10
5.4 Backscatter coefficient .11
5.4.1 General .11
5.4.2 Determining the boundary condition for the derivation of the particle
backscatter coefficient .11
5.5 Particle extinction coefficient . 12
5.6 Optical depth . 13
5.7 Shape classification / linear depolarisation ratio . 13
5.8 Obtaining information on particle size distribution .14
6 Measured and target variables of the backscatter lidar . 14
6.1 General .14
6.2 Measured variables . 15
6.3 Target variables . 15
6.3.1 Backscatter profiles . 15
6.3.2 Identification of layers and their boundaries . 15
6.3.3 Cloud base, cloud penetration depth and vertical extension .16
6.3.4 Shape classification by analysing the linear depolarisation ratio .17
6.4 Lidar Ratio .17
6.5 Auxiliary variable .19
6.6 Definition of performance characteristics .19
6.6.1 General .19
6.6.2 Temporal resolution .19
6.6.3 Range resolution . .19
6.6.4 Spatial averaging . 20
6.6.5 Signal-to-noise ratio . 20
6.6.6 Operating range . 22
6.6.7 Completeness, POD and FAR . 23
6.7 Conventional ranges . 23
7 Systems and system components .24
7.1 General .24
7.2 Radiation source(s) .24
7.3 Transmitting and receiving optics . . 25
7.4 Filters . 25
7.5 Detectors. 26
7.5.1 General . 26
7.5.2 Photon counting . 26
7.5.3 Analogue data acquisition .27
7.6 Data acquisition, control system .27
7.7 Mechanical structure, infrastructure . 28

iii
7.8 Additional system components . 28
7.9 Typical lidar design . 28
7.9.1 General . 28
7.9.2 Bistatic and monostatic lidars. 29
7.9.3 Ceilometers .32
7.9.4 Single wavelength lidar systems .32
7.9.5 Multi-wavelength systems.32
7.9.6 Polarisation lidar . . 33
8 Measurement planning and site requirements .34
8.1 General considerations. 34
8.2 Safety. 35
8.3 Adjusting the measurement system to atmospheric conditions. 35
8.4 Limiting conditions for general operation . 36
8.5 Maintenance and operational test. 36
8.5.1 General . 36
8.5.2 Maintenance . 36
9 Factors and uncertainty considerations .36
9.1 General . 36
9.2 “Frozen” atmosphere . 39
9.3 Uncertainty considerations related to β and cloud base . 40
att
9.4 Uncertainty considerations relating to the particle backscatter coefficient .41
9.5 Depolarisation lidar uncertainty considerations .42
9.6 Effect of atmospheric pressure and temperature .43
10 Quality assurance and system monitoring .43
10.1 Adjustments .43
10.2 Functional tests .43
10.3 Maintenance . . 44
10.4 Calibration . 44
10.5 Quality assurance of the attenuated backscatter lidar .45
10.6 Performance tests in the laboratory or at test sites. 46
10.6.1 General . 46
10.6.2 Depolarisation . 49
10.7 Individual or specific parameter tests . 49
Annex A (informative) Examples and theory . 51
Annex B (informative) Mathematical considerations .62
Bibliography .65

iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5,
Meteorology, and by the World Meteorological Organization (WMO) as a common ISO/WMO Standard under
the Agreement on Working Arrangements signed between the WMO and ISO in 2008.
A list of all parts in the ISO 28902 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
Light detection and ranging (lidar) is an active remote sensing method. The special advantage of lidar
methods are that they permit spatially resolved observations, the measurements are non-contact and can
be conducted in arbitrary directions.
In meteorological and environmental applications light in the ultraviolet to infrared spectral range is used to
probe the atmosphere and measure different parameters depending on the task the specific lidar for which
the system has been designed. The measured quantities result from the scattering and absorption process
of the emitted light on aerosols and molecules. Atmospheric lidar systems are designed to provide range-
resolved profiles of target parameters like humidity and wind speed and provide target parameters like
aerosol optical properties and cloud base heights derived from the actually measured backscatter signal.
This makes lidar systems suitable for several measurement tasks that cannot be carried out adequately with
point-like measuring in-situ methods. The applications include e.g. measurements where spatially resolved
information is important; measurements in which large areas and angular ranges are to be sampled
rapidly and representatively; and measurements at sites and in directions that are difficult to access using
conventional techniques. Examples that illustrate these applications are discussed in Annex A.
This technology has made considerable strides since the first lidar applications in the 1960s. There now
exist commercial systems that can be used in the meteorological field for a variety of measurement tasks.
This document describes atmospheric backscatter lidar, which is based on elastic scattering. It explains
the lidar’s mode of operation and discusses how reliable results can be obtained. In addition, the
method’s parameters are described, and typical applications are discussed. Theoretical and mathematical
considerations for some parameters listed in this document are defined in Annex B. Technical, safety and
operational issues for specific applications that are not covered in detail in this document; References [1] to
[7] cover these aspects.
NOTE To improve readability within this document, “backscatter lidar” is used instead of “atmospheric
backscatter lidar”.
References exist for the following remote sensing standardized methods:
[8]
— ground-based remote sensing of visual range by lidar, see ISO 28902-1 ;
[9]
— ground-based remote sensing of wind by heterodyne pulsed Doppler lidar, see ISO 28902-2 ;
[10]
— ground-based remote sensing of wind by continuous-wave Doppler lidar, seeISO 28902-3 ;
— system performance of ground-based weather radar systems measuring the atmosphere using
[11]
frequencies between 2 GHz and 10 GHz, see ISO 19926-1 ;
[12]
— gaseous air pollution measurement with DAS LIDAR, see VDI 4210-1 .

vi
International Standard ISO 28902-4:2026(en)
Air quality — Environmental meteorology —
Part 4:
Ground-based remote sensing of meteorological parameters
by particle backscatter lidar
IMPORTANT — The electronic file of this document contains colours which are considered to be
useful for the correct understanding of the document. Users should therefore consider printing this
document using a colour printer.
1 Scope
This document specifies the determination of height-resolved profiles of atmospheric backscattering by
means of active optical sounding. The measurements allow the following properties of the atmosphere up to
several kilometres above ground to be derived:
— cloud bases;
— upper boundaries of optically thin clouds;
— upper and lower boundaries and internal structures of particle layers:
— height of structures, e.g. inversions, boundary layer height, mixing layer height (under suitable
conditions);
— attenuated backscatter of the particles;
— particle backscatter and extinction coefficients (requires further assumptions).
The document also addresses the depolarisation lidar and the use of multi-wavelength systems. This allows
further parameters to be determined:
— particle size classification (Ångström exponent, colour ratio);
— shape classification (linear depolarisation degree).
The following fields of application are particularly important:
— air quality monitoring (vertical structure of the boundary layer);
[8]
— aviation safety (cloud base and visual range) (see ISO 28902-1 );
— particle content and transport (e.g. volcanic dust);
— weather forecasting and climate modelling (e.g. atmospheric boundary layer, cloud base, cloud
microphysics);
— satellite remote sensing (validation).
Examples that illustrate these applications are discussed in Annex A.
The benefits of scanning systems for parameters mentioned above are also discussed in Clause A.7.
In addition, particle backscatter lidars that measure at least two carefully selected wavelengths can be
used to determine atmospheric gas concentrations. This is known as the differential absorption lidar “DIAL”
[12]
technique. This technique is not part of this document and has been described in VDI 4210-1 .

This document does not specify extended lidar techniques that monitor the following parameters
quantitatively: inelastic scattering effects such as, Raman scattering, Doppler broadening, Doppler shift,
multiple scattering, modulation techniques, and spectral separation of molecular and particle backscattering
[high spectral resolution lidar (HSRL)]. Some of these extended techniques are or will be described in other
parts of the ISO 28902 series.
This document does not address special features of airborne or satellite-borne systems.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
lidar
light detection and ranging
active distance-resolving remote sensing method, in which the travel time of light from the source to
scattering objects and back is used for distance measurement and the backscattering intensity is analysed
Note 1 to entry: In this document, only atmospheric applications are presented.
3.2
lidar system
measurement system based on lidar technology that uses at least one pulsed light source and one detection
instrument with suitable temporal resolution
Note 1 to entry: In this document, ceilometers are also referred to as lidar systems.
[SOURCE: Reference [13]]
3.3
laser divergence
angle defined by the beam diameter at a particular distance
3.4
field of view
FOV
Φ
field seen by the receiver (per channel) or region of atmospheric volume (object field) from which the
channel detects radiation
Note 1 to entry: Usually quoted as a solid angle. It is determined by the diameter of the field aperture and the focal
length of the receiving optics.
3.5
particle
aerosol particle
airborne (or only very slowly descending) particles that are either solid, liquid, or both, and whose number,
size, shape and material can vary greatly in space and time
Note 1 to entry: Particles can be of either natural or anthropogenic origin.

Note 2 to entry: Primary particles enter the air; secondary particles form in the air through condensation or coagulation
of inorganic or organic molecules. Solid particles include swirled-up dust, ash particles from volcanoes and fires, sea
salt, ice and snow crystals, and organic particles such as pollen, spores, viruses, or bacteria. Liquid particles can be
small water droplets forming on condensation nuclei, or sulphuric acid droplets forming in volcanic gases.
Note 3 to entry: The term “particulate” is generally used in the context of several separate particles, a particle
ensemble. Particulate matter (PM) is used to classify particles in air quality. Since it is more common to use particles in
atmospheric lidar systems (3.2), such as particle backscatter lidar, the term "particulate" is not used in this document.
3.6
elastic scattering of photons
form of scattering in which the wavelength of the scattered photon is conserved in the centre-of-mass frame,
but its direction of propagation is modified.
3.7
inelastic scattering of photons
fundamental scattering process in which the wavelength of scattered photon is not conserved
Note 1 to entry: In an inelastic scattering process, some of the energy of the incident particle is lost or increased.
3.8
particle scattering
elastic scattering of photons (3.6) of electromagnetic radiation by atmospheric particles (e.g. desert dust, ice
crystals, cloud droplets)
[14]
Note 1 to entry: For spherical particles, the Mie theory applies .
3.9
molecular scattering
elastic scattering of photons (3.6) of electromagnetic radiation by air molecules
Note 1 to entry: Usually Rayleigh scattering of ‘light’, wavelengths between approximately 300 nm and 3 µm.
3.10
backscatter coefficient
wavelength-dependent fraction of the light that is scattered backwards elastically by atmospheric particles
and air molecules per unit of solid angle and per unit of distance
3.11
extinction coefficient
wavelength-dependent fraction of the light intensity that is lost through scattering and absorption processes
along the light path, per unit of distance
3.12
lidar ratio
ratio of the extinction coefficient (3.11) and the backscatter coefficient (3.10)
Note 1 to entry: Quoted separately for particles and air molecules.
3.13
backscatter ratio
ratio of the backscatter coefficient (3.10) and the molecular backscatter coefficient
3.14
Ångström exponent
parameter describing the wavelength-dependence of the extinction coefficient (3.11)
Note 1 to entry: Ångström exponent is usually quoted for particles.

3.15
linear depolarisation ratio
ratio of parallel and cross polarised backscattered light
Note 1 to entry: The emission from the lasers used for backscatter lidar is normally linearly-polarised.
Note 2 to entry: The backscattered light from non-spherical particles will have a modified polarisation that is strongly
dependent on particle size, shape and laser wavelength.
Note 3 to entry: Measurements are made of the backscatter signal in two planes, one parallel to the plane of polarisation
of the emitted laser pulse and the other perpendicular to that plane.
3.16
penetration depth
measure of how deep light can penetrate a material
Note 1 to entry: It is defined as the depth at which the intensity of the radiation inside the cloud or fog falls to 1/e
(about 37 %) of its original value at (or more properly, just behind) the surface.
Note 2 to entry: The penetration depth will generally be a function of wavelength.
Note 3 to entry: The Beer-Lambert law describes the exponential decrease of the intensity of an electromagnetic wave
within the atmosphere when absorption or scattering is present.
4 Symbols and abbreviated terms
4.1 Symbols
A Effective area of the receiving telescope m
c Speed of light m/s
C (λ) System constant (lidar constant) J m
s
k (λ) Calibration constant –
pol
LR(x,λ) Lidar ratio sr
LR (x,λ) Lidar ratio that is affected by multiple scattering sr
eff
LR (x,λ) Lidar ratio for air molecules sr
m
LR (x,λ) Lidar ratio for particles sr
p
N Number of laser pulses –
p
N Number of samples per laser pulse (function of time) –
x
O(x,λ) Equipment-specific overlap function –
P(x,λ) Total backscatter signal J
ΔP (x,λ) Noise of the useful signal J
sig
P (x,λ) Background signal J
h
P (x,λ) Useful signal J
sig
P (λ) Energy (mean power times pulse duration) of the emitted laser pulse J
P Normalized measured signal –
n
Fraction of the backscatter signal polarised parallel to the polarisation of the emitted J
Px,

||
laser pulse
Fraction of the backscatter signal polarised perpendicular to the polarisation of the J
Px,


emitted laser pulse
p(x) Air pressure hPa
R(x,λ) Backscatter ratio –
R Gas constant for dry air J/(kg∙K)
d
R Signal-to-noise ratio –
SN
S(x,λ) Lidar signature J m
T(x) Temperature K
T (x) Virtual temperature K
v
q(x) Specific humidity g/kg
Δt Temporal resolution s
t Travel time after emitting the light pulse s
l
Δt Temporal width of the receiving window s
s
V Meteorological optical range m
MOR
x Distance to the backscatter location m
Δx Range resolution m
x Conventional range, index i={a, b, c} m
CRi
x Reference height determined by the boundary condition for the Klett inversion of the m
Ref
backscatter signal
x Maximum detection range m
max
α(x,λ) Extinction coefficient (total extinction coefficient) 1/m
α (x,λ) Molecular extinction coefficient 1/m
m
α (x,λ) Particle extinction coefficient 1/m
p
β(x,λ) Backscatter coefficient (total backscatter coefficient) 1/(m∙sr)
β (x,λ) Molecular backscatter coefficient 1/(m∙sr)
m
β (x,λ) Particle backscatter coefficient 1/(m∙sr)
p
β (x,λ) Attenuated backscatter coefficient 1/(m∙sr)
att
δ (x,λ) Laser divergence rad
L
δ (x,λ) Molecular depolarisation ratio –
m
δ (x,λ) Linear depolarisation ratio of the particles –
P
δ (x,λ) Linear depolarisation ratio of the backscatter volume –
v
η(λ) Total efficiency of the lidar system –
η Multiple scattering factor –
ms
κ(x,λ ,λ ) Ångström-Exponent –
1 2
κ (x,λ ,λ ) Parameter analogous to the Ångström exponent, which takes into account the wave- –
β 1 2
length-dependence of the backscatter coefficient
λ Wavelength m
ξ Distance variable m
ρ Air density kg/m
τ(λ) Optical depth of the atmosphere –
τ (λ) Molecular optical depth of the atmosphere –
m
τ (λ) Particle optical depth of the atmosphere –
p
χ(x,λ ,λ ) Colour ratio –
1 2
4.2 Abbreviated terms
ADC Analogue-to-digital converter
AOD Aerosol optical depth
APD Avalanche photo diode
ATC Air traffic controller
CBH Cloud base height
CPD Cloud penetration depth
FAR False alarm rate
FOV Field of view
HSRL High spectral resolution lidar
IR Infrared spectral range
MOR Meteorological optical range
NIR Near infrared spectral range
POD Probability of detection
PMT Photomultiplier tube
SNR Signal-to-noise ratio
UV Ultraviolet spectral range
VIS Visible spectral range
VOR Vertical optical range
5 Fundamental principles of the backscatter lidar
5.1 General
All lidar methods discussed in this document have in common that a short laser light pulse is emitted into
the atmosphere, and the light scattered backwards is detected, temporally resolved, and analysed. Using the
travel time t from the light pulse’s emission and the speed of light c, allows the distance to the backscattering
l
location x to be calculated:
c
xt (1)
l
The factor ½ arises from the forward and back path travelled by emitted light before being detected by the
lidar system.
In the arrangement shown schematically in Figure 1, after emitting a laser pulse of wavelength λ , the
detector records successive signals Px, given by Formula (2) (lidar equation) for elastic (simple)

backscattering (see Reference [13]).
Key
x start of the overlap between the laser pulse region and the receiver’s field of view (FOV)
x distance at which the geometric overlap is complete (see also 5.2.2)
x bottom height of an aerosol layer
x top height of an aerosol layer
δ laser divergence (corresponds to the angle of aperture of the emitted radiation)
L
Φ FOV
Figure 1 — Schematic diagram of the lidar principle for a bistatic system
It is assumed that a photon recorded by the lidar’s receiving system has experienced exactly one scattering
event (in the backward direction). Multiple scattering occurs when the medium probed by the laser beam
is dense. Then the probability becomes significant that a photon scattered by a first scatterer is further
scattered by a second and additional scatterers. Multiple scattering typically happens in clouds. The
detection of multiple scattering is enhanced by a large field-of-view. The result is an underestimate of the

medium’s optical depth if the analysis assumes single scattering. The determination of cloud bases, however,
is not significantly impaired by this assumption.
For the usual small fields of view in concert with the high spatial resolution and the small optical depths of
the atmosphere, makes neglecting multiple scattering reasonable. The lidar equation is then simplified to
(see Reference [15]):
AO·· x,
  x
c
 
Px,·t ··P ·,xdexp 2 ,
P (2)
     
s 0  h
 
x
where
x
is the distance (m);
λ
is the wavelength (m);
is the temporal width of the receiving window (s);
∆t
s
-1
c
is the speed of light (ms );
is the energy (mean power multiplied with pulse duration) of the emitted laser pulse (often
P 

1)
polarised) (J);
A
is the effective area of the receiving telescope (m );
is the overall efficiency of the lidar system (can be polarisation-dependent, see Reference [9]);


is the distance-dependent overlap function between transmitted beam and FOV;
Ox,

is the backscatter coefficient (scatter coefficient for backscattering, i.e., at 180° based on the
x,

-1 -1
solid angle) (m sr );
-1
is the extinction coefficient (m ).
,

The signal consists only of a background component P and the useful signal P , see Formula (3).
h sig
Thus:
PP P (3)
sigh
Both the background components and signal components have their own noise components. This is further
discussed in 6.6.5 and Clause 9.
The exponential term goes into Formula (2) to the second power since the extinction of the transmitted
beam takes place both in the forward journey (from the laser to the scattering volume) and on the return
journey (from the scattering volume to the receiving telescope).
The optical depth x, between the lidar and the backscattering volume at a distance x is obtained from

the extinction coefficient of the atmosphere x, by integrating over the distance variable ξ .

x
xd,,   (4)
 

[8]
NOTE The lidar signature Sx, was used differently in the ISO 28902-1:2012, as the background signal was

ignored.
1) Energy is power multiplied by time. In the emitted signal, the pulse duration is relevant, and in the received signal,
the temporal width of the receiving window Formula (2) is expressed in units of energy, nevertheless the usual letter P is
retained.
Formula (2) and Formula (4) yield the range-corrected lidar backscatter signal which is referred to in the
following as the lidar signature Sx, :

Sx,,Px xC ·,Ox ·,xx·,exp 2 (5)
     
sigs
CP Ax   (6)
  
s 0
with C  a wavelength-dependent system parameter. The distance-dependent and equipment-specific

s
overlap function Ox, , defined between 0 and 1 (see 5.2.2), is close to 0 for small distances, increases in a

transition zone, and for large distances should reach a constant value of 1.
Formula (2) does include the complex and significant aspect of background radiation. This is particularly
important under daytime conditions. The background light and noise contribution are further discussed in
Clause 9.
It is important to distinguish between the performance of ceilometers based on laser-diode transmitters
and the more powerful lidar systems based on the use of Diode-Pumped Solid-State lasers or flash-lamp
pumped lasers. Ceilometers are specifically designed to measure the cloud-base and aerosol layers. The
more powerful lidars can measure detailed atmospheric structure, such as aerosol characterisation, up to
much higher altitudes but still limited by the obstruction of optically thick clouds.
It is also assumed that the laser pulse duration is shorter than the equipment-based temporal width of the
receiving window ∆t , with which the signal Px, can be recorded. If this condition is not met, to use

s
Formula (2), it shall be formulated differently and a deconvolution procedure shall be applied (see
References [16], [17] and [18]).
5.2 Choice of suitable wavelengths
5.2.1 General
Backscatter lidar systems commonly use wavelengths in the range from 350 nm to 10 µm. The following
factors determine the choice of wavelength:
— target variables and required accuracies;
— atmospheric transmission;
— fields of application and ambient monitoring conditions;
— lasers and eye safety;
— system costs, availability and maintenance intensity.
All systems use elastic backscattering by air molecules and particles (aerosols or hydrometeors).
In general, the amount of backscattered light from droplets, crystals, and particles depends on the relation
between their size and the laser wavelength. The relationship is most sensitive when wavelength and
particle size are close together, and in these cases, it can be actively used to characterize particles, when
several wavelengths are used (multiwavelength lidars). By using short wavelengths in the UV spectral range,
it is possible to detect smaller anthropogenic particles with particular sensitivity, whilst wavelengths in the
range from 900 nm to 1 500 nm are more suitable for clouds and sand dust.
The overall radius of particles lies between a few nanometres for small soot particles and 100 µm for large
volcanic ash and mineral particles.
...

International
Standard
ISO 28902-4
First edition
Air quality — Environmental
2026-08
meteorology —
Part 4:
Ground-based remote sensing of
meteorological parameters by
particle backscatter lidar
Qualité de l'air — Météorologie de l'environnement —
Partie 4: Télédétection basée sur le sol des paramètres
météorologiques par lidar à rétrodiffusion des particules
Reference number
© ISO 2026
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
4 Symbols and abbreviated terms. 4
4.1 Symbols .4
4.2 Abbreviated terms .6
5 Fundamental principles of the backscatter lidar . 7
5.1 General .7
5.2 Choice of suitable wavelengths .9
5.2.1 General .9
5.2.2 Overlap function .10
5.3 Attenuated backscatter coefficient .10
5.4 Backscatter coefficient .11
5.4.1 General .11
5.4.2 Determining the boundary condition for the derivation of the particle
backscatter coefficient .11
5.5 Particle extinction coefficient . 12
5.6 Optical depth . 13
5.7 Shape classification / linear depolarisation ratio . 13
5.8 Obtaining information on particle size distribution .14
6 Measured and target variables of the backscatter lidar . 14
6.1 General .14
6.2 Measured variables . 15
6.3 Target variables . 15
6.3.1 Backscatter profiles . 15
6.3.2 Identification of layers and their boundaries . 15
6.3.3 Cloud base, cloud penetration depth and vertical extension .16
6.3.4 Shape classification by analysing the linear depolarisation ratio .17
6.4 Lidar Ratio .17
6.5 Auxiliary variable .19
6.6 Definition of performance characteristics .19
6.6.1 General .19
6.6.2 Temporal resolution .19
6.6.3 Range resolution . .19
6.6.4 Spatial averaging . 20
6.6.5 Signal-to-noise ratio . 20
6.6.6 Operating range . 22
6.6.7 Completeness, POD and FAR . 23
6.7 Conventional ranges . 23
7 Systems and system components .24
7.1 General .24
7.2 Radiation source(s) .24
7.3 Transmitting and receiving optics . . 25
7.4 Filters . 25
7.5 Detectors. 26
7.5.1 General . 26
7.5.2 Photon counting . 26
7.5.3 Analogue data acquisition .27
7.6 Data acquisition, control system .27
7.7 Mechanical structure, infrastructure . 28

iii
7.8 Additional system components . 28
7.9 Typical lidar design . 28
7.9.1 General . 28
7.9.2 Bistatic and monostatic lidars. 29
7.9.3 Ceilometers .32
7.9.4 Single wavelength lidar systems .32
7.9.5 Multi-wavelength systems.32
7.9.6 Polarisation lidar . . 33
8 Measurement planning and site requirements .34
8.1 General considerations. 34
8.2 Safety. 35
8.3 Adjusting the measurement system to atmospheric conditions. 35
8.4 Limiting conditions for general operation . 36
8.5 Maintenance and operational test. 36
8.5.1 General . 36
8.5.2 Maintenance . 36
9 Factors and uncertainty considerations .36
9.1 General . 36
9.2 “Frozen” atmosphere . 39
9.3 Uncertainty considerations related to β and cloud base . 40
att
9.4 Uncertainty considerations relating to the particle backscatter coefficient .41
9.5 Depolarisation lidar uncertainty considerations .42
9.6 Effect of atmospheric pressure and temperature .43
10 Quality assurance and system monitoring .43
10.1 Adjustments .43
10.2 Functional tests .43
10.3 Maintenance . . 44
10.4 Calibration . 44
10.5 Quality assurance of the attenuated backscatter lidar .45
10.6 Performance tests in the laboratory or at test sites. 46
10.6.1 General . 46
10.6.2 Depolarisation . 49
10.7 Individual or specific parameter tests . 49
Annex A (informative) Examples and theory . 51
Annex B (informative) Mathematical considerations .62
Bibliography .65

iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5,
Meteorology, and by the World Meteorological Organization (WMO) as a common ISO/WMO Standard under
the Agreement on Working Arrangements signed between the WMO and ISO in 2008.
A list of all parts in the ISO 28902 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
Light detection and ranging (lidar) is an active remote sensing method. The special advantage of lidar
methods are that they permit spatially resolved observations, the measurements are non-contact and can
be conducted in arbitrary directions.
In meteorological and environmental applications light in the ultraviolet to infrared spectral range is used to
probe the atmosphere and measure different parameters depending on the task the specific lidar for which
the system has been designed. The measured quantities result from the scattering and absorption process
of the emitted light on aerosols and molecules. Atmospheric lidar systems are designed to provide range-
resolved profiles of target parameters like humidity and wind speed and provide target parameters like
aerosol optical properties and cloud base heights derived from the actually measured backscatter signal.
This makes lidar systems suitable for several measurement tasks that cannot be carried out adequately with
point-like measuring in-situ methods. The applications include e.g. measurements where spatially resolved
information is important; measurements in which large areas and angular ranges are to be sampled
rapidly and representatively; and measurements at sites and in directions that are difficult to access using
conventional techniques. Examples that illustrate these applications are discussed in Annex A.
This technology has made considerable strides since the first lidar applications in the 1960s. There now
exist commercial systems that can be used in the meteorological field for a variety of measurement tasks.
This document describes atmospheric backscatter lidar, which is based on elastic scattering. It explains
the lidar’s mode of operation and discusses how reliable results can be obtained. In addition, the
method’s parameters are described, and typical applications are discussed. Theoretical and mathematical
considerations for some parameters listed in this document are defined in Annex B. Technical, safety and
operational issues for specific applications that are not covered in detail in this document; References [1] to
[7] cover these aspects.
NOTE To improve readability within this document, “backscatter lidar” is used instead of “atmospheric
backscatter lidar”.
References exist for the following remote sensing standardized methods:
[8]
— ground-based remote sensing of visual range by lidar, see ISO 28902-1 ;
[9]
— ground-based remote sensing of wind by heterodyne pulsed Doppler lidar, see ISO 28902-2 ;
[10]
— ground-based remote sensing of wind by continuous-wave Doppler lidar, seeISO 28902-3 ;
— system performance of ground-based weather radar systems measuring the atmosphere using
[11]
frequencies between 2 GHz and 10 GHz, see ISO 19926-1 ;
[12]
— gaseous air pollution measurement with DAS LIDAR, see VDI 4210-1 .

vi
International Standard ISO 28902-4:2026(en)
Air quality — Environmental meteorology —
Part 4:
Ground-based remote sensing of meteorological parameters
by particle backscatter lidar
IMPORTANT — The electronic file of this document contains colours which are considered to be
useful for the correct understanding of the document. Users should therefore consider printing this
document using a colour printer.
1 Scope
This document specifies the determination of height-resolved profiles of atmospheric backscattering by
means of active optical sounding. The measurements allow the following properties of the atmosphere up to
several kilometres above ground to be derived:
— cloud bases;
— upper boundaries of optically thin clouds;
— upper and lower boundaries and internal structures of particle layers:
— height of structures, e.g. inversions, boundary layer height, mixing layer height (under suitable
conditions);
— attenuated backscatter of the particles;
— particle backscatter and extinction coefficients (requires further assumptions).
The document also addresses the depolarisation lidar and the use of multi-wavelength systems. This allows
further parameters to be determined:
— particle size classification (Ångström exponent, colour ratio);
— shape classification (linear depolarisation degree).
The following fields of application are particularly important:
— air quality monitoring (vertical structure of the boundary layer);
[8]
— aviation safety (cloud base and visual range) (see ISO 28902-1 );
— particle content and transport (e.g. volcanic dust);
— weather forecasting and climate modelling (e.g. atmospheric boundary layer, cloud base, cloud
microphysics);
— satellite remote sensing (validation).
Examples that illustrate these applications are discussed in Annex A.
The benefits of scanning systems for parameters mentioned above are also discussed in Clause A.7.
In addition, particle backscatter lidars that measure at least two carefully selected wavelengths can be
used to determine atmospheric gas concentrations. This is known as the differential absorption lidar “DIAL”
[12]
technique. This technique is not part of this document and has been described in VDI 4210-1 .

This document does not specify extended lidar techniques that monitor the following parameters
quantitatively: inelastic scattering effects such as, Raman scattering, Doppler broadening, Doppler shift,
multiple scattering, modulation techniques, and spectral separation of molecular and particle backscattering
[high spectral resolution lidar (HSRL)]. Some of these extended techniques are or will be described in other
parts of the ISO 28902 series.
This document does not address special features of airborne or satellite-borne systems.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
lidar
light detection and ranging
active distance-resolving remote sensing method, in which the travel time of light from the source to
scattering objects and back is used for distance measurement and the backscattering intensity is analysed
Note 1 to entry: In this document, only atmospheric applications are presented.
3.2
lidar system
measurement system based on lidar technology that uses at least one pulsed light source and one detection
instrument with suitable temporal resolution
Note 1 to entry: In this document, ceilometers are also referred to as lidar systems.
[SOURCE: Reference [13]]
3.3
laser divergence
angle defined by the beam diameter at a particular distance
3.4
field of view
FOV
Φ
field seen by the receiver (per channel) or region of atmospheric volume (object field) from which the
channel detects radiation
Note 1 to entry: Usually quoted as a solid angle. It is determined by the diameter of the field aperture and the focal
length of the receiving optics.
3.5
particle
aerosol particle
airborne (or only very slowly descending) particles that are either solid, liquid, or both, and whose number,
size, shape and material can vary greatly in space and time
Note 1 to entry: Particles can be of either natural or anthropogenic origin.

Note 2 to entry: Primary particles enter the air; secondary particles form in the air through condensation or coagulation
of inorganic or organic molecules. Solid particles include swirled-up dust, ash particles from volcanoes and fires, sea
salt, ice and snow crystals, and organic particles such as pollen, spores, viruses, or bacteria. Liquid particles can be
small water droplets forming on condensation nuclei, or sulphuric acid droplets forming in volcanic gases.
Note 3 to entry: The term “particulate” is generally used in the context of several separate particles, a particle
ensemble. Particulate matter (PM) is used to classify particles in air quality. Since it is more common to use particles in
atmospheric lidar systems (3.2), such as particle backscatter lidar, the term "particulate" is not used in this document.
3.6
elastic scattering of photons
form of scattering in which the wavelength of the scattered photon is conserved in the centre-of-mass frame,
but its direction of propagation is modified.
3.7
inelastic scattering of photons
fundamental scattering process in which the wavelength of scattered photon is not conserved
Note 1 to entry: In an inelastic scattering process, some of the energy of the incident particle is lost or increased.
3.8
particle scattering
elastic scattering of photons (3.6) of electromagnetic radiation by atmospheric particles (e.g. desert dust, ice
crystals, cloud droplets)
[14]
Note 1 to entry: For spherical particles, the Mie theory applies .
3.9
molecular scattering
elastic scattering of photons (3.6) of electromagnetic radiation by air molecules
Note 1 to entry: Usually Rayleigh scattering of ‘light’, wavelengths between approximately 300 nm and 3 µm.
3.10
backscatter coefficient
wavelength-dependent fraction of the light that is scattered backwards elastically by atmospheric particles
and air molecules per unit of solid angle and per unit of distance
3.11
extinction coefficient
wavelength-dependent fraction of the light intensity that is lost through scattering and absorption processes
along the light path, per unit of distance
3.12
lidar ratio
ratio of the extinction coefficient (3.11) and the backscatter coefficient (3.10)
Note 1 to entry: Quoted separately for particles and air molecules.
3.13
backscatter ratio
ratio of the backscatter coefficient (3.10) and the molecular backscatter coefficient
3.14
Ångström exponent
parameter describing the wavelength-dependence of the extinction coefficient (3.11)
Note 1 to entry: Ångström exponent is usually quoted for particles.

3.15
linear depolarisation ratio
ratio of parallel and cross polarised backscattered light
Note 1 to entry: The emission from the lasers used for backscatter lidar is normally linearly-polarised.
Note 2 to entry: The backscattered light from non-spherical particles will have a modified polarisation that is strongly
dependent on particle size, shape and laser wavelength.
Note 3 to entry: Measurements are made of the backscatter signal in two planes, one parallel to the plane of polarisation
of the emitted laser pulse and the other perpendicular to that plane.
3.16
penetration depth
measure of how deep light can penetrate a material
Note 1 to entry: It is defined as the depth at which the intensity of the radiation inside the cloud or fog falls to 1/e
(about 37 %) of its original value at (or more properly, just behind) the surface.
Note 2 to entry: The penetration depth will generally be a function of wavelength.
Note 3 to entry: The Beer-Lambert law describes the exponential decrease of the intensity of an electromagnetic wave
within the atmosphere when absorption or scattering is present.
4 Symbols and abbreviated terms
4.1 Symbols
A Effective area of the receiving telescope m
c Speed of light m/s
C (λ) System constant (lidar constant) J m
s
k (λ) Calibration constant –
pol
LR(x,λ) Lidar ratio sr
LR (x,λ) Lidar ratio that is affected by multiple scattering sr
eff
LR (x,λ) Lidar ratio for air molecules sr
m
LR (x,λ) Lidar ratio for particles sr
p
N Number of laser pulses –
p
N Number of samples per laser pulse (function of time) –
x
O(x,λ) Equipment-specific overlap function –
P(x,λ) Total backscatter signal J
ΔP (x,λ) Noise of the useful signal J
sig
P (x,λ) Background signal J
h
P (x,λ) Useful signal J
sig
P (λ) Energy (mean power times pulse duration) of the emitted laser pulse J
P Normalized measured signal –
n
Fraction of the backscatter signal polarised parallel to the polarisation of the emitted J
Px,

||
laser pulse
Fraction of the backscatter signal polarised perpendicular to the polarisation of the J
Px,


emitted laser pulse
p(x) Air pressure hPa
R(x,λ) Backscatter ratio –
R Gas constant for dry air J/(kg∙K)
d
R Signal-to-noise ratio –
SN
S(x,λ) Lidar signature J m
T(x) Temperature K
T (x) Virtual temperature K
v
q(x) Specific humidity g/kg
Δt Temporal resolution s
t Travel time after emitting the light pulse s
l
Δt Temporal width of the receiving window s
s
V Meteorological optical range m
MOR
x Distance to the backscatter location m
Δx Range resolution m
x Conventional range, index i={a, b, c} m
CRi
x Reference height determined by the boundary condition for the Klett inversion of the m
Ref
backscatter signal
x Maximum detection range m
max
α(x,λ) Extinction coefficient (total extinction coefficient) 1/m
α (x,λ) Molecular extinction coefficient 1/m
m
α (x,λ) Particle extinction coefficient 1/m
p
β(x,λ) Backscatter coefficient (total backscatter coefficient) 1/(m∙sr)
β (x,λ) Molecular backscatter coefficient 1/(m∙sr)
m
β (x,λ) Particle backscatter coefficient 1/(m∙sr)
p
β (x,λ) Attenuated backscatter coefficient 1/(m∙sr)
att
δ (x,λ) Laser divergence rad
L
δ (x,λ) Molecular depolarisation ratio –
m
δ (x,λ) Linear depolarisation ratio of the particles –
P
δ (x,λ) Linear depolarisation ratio of the backscatter volume –
v
η(λ) Total efficiency of the lidar system –
η Multiple scattering factor –
ms
κ(x,λ ,λ ) Ångström-Exponent –
1 2
κ (x,λ ,λ ) Parameter analogous to the Ångström exponent, which takes into account the wave- –
β 1 2
length-dependence of the backscatter coefficient
λ Wavelength m
ξ Distance variable m
ρ Air density kg/m
τ(λ) Optical depth of the atmosphere –
τ (λ) Molecular optical depth of the atmosphere –
m
τ (λ) Particle optical depth of the atmosphere –
p
χ(x,λ ,λ ) Colour ratio –
1 2
4.2 Abbreviated terms
ADC Analogue-to-digital converter
AOD Aerosol optical depth
APD Avalanche photo diode
ATC Air traffic controller
CBH Cloud base height
CPD Cloud penetration depth
FAR False alarm rate
FOV Field of view
HSRL High spectral resolution lidar
IR Infrared spectral range
MOR Meteorological optical range
NIR Near infrared spectral range
POD Probability of detection
PMT Photomultiplier tube
SNR Signal-to-noise ratio
UV Ultraviolet spectral range
VIS Visible spectral range
VOR Vertical optical range
5 Fundamental principles of the backscatter lidar
5.1 General
All lidar methods discussed in this document have in common that a short laser light pulse is emitted into
the atmosphere, and the light scattered backwards is detected, temporally resolved, and analysed. Using the
travel time t from the light pulse’s emission and the speed of light c, allows the distance to the backscattering
l
location x to be calculated:
c
xt (1)
l
The factor ½ arises from the forward and back path travelled by emitted light before being detected by the
lidar system.
In the arrangement shown schematically in Figure 1, after emitting a laser pulse of wavelength λ , the
detector records successive signals Px, given by Formula (2) (lidar equation) for elastic (simple)

backscattering (see Reference [13]).
Key
x start of the overlap between the laser pulse region and the receiver’s field of view (FOV)
x distance at which the geometric overlap is complete (see also 5.2.2)
x bottom height of an aerosol layer
x top height of an aerosol layer
δ laser divergence (corresponds to the angle of aperture of the emitted radiation)
L
Φ FOV
Figure 1 — Schematic diagram of the lidar principle for a bistatic system
It is assumed that a photon recorded by the lidar’s receiving system has experienced exactly one scattering
event (in the backward direction). Multiple scattering occurs when the medium probed by the laser beam
is dense. Then the probability becomes significant that a photon scattered by a first scatterer is further
scattered by a second and additional scatterers. Multiple scattering typically happens in clouds. The
detection of multiple scattering is enhanced by a large field-of-view. The result is an underestimate of the

medium’s optical depth if the analysis assumes single scattering. The determination of cloud bases, however,
is not significantly impaired by this assumption.
For the usual small fields of view in concert with the high spatial resolution and the small optical depths of
the atmosphere, makes neglecting multiple scattering reasonable. The lidar equation is then simplified to
(see Reference [15]):
AO·· x,
  x
c
 
Px,·t ··P ·,xdexp 2 ,
P (2)
     
s 0  h
 
x
where
x
is the distance (m);
λ
is the wavelength (m);
is the temporal width of the receiving window (s);
∆t
s
-1
c
is the speed of light (ms );
is the energy (mean power multiplied with pulse duration) of the emitted laser pulse (often
P 

1)
polarised) (J);
A
is the effective area of the receiving telescope (m );
is the overall efficiency of the lidar system (can be polarisation-dependent, see Reference [9]);


is the distance-dependent overlap function between transmitted beam and FOV;
Ox,

is the backscatter coefficient (scatter coefficient for backscattering, i.e., at 180° based on the
x,

-1 -1
solid angle) (m sr );
-1
is the extinction coefficient (m ).
,

The signal consists only of a background component P and the useful signal P , see Formula (3).
h sig
Thus:
PP P (3)
sigh
Both the background components and signal components have their own noise components. This is further
discussed in 6.6.5 and Clause 9.
The exponential term goes into Formula (2) to the second power since the extinction of the transmitted
beam takes place both in the forward journey (from the laser to the scattering volume) and on the return
journey (from the scattering volume to the receiving telescope).
The optical depth x, between the lidar and the backscattering volume at a distance x is obtained from

the extinction coefficient of the atmosphere x, by integrating over the distance variable ξ .

x
xd,,   (4)
 

[8]
NOTE The lidar signature Sx, was used differently in the ISO 28902-1:2012, as the background signal was

ignored.
1) Energy is power multiplied by time. In the emitted signal, the pulse duration is relevant, and in the received signal,
the temporal width of the receiving window Formula (2) is expressed in units of energy, nevertheless the usual letter P is
retained.
Formula (2) and Formula (4) yield the range-corrected lidar backscatter signal which is referred to in the
following as the lidar signature Sx, :

Sx,,Px xC ·,Ox ·,xx·,exp 2 (5)
     
sigs
CP Ax   (6)
  
s 0
with C  a wavelength-dependent system parameter. The distance-dependent and equipment-specific

s
overlap function Ox, , defined between 0 and 1 (see 5.2.2), is close to 0 for small distances, increases in a

transition zone, and for large distances should reach a constant value of 1.
Formula (2) does include the complex and significant aspect of background radiation. This is particularly
important under daytime conditions. The background light and noise contribution are further discussed in
Clause 9.
It is important to distinguish between the performance of ceilometers based on laser-diode transmitters
and the more powerful lidar systems based on the use of Diode-Pumped Solid-State lasers or flash-lamp
pumped lasers. Ceilometers are specifically designed to measure the cloud-base and aerosol layers. The
more powerful lidars can measure detailed atmospheric structure, such as aerosol characterisation, up to
much higher altitudes but still limited by the obstruction of optically thick clouds.
It is also assumed that the laser pulse duration is shorter than the equipment-based temporal width of the
receiving window ∆t , with which the signal Px, can be recorded. If this condition is not met, to use

s
Formula (2), it shall be formulated differently and a deconvolution procedure shall be applied (see
References [16], [17] and [18]).
5.2 Choice of suitable wavelengths
5.2.1 General
Backscatter lidar systems commonly use wavelengths in the range from 350 nm to 10 µm. The following
factors determine the choice of wavelength:
— target variables and required accuracies;
— atmospheric transmission;
— fields of application and ambient monitoring conditions;
— lasers and eye safety;
— system costs, availability and maintenance intensity.
All systems use elastic backscattering by air molecules and particles (aerosols or hydrometeors).
In general, the amount of backscattered light from droplets, crystals, and particles depends on the relation
between their size and the laser wavelength. The relationship is most sensitive when wavelength and
particle size are close together, and in these cases, it can be actively used to characterize particles, when
several wavelengths are used (multiwavelength lidars). By using short wavelengths in the UV spectral range,
it is possible to detect smaller anthropogenic particles with particular sensitivity, whilst wavelengths in the
range from 900 nm to 1 500 nm are more suitable for clouds and sand dust.
The overall radius of particles lies between a few nanometres for small soot particles and 100 µm for large
volcanic ash and mineral particles. The droplet sizes in clouds and fog are approximately above 1 µm in
diameter; ice crystals are typically from 10 µm to 100 µm in diameter.
The UV wavelength range below 350 nm is not commonly used in backscatter lidars, since decreasing
wavelengths are associated with a rapid increase in molecular (Rayleigh) scattering and below 320 nm trace

[12]
gases such as ozone exhibit strong absorption (see VDI 4210-1 ). Moreover, there is a limited availability
of permanently operating laser sources. The 350 nm to 355 nm UV range is used by several backscatter and
Raman lidars. For this range, eye-safety restrictions are less restrictive while the availability and quality of
laser sources and suitable detectors are high.
In the visible spectral range between 400 nm and 700 nm, eye-safety regulations are, in general, more
[1]
restrictive. The short reaction time of the eye-shut reflex is used to develop specific eye-safe backscatter
lidar systems. Data generated using visible laser wavelengths is more comparable to that obtained by the
human eye, e.g., when making visibility measurements, unlike in the NIR, or UV range, where correction
factors can be required. A common wavelength in t
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