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Communications Surveys & Tutorials
Optical Communication in Space: Challenges and
Mitigation Techniques
Hemani Kaushal
1
and Georges Kaddoum
2
1
Department of Electrical, Electronics and Communication Engineering, The NorthCap University, Gurgaon,
Haryana, India-122017.
2
Département de génie électrique, École de technologie supérieure, Montréal (QC), Canada.
Abstract—In recent years, free space optical (FSO)
communication has gained significant importance owing to
its unique features: large bandwidth, license free spectrum, high
data rate, easy and quick deployability, less power and low
mass requirements. FSO communication uses optical carrier
in the near infrared (IR) band to establish either terrestrial
links within the Earth’s atmosphere or inter-satellite/deep
space links or ground-to-satellite/satellite-to-ground links. It
also finds its applications in remote sensing, radio astronomy,
military, disaster recovery, last mile access, backhaul for
wireless cellular networks and many more. However, despite
of great potential of FSO communication, its performance is
limited by the adverse effects (viz., absorption, scattering and
turbulence) of the atmospheric channel. Out of these three
effects, the atmospheric turbulence is a major challenge that
may lead to serious degradation in the bit error rate (BER)
performance of the system and make the communication link
infeasible. This paper presents a comprehensive survey on
various challenges faced by FSO communication system for
ground-to-satellite/satellite-to-ground and inter-satellite links. It
also provide details of various performance mitigation techniques
in order to have high link availability and reliability. The first
part of the paper will focus on various types of impairments
that pose a serious challenge to the performance of optical
communication system for ground-to-satellite/satellite-to-ground
and inter-satellite links. The latter part of the paper will provide
the reader with an exhaustive review of various techniques
both at physical layer as well as at the other layers (link,
network or transport layer) to combat the adverse effects of
the atmosphere. It also uniquely presents a recently developed
technique using orbital angular momentum for utilizing the high
capacity advantage of optical carrier in case of space-based and
near-Earth optical communication links. This survey provides
the reader with comprehensive details on the use of space-based
optical backhaul links in order to provide high capacity and
low cost backhaul solutions.
Index Terms—Free space optical communication, atmospheric
turbulence, aperture averaging, diversity, adaptive optics,
advanced modulation and coding techniques, hybrid RF/FSO,
ARQ, routing protocols, orbital angular momentum, FSO
backhaul.
I. INTRODUCTION
A. FSO Communication - An Overview
In the recent few years, tremendous growth and
advancements have been observed in information and
communication technologies. With the increasing usage of
high speed internet, video-conferencing, live streaming etc.,
the bandwidth and capacity requirements are increasing
drastically. This ever growing demand of increase in data and
multimedia services has led to congestion in conventionally
used radio frequency (RF) spectrum and arises a need to
shift from RF carrier to optical carrier. Unlike RF carrier
where spectrum usage is restricted, optical carrier does not
require any spectrum licensing and therefore, is an attractive
prospect for high bandwidth and capacity applications. Optical
wireless communication (OWC) is the technology that uses
optical carrier to transfer information from one point to
another through an unguided channel which may be an
atmosphere or free space. OWC is considered as a next frontier
for high speed broadband connection as it offers extremely
high bandwidth, ease of deployment, unlicensed spectrum
allocation, reduced power consumption (∼1/2 of RF), reduced
size (∼1/10 of the RF antenna diameter) and improved channel
security [1]. It provides LOS communication owing to its
narrow transmit beamwidth and works in visible and IR
spectrum. The basic principle of OWC is similar to fiber
optic communication except that unlike fiber transmission, in
this case the modulated data is transmitted through unguided
channel instead of guided optical fiber. The initial work on
OWC has started almost 50 years back for defense and space
applications where US military used to send telegraph signal
from one point to another using sunlight powered devices.
In year 1876, Alexander Graham Bell demonstrated his first
wireless telephone system [2], [3] by converting sound waves
to electrical telephone signals and transmitted the voice signal
over few feets using sunlight as carrier. The device was called
“photo-phone” as it was the world’s first wireless telephone
system. Thereafter, with the discovery of first working laser
at Hughes Research Laboratories, Malibu, California in 1960
[4], a great advancement was observed in FSO technology.
The OWC can be classified into two broad categories,
namely indoor and outdoor optical wireless communications.
Indoor OWC uses IR or visible light for communicating within
a building where the possibility of setting up a physical
wired connection is cumbersome [5]–[12]. Indoor OWC is
classified into four generic system configurations i.e., directed
line-of-sight (LOS), non-directed LOS, diffused and tracked.
Outdoor OWC is also termed as free space optical (FSO)
communication. The FSO communication systems are further
classified into terrestrial and space optical links that include
building-to-building, ground-to-satellite, satellite-to-ground,
satellite-to-satellite, satellite-to-airborne platforms (unmanned
aerial vehicles (UAVs) or balloons), [13]–[15] etc. Fig. 1
illustrates the classification of OWC system. This survey is
arXiv:1705.10630v1 [cs.IT] 28 May 2017
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Communications Surveys & Tutorials
2
focused around space optical links which include both ground-
to-satellite/ satellite-to-ground links, inter-satellite links and
deep space links.
Optical Wireless Communication System
Indoor System
Outdoor System
(FSO)
Terrestrial
Links
Space
Links
Inter-
Orbital
Links (IOL)
Inter-
Satellite
Links (ISL)
Deep Space
Links
(DSL)
Directed
LOS
Non-
Directed
LOS
Diffused Tracked
Figure 1. Classification of the optical wireless communication system
Advancement in space technology and development of more
sophisticated space-based instruments opened a new chapter
for optical space-based communication. Due to increasing
demands for high data rate and large communication
capacity, researchers are actively working to build all optical
communication architecture that includes ground-to-satellite
optical communication links which are connected to satellite
optical network and satellite-to-ground optical links as shown
in Fig. 2.
GEO Satellite
GEO Satellite
GEO Satellite
LEO Satellite
Ground Station
Satellite
UAV
Mobile User
Satellite
Mobile User
Satellite
Figure 2. Space FSO links
The first theoretical study on optical uplink transmission
from ground-to-satellite has been studied by Fried in
1967 [16]. Few years later, an uplink transmission
using ground-based continuous-wave (CW) argon laser
towards geodetic Earth orbiting satellite-II (GEOS-II)
was demonstrated in [17]. Thereafter, various theoretical
studies were suggested [14], [18]–[20] and successful
experiments [21]–[23] were performed to investigate optical
ground-to-satellite and inter-satellite communications. In early
1990, a relay mirror experiment (RME) was conducted using
three laser beams that propagated from ground-to-satellite
and were retro-reflected from the RME spacecraft orbiting at
an altitude of 350 km [24]. The beam intensity profile was
measured for investigating the temporal nature of atmospheric
turbulence on the optical beam. In 1992, an uplink optical
communication to deep space vehicle was demonstrated
through Galileo optical experiment (GOPEX) that transmitted
a pulsed laser signal from two optical ground stations (OGS)
mounted at California and New Mexico [25]. The results
demonstrated the distortion of uplink beam due to atmospheric
turbulence. Later in 1995, the first ground-to-space two
way communication link was demonstrated in ground/orbiter
lasercom demonstration (GOLD) using argon ion laser [26],
[27]. A comparison of theoretical and experimental data for
single and multiple uplink beams was carried out in the
GOLD demonstration. A bi-directional Earth-to-moon laser
link was demonstrated with adaptive optics to mitigate the
effect of atmospheric turbulence in [28]. The first inter-satellite
laser communication link was successfully demonstrated by
European Space Agency (ESA) between two satellites SPOT-4
and ARTEMIS for optical data-relay services at 50 Mbps
[29]. They built an OGS and commission Semi-conductor
Inter Satellite Link Experiment (SILEX) terminals in space.
Later, successful bi-directional optical link between KIRARI,
the Japanese satellite (officially called Optical Inter-Orbit
Communications Engineering Test Satellite - OICETS) and
ESA’s Artemis was demonstrated in 2005 [30]. An optical
link between two LEO orbiting satellites, Terra SAR-X and
NFIRE, at 5.5 Gbps on a total distance of 5500 km and at a
speed of 25, 000 km/hr has been established in 2008. The first
successful ground-satellite optical link was conducted between
the OGS and ETS-VI satellite in Konegi, Japan [31].
Several other experiments were performed in military and
aerospace laboratories that demonstrated ground-to-satellite,
satellite-to-satellite and satellite-to-ground optical links. It
has resulted in various successful missions like (i) airborne
flight test system (AFTS)- a link between aircraft and ground
station at New Mexico [32], (ii) laser cross link system
(LCLS)- full duplex space-to-space link for geosynchronous
system [33], (iii) optical communication demonstrator (OCD)-
laboratory prototype for demonstrating high speed data
transfer from satellite-to-ground, (iv) stratospheric optical
payload experiment STROPEX (CAPANINA Project)- high bit
rate optical downlink from airborne station to transportable
optical ground station [34], (v) Mars laser communications
demonstration (MLCD)- provides up to 10 Mbps data transfer
between Earth and Mars [35], and (vi) airborne laser optical
link (LOLA)- first demonstration of a two-way optical link
between high altitude aircraft and GEO satellite (ARTEMIS)
[36]. Another mission by NASA is laser communication relay
demonstration (LCRD) that will be launched in 2017 will
demonstrate optical relay services for near earth and deep
space communication missions [37]
This paper presents a comprehensive survey of FSO
communication with primary focus on ground-to-satellite,
satellite-to-ground and inter-satellite links. The issues involved
in laser uplink are different from that of downlink
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Communications Surveys & Tutorials
3
or inter-satellite links. In case of laser uplink from
ground-to-satellite, the beam comes immediately in contact
with the atmosphere and therefore, suffer more from distortion
and pointing instability due to spatial and temporal changes in
the refractive index of the atmosphere. On the other hand, the
downlink communication from satellite-to-ground causes the
optical beam to spread geometrically (i.e., caused primarily
due to beam divergence loss) and very little spread is due to
atmospheric effect or variation in the beam steering. For this
reason, the effect of atmospheric turbulence is generally very
small on the downlink propagation as the beam goes through
a non-atmospheric path until it reaches about 30 km from
the Earth’s surface. In case of inter-satellite links, the major
challenge is laser beam pointing to or from a moving platform.
For this reason, a very tight acquisition, tracking and pointing
(ATP) systems are required for the optical beam to reliably
reach the receiver. This survey paper provides an exhaustive
coverage of various challenges and their mitigation techniques
for space-based optical communication links.
B. Advantages of FSO Communication over RF
Communication
FSO communication system offers several advantages
over RF system. The major difference between FSO and
RF communication arises from the large difference in the
wavelength. For FSO system, under clear weather conditions
(visibility > 10 miles), the atmospheric transmission window
lies in the near IR wavelength range between 700 nm
to 1600 nm. The transmission window for RF system lies
between 30 mm to 3 m. Therefore, RF wavelength is thousand
of times larger than optical wavelength. This high ratio of
wavelength leads to some interesting differences between the
two systems as given below:
(I) High bandwidth: It is a well known fact that an
increase in carrier frequency increases the information
carrying capacity of a communication system. In RF
and microwave communication systems, the allowable
bandwidth can be up to 20% of the carrier frequency. In
optical communication, even if the bandwidth is taken to
be 1% of carrier frequency (≈10
16
Hz), the allowable
bandwidth will be 100 THz. This makes the usable
bandwidth at an optical frequency in the order of THz
which is almost 10
5
times that of a typical RF carrier
[38], [39].
(II) Less power and mass requirements: The beam
divergence is proportional to λ/D
R
, where λ is the carrier
wavelength and D
R
the aperture diameter. Thus, the beam
spread offered by the optical carrier is narrower than
that of the RF carrier. This leads to an increase in the
intensity of signal at the receiver for a given transmitted
power. Fig. 3 shows the comparison of beam divergence
for optical and RF signals when sent back from Mars
towards Earth.
Thus, a smaller wavelength of optical carrier permits the
FSO designer to come up with a system that has smaller
antenna than RF system to achieve the same gain (as
antenna gain scales inversely proportional to the square
~
100 Earth Diameter
Beam Divergence (θ
div
) = 2.44 (λ/D
R
)
~ 0.1 Earth Diameter
Earth
Earth
Mars
Mars
RF link
Optical Link
Figure 3. Comparison of optical and RF beam divergence from Mars towards
Earth [40]
of operating wavelength). The typical size for the optical
system is 0.3 m vs 1.5 m for RF spacecraft antenna [26].
Table I gives the power and mass comparison between
optical and RF communication systems using 10 W and
50 W for optical and Ka band systems, respectively at
2.5 Gbps.
Link
Optical
RF
GEO-LEO
Antenna Diameter
Mass
Power
10.2 cm (1.0)
65.3 kg (1.0)
93.8 W (1.0)
2.2 m (21.6)
152.8 kg (2.3)
213.9 W (2.3)
GEO-GEO
Antenna Diameter
Mass
Power
13.5 cm (1.0)
86.4 kg (1.0)
124.2 W (1.0)
2.1 m (15.6)
145.8 kg (1.7)
204.2 W (1.6)
LEO-LEO
Antenna Diameter
Mass
Power
3.6 cm (1.0)
23.0 kg (1.0)
33.1 W (1.0)
0.8 m (22.2)
55.6 kg (2.4)
77.8 W (2.3)
Table I
COMPARISON OF POWER AND MASS FOR GEOSTATIONARY EARTH ORBIT
(GEO) AND LOW EARTH ORBIT (LEO) LINKS USING OPTICAL AND RF
COMMUNICATION SYSTEMS (VALUES IN PARENTHESES ARE NORMALIZED
TO THE OPTICAL PARAMETERS) [41]
High directivity: The directivity of antenna is closely
related to its gain. The advantage of optical carrier over
RF carrier can be seen from the ratio of antenna gain as
given in the equation below [42]
Gain
(optical)
Gain
(RF)
≈
4π/θ
2
div(optical)
4π/θ
2
div(RF)
, (1)
where θ
div(optical)
and θ
div(RF)
are the optical and RF
beam divergences, respectively and are proportional to
λ/D
R
. Since the optical wavelength is very small, a very
high directivity and improved gain are obtained.
(III) (IV) Unlicensed spectrum: In the RF system,
interference from adjacent carrier is the major problem
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Communications Surveys & Tutorials
4
due to spectrum congestion. This requires the need of
spectrum licensing by regulatory authorities. But on the
other hand, the optical system is free from spectrum
licensing till now. This reduces the initial set up cost and
development time [43].
(V) High Security: FSO communication can not be
detected by spectrum analyzers or RF meters as FSO
laser beam is highly directional with very narrow beam
divergence. Any kind of interception is therefore very
difficult. Unlike RF signal, FSO signal cannot penetrate
walls, therefore can prevent eavesdropping [44].
In addition to the above advantages, FSO communication
offers secondary benefits as being : (i) easily expandable and
reducing the size of network segments, (ii) light weight and
compact, (iii) easy and quick deployable, and (iv) able to
be used where fiber optic cables cannot be used. However,
despite of many advantages, FSO communication system has
its own drawbacks over RF system. The main disadvantage
is the requirement of tight ATP system due to the narrow
beam divergence. Also, FSO communication is dependent
upon unpredictable atmospheric conditions that can degrade
the performance of the system. Another limiting factor, is
the position of the Sun relative to the laser transmitter and
receiver. In a particular alignment, solar background radiations
can increase and that will lead to poor system performance
[45], [46]. This undoubtedly poses a great challenge to FSO
system designers.
C. Choice of wavelength in FSO communication
Wavelength selection in FSO communication is a very
important design parameter as it affects link performance
and detector sensitivity of the system. Since antenna gain
is inversely proportional to operating wavelength, therefore,
it is more beneficial to operate at lower wavelengths.
However, higher wavelengths provide better link quality
and lower pointing-induced signal fades [47]. Therefore, a
careful optimization of operating wavelength in the design
of FSO link helps in achieving a better performance.
The choice of wavelength strongly depends on atmospheric
effects, attenuation and background noise power. Further,
the availability of transmitter and receiver components, eye
safety regulations and cost deeply impacts the selection of
wavelength in the FSO design process.
The International Commission on Illumination [48] has
classified optical radiations into three categories: IR-A
(700 nm to 1400 nm), IR-B (1400 nm to 3000 nm) and
IR-C (3000 nm to 1 mm). It can be sub-classified into : (i)
near-infrared (NIR) ranging from 750 nm to 1450 nm is a
low attenuation window and mainly used for fiber optics, (ii)
short-infrared (SIR) ranging from 1400 nm to 3000 nm out of
which 1530 nm to 1560 nm is a dominant spectral range for
long distance communication, (iii) mid-infrared (MIR) ranging
from 3000 nm to 8000 nm is used in military applications
for guiding missiles, (iv) long-infrared (LIR) ranging from
8000 nm to 15 µm is used in thermal imaging, and (v)
far-infrared (FIR) is ranging from 15 µm to 1 mm. Almost
all commercially available FSO systems are using NIR and
SIR wavelength ranges since these wavelengths are also used
in fiber optic communication and their components are readily
available in market.
For space-based optical applications, the choice of operating
wavelength depends upon the trade-off between receiver
sensitivity and pointing bias due to thermal variations
across the Earth’s surface. Generally, longer wavelengths are
preferred as they cause reduction in solar background and solar
scattering from the surface of the Earth. Lasers currently being
considered and developed for space communication are in the
range of 500 nm to 2000 nm. Table II summarizes various
wavelengths used in practical space-based optical systems.
The wavelength selection for FSO communication has to be
eye and skin safe as certain wavelengths between 400 nm to
1500 nm can cause potential eye hazards or damages to the
retina [49]. Under International Electrotechanical Commission
(IEC), lasers are classified into four groups from Class 1 to
Class 4 depending upon their power and possible hazards [50].
Most of the FSO system use Class 1 and 1M lasers. For same
safety class, FSO system operating at 1500 nm can transmit
more than 10 times optical power than system operating at
shorter operating wavelengths like 750 nm or 850 nm. It is
because cornea, the outer layer of the eye absorb the energy
of the light at 1550 nm and does not allow it to focus on
retina. Maximum possible exposure (MPE) [51] specifies a
certain laser power level up to which person can be exposed
without any hazardous effect on eye or skin.
D. Related Surveys
Although, FSO communication has been studied in various
literatures before, however most of these surveys are centered
around terrestrial FSO links and very less surveys are available
for space-based optical links. For example, a survey paper
by Khalighi and Uysal [67] has elaborated various issues
in FSO link according to communication theory prospective.
They have presented different types of losses encountered in
terrestrial FSO communication, details on FSO transceiver,
channel coding, modulation and ways to mitigate fading
effects of atmospheric turbulence. Similarly, Bloom et al. [68]
have quantitatively covered various aspects that affect the
performance of terrestrial FSO link - atmospheric attenuation,
scintillation, alignment or building motion, solar interference
and line-of-sight obstructions. Another survey by Demers
et al. in [69] solely focused on FSO communication for
next generation cellular networks. An introductory paper on
terrestrial FSO communication by Ghassemlooy et al. [13] and
Henniger et al. [43] provide an overview of various challenges
faced in the design of FSO communication. In [70], the
authors laid emphasis on deep space optical communication
requirements and its future prospective. Similarly, in [71],
the authors presented various trends and key initiatives used
in deep space optical links. They have discussed the optical
communication road-map for meeting future requirements and
performance benefits in deep space optical links. Our survey is
also related to space-based optical communication with focus
on various challenges, current status and latest research trends
in this field. In our work, we are intending to provide the
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Communications Surveys & Tutorials
5
Mission Laser Wavelength Other parameters
Application
Semiconductor
Inter-satellite Link
Experiment (SILEX) [52]
AlGaAs laser
diode
830 nm 60 mW, 25 cm telescope size, 50
Mbps, 6 µrad divergence, direct
detection
Inter-satellite
communication
Ground/Orbiter
Lasercomm
Demonstration (GOLD)
[27]
Argon-ion
laser/GaAs laser
Uplink: 514.5 nm
Downlink: 830 nm
13 W, 0.6 m and 1.2m transmitter
and receiver telescopes size,
respectively, 1.024 Mbps, 20 µrad
divergence
Ground-to-satellite link
RF Optical System for
Aurora (ROSA) [53]
Diode pumped
Nd:YVO4 laser
1064 nm 6 W, 0.135 m and 10 m
transmitter and receiver telescopes
size, respectively, 320 kbps
Deep space missions
Deep Space Optical Link
Communications
Experiment (DOLCE)
[54]
Master oscillator
power amplifier
(MOPA)
1058 nm 1 W, 10-20 Mbps
Inter-satellite/deep space
missions
Mars Orbiter Laser
Altimeter (MOLA) [55]
Diode pumped Q
switched
Cr:Nd:YAG
1064 nm 32.4 W, 420 µrad divergence, 10
Hz pulse rate, 618 bps, 850 µrad
receiver field-of-view (FOV)
Altimetry
General Atomics
Aeronautical Systems,
Inc. (GA-ASI) & TESAT
[56]
Nd:YAG 1064 nm 2.6 Gbps
Remotely piloted aircraft
(RPA) to LEO
Altair UAV-to-ground
Lasercomm
Demonstration [57]
Laser diode 1550 nm 200 mW, 2.5 Gbps, 19.5 µrad
jitter error, 10 cm and 1 m uplink
and downlink telescopes size,
respectively
UAV-to-ground link
Mars Polar Lander [58] AlGaAs laser
diode
880 nm 400 nJ energy in 100 nsec pulses,
2.5 kHz rate, 128 kbps
Spectroscopy
Cloud-Aerosol Lidar and
Infrared Pathfinder
Satellite Observation
(CALIPSO) [59]
Nd:YAG 532 nm/1064 nm 115 mJ energy, 20 Hz rate, 24 ns
pulse
Altimetry
KIrari’s Optical Downlink
to Oberpfaffenhofen
(KIODO) [60]
AlGaAs laser
diode
847 nm/810 nm 50 Mbps, 40 cm and 4 m
transmitter and receiver telescopes
size, respectively, 5µrad
divergence
Satellite-to-ground
downlink
Airborne Laser Optical
Link (LOLA) [36]
Lumics fiber
laser diode
800 nm 300 mW, 50 Mbps
Aircraft and GEO
satellite link
Tropospheric Emission
Spectrometer (TES) [61]
Nd:YAG 1064 nm 360 W, 5 cm telescope size, 6.2
Mbps
Interferometry
Galileo Optical
Experiment (GOPEX)
[25]
Nd:YAG 532 nm 250 mJ, 12 ns pulse width, 110
µrad divergence, 0.6 m primary
and 0.2 m secondary transmitter
telescope size, 12.19 x 12.19 mm
charge coupled device (CCD)
array receiver
Deep space missions
Engineering Test Satellite
VI (ETS-VI) [62]
AlGaAs laser
diode (downlink)
Argon laser
(uplink)
Uplink: 510 nm
Downlink: 830 nm
13.8 mW, 1.024 Mbps
bidirectional link, direct detection,
7.5 cm spacecraft telescope size,
1.5 m Earth station telescope
Bi-directional
ground-to-satellite link
Optical Inter-orbit
Communications
Engineering Test Satellite
(OICETS) [63]
Laser Diode 819 nm 200 mW, 2.048 Mbps, direct
detection, 25 cm telescope size
Bi-directional Inter-orbit
link
Solid State Laser
Communications in Space
(SOLACOS) [64]
Diode pumped
Nd:YAG
1064 nm 1 W, 650 Mbps return channel and
10 Mbps forward channel, 15 cm
telescope size, coherent reception
GEO-GEO link
Short Range Optical
Inter-satellite Link
(SROIL) [65]
Diode pumped
Nd:YAG
1064 nm 40 W, 1.2 Gbps, 4 cm telescope
size, BPSK homodyne detection
Inter-satellite link
Mars Laser
Communications
Demonstration (MLCD)
[66]
Fiber laser 1064 nm and
1076 nm
5 W, 1- 30 Mbps, 30 cm
transmitter telescope size and 5 m
and 1.6 m receiver telescope size,
64 PPM
Deep space missions
Table II
WAVELENGTHS USED IN PRACTICAL FSO COMMUNICATION SYSTEMS
![Table IV MODTRAN SIMULATED ATMOSPHERIC TRANSMITTANCE FOR VARIOUS WAVELENGTH AT 5 KM OF VISUAL RANGE [84]](/figures/table-iv-modtran-simulated-atmospheric-transmittance-for-2g6y0dip.webp)
![Table III MOLECULAR ABSORPTION AT TYPICAL WAVELENGTHS [80]](/figures/table-iii-molecular-absorption-at-typical-wavelengths-80-2c33wdz6.webp)
