Wavefront distortion of data-carrying laser beam propagating through the atmosphere has been reported to have detrimental effects on the performance of Free-Space Optical (FSO) communication systems. Optical intensity fluctuation models generally assume clear air optical turbulence where atmospheric aerosols warming effects are neglected. This variation of the refractive index structure parameter (Cn2) of the atmosphere due to the aerosol induced warming and its influence on Bit Error Rate (BER) performance of FSO systems are studied in this paper using high-resolution radiosonde and multi-satellite observations of aerosols and atmospheric thermodynamics. Based on an approximate mathematical expression built on Gauss-Laguerre quadrature rule, and a radiative transfer model-based analysis, the BER of a Differential Phase Shift Keying (DPSK) FSO communication link through Exponentiated Weibull modelled turbulence with aperture averaging has been investigated. Our results show significant signal deterioration with the aerosol-induced turbulence taking a toll on the signal to noise ratio (SNR) over more than 15 dB. BER analysis under different receiver aperture dimensions is performed with the selected intensity fluctuation model. We show that aperture averaging does not have significant influence on the performance enhancement under aerosol perturbed atmospheric conditions.
Accurate characterization of atmospheric refractive index fluctuations (optical turbulence) is important in applications such as Free-Space Optical (FSO) communication and laser remote sensing, where atmospheric optical propagation is involved. In this paper, we present the statistics of near-surface optical turbulence derived from three-year sonic anemometer-thermometer observations at a semi-arid, flat terrain in peninsular India. Using concurrent and collocated measurements of meteorological fields and atmospheric aerosols (both scattering and absorbing type), the role of atmospheric boundary layer dynamics and aerosols in modulating the magnitude, evolution, and temporal variations (over diurnal and seasonal scales) of refractive index structure parameter (Cn2) are discussed. Absorption of solar radiation and the resulting atmospheric heating by aerosol particles will modify the land-atmosphere temperature gradient that regulates the near-surface Cn2. We discuss such a scenario using aerosol black carbon measurements close to the surface of the Earth and highlight the crucial role of ABL dynamics in controlling the influence of such aerosol radiative heating effects on Cn2. These results will be helpful in improving the weather model simulations of optical turbulence over semi-arid regions.
Free space optical (FSO) communication is a line of sight technology capable of carrying large volume of data using laser signals through the atmosphere. This unguided propagation of laser beams through the atmosphere confronts with turbulent fluctuations and suspended aerosol particles on its en route to the receiver. Random fluctuations in the atmospheric refractive index causes variations in the propagation constant and thereby affects the optical pulse propagation. We examine the local atmospheric warming effects of absorbing aerosols on the atmospheric refractive index fluctuation statistics and its influence on the group velocity dispersion (GVD) parameter. Black Carbon (BC) aerosols increase local temperature through solar absorption, which will be amplified when they reside in the upper atmosphere for longer duration, owing to the reduced atmospheric density prevailing at higher altitudes. To elucidate the implications of elevated BC layer heating on FSO links, vertical BC mass concentration was measured using an Aethalometer (Model AE-42, of Magee Scientific, USA) mounted on a hydrogen filled balloon. Long term analysis of multi-satellite observations along with in-situ measurements of aerosol parameters show dependence of GVD on aerosol induced local atmospheric warming. Effect of warming on outage probability of FSO systems employing chirped Gaussian pulses are also presented.
Performance of terrestrial and vertical Free-Space Optical (FSO) communication systems are strongly influenced by the atmospheric boundary layer (ABL) dynamics. In addition to the diurnal variations in the refractive index structure parameter (Cn2 ) caused by the wind speed and the temperature difference between the surface and the near-surface air mass, any other unprecedented temperature variations can lead to Cn2 variations. Even though the prevailing ‘simple’ Cn2 models capture the overall trend in vertical Cn2 variations, they fail to capture the effects of temperature inversions, especially in tropical regions. Influence of absorbing aerosols like Black Carbon (BC), which can improve the atmospheric stability by forming strong temperature inversion layers, are not considered in these models. BC can reduce the optical beam intensity by scattering and absorption and can also cause variations in refractive index by modifying the local temperature. The uncertainties in the implications of BC will be large, owing to their large spatio-temporal and vertical variations. Using high-resolution balloon measurements and multi-satellite observations coupled with a radiative transfer model, we substantiate the strong influence of absorbing aerosols on the vertical distribution of Cn2 . We report how vertical profiling of absorbing aerosols can be used to estimate altitudes with low refractive index fluctuations. The manifestations of high-altitude aerosol-induced atmospheric warming in FSO systems are also pointed out. We conclude by discussing how mass concentration of BC, a good tracer for ABL dynamics, is correlated with the near-surface refractive index fluctuations.
The LIDAR equation contains four unknown variables in a two-component atmosphere where the effects
caused by both molecules and aerosols have to be considered. The inversion of LIDAR returns to retrieve aerosol
extinction profiles, thus, calls for some functional relationship to be assumed between these two. The Klett's method,
assumes a functional relationship between the extinction and backscatter. In this paper, we apply a different technique,
called the optical depth solution, where we made use of the total optical depth or transmittance of the atmosphere along
the LIDAR-measurement range. This method provides a stable solution to the LIDAR equation. In this study, we apply
this technique to the data obtained using a micro pulse LIDAR (MPL, model 1000, Science and Engineering Services
Inc) to retrieve the vertical distribution of aerosol extinction coefficient. The LIDAR is equipped with Nd-YLF laser at
an operating wavelength of 523.5 nm and the data were collected over Bangalore. The LIDAR data are analyzed to get to
weighted extinction coefficient profiles or the weighted sum of aerosol and molecular extinction coefficient profiles.
Simultaneous measurements of aerosol column optical depth (at 500 nm) using a Microtops sun photometer were used in
the retrievals. The molecular extinction coefficient is determined assuming standard atmospheric conditions. The aerosol
extinction coefficient profiles are determined by subtracting the molecular part from the weighted extinction coefficient
profiles. The details of the method and the results obtained are presented.
In this paper, we report the results of extensive, and all-season, collocated, measurements of several aerosol parameters
[such as spectral aerosol optical depth (AOD) at 10 bands spanning from UV to IR; mass size distribution and mass
concentration of composite aerosols; as well as mass concentration and mass mixing ratio of aerosol black carbon (BC)]
for over a 4-year period (January 2000 to December 2003), from an unindustrialized coastal location, Trivandrum
(8.55°N, 76.9°E), close to the southern tip of Indian peninsula and use these properties to estimate the aerosol short wave
radiative forcing. The results show that the top of the atmosphere (TOA) forcing is significantly positive during winter
while it changes to negative during monsoon and post monsoon seasons. The surface forcing decreases from winter to
summer. Consequently, the net atmospheric absorption decreases from a high value in winter to low values during
monsoon.
We have compared the spectral aerosol optical depth (AOD) and aerosol fine mode fraction (AFMF) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) with those of Aerosol Robotic Network (AERONET) at Kanpur (26.45N, 80.35E), northern India for the pre-monsoon season (March to June, 2001-2005). We found that MODIS systematically overestimates AOD during pre-monsoon season (known to be influenced by dust transport from north-west of India). The errors in AOD were correlated with the MODIS top-of-atmosphere apparent surface reflectance in 2.1 μm channel (ρ*2.1). MODIS aerosol algorithm uses (ρ*2.1) to derive the surface reflectance in visible channels (ρ0.47, ρ0.66) using an empirical mid IR-visible correlation (ρ0.47= ρ2.1/4, ρ0.66 ρ2.1/2). The large uncertainty in estimating surface reflectance in visible channels (Δρ0.66±0.04, Δρ0.47±0.02) at higher values of ρ*2.1 (ρ*2.1>0.18) leads to higher aerosol contribution in the total reflected radiance at top-of-atmosphere to compensate for the reduced surface reflectance in visible channels and thus leads to overestimation of AOD. This was also reflected in the very low values of AFMF during pre-monsoon whose accuracy depends on the aerosol path radiance in 0.47 and 0.66 μm channels and aerosol models. The errors in AOD were also high in the scattering angle range 110°-140°, where the effect of dust non-spherity on its optical properties is significant. The direct measurements of spectral surface reflectance are required over the Indo-Gangetic basin in order to validate the mid IR-visible relationship. MODIS aerosol models should also be modified to incorporate the effect of non-spherity of dust aerosols.
Characteristics of atmospheric aerosols and their radiative impacts show large variations in space and time, leading to
large uncertainties in the climate impact assessment. Despite the concerted efforts in the last decade through several
field campaigns, the uncertainty still persists. This essentially arises due to lack of a comprehensive data on several
aerosol parameters with adequate spatial resolution and for long duration. This is particularly true for the Indian region,
with large geographical diversities, large density of population, diverse living habits and rapid industrialization. With a
view to addressing to this problem and to evolve a regional scale database for assessing the radiative impacts of
aerosols, an integrated, multi-platform field campaign, ICARB, was carried out during March-May 2006, under the
ISRO's Geosphere Biosphere Programme. The campaign involved participation by more than 100 scientists from about
35 different institutions across the country and collocated observation over the mainland, over oceans and same time
measurements of altitude profiles using aircrafts. A campaign of this magnitude is carried out for the first time in this
region, and perhaps for the first time globally. The mainland observations included a network of aerosol observatories
over Indian landmass and from islands in the Arabian Sea and Bay of Bengal, while a dedicated scientific cruise of 64-
days duration covered the vast oceanic regions around India. Altitude profiles were obtained using research aircraft of
the National Remote Sensing Agency, from 5 different bases. The details of the campaign and the preliminary findings
will be presented.
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