Author Topic: TODAY: CAOS: PhD Thesis Colloquium: 03 March 2020 (Tuesday): 3:30 pm  (Read 1210 times)

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TODAY: CAOS: PhD Thesis Colloquium: 03 March 2020 (Tuesday): 3:30 pm
Title: "Effects of aerosols and atmospheric boundary layer dynamics on refractive index fluctuations: implications for Free-Space Optical communication"

Candidate: Mr. Anand N

Date: 03 March 2020 (Tuesday)

Time: 3:30 PM

Venue: CAOS Seminar Hall

Tea/Coffee at 3:15 pm

ALL ARE WELCOME

Abstract:

Laser communication through atmospheric channels is an emerging wireless technology,commonly known as Free-Space Optical (FSO) communication. It facilitates unprecedented channel capacity and very large bandwidth favoring huge-volume data transfer across spatially separated locations. Such systems essentially consist of a laser beam pointed towards a distant photodetector and are devoid of bulky copper cables or optical fibers. However, the performance of FSO links depend largely on the atmospheric channel state. There are two main processes leading to signal degradation in FSO links: attenuation due to scattering and/or absorption and intensity fluctuations due to scintillation and/or beam wander (represented using the refractive index structure parameter Cn2). While the attenuation effects can be quantified and compensated, it is very difficult to quantify Cn2 owing to the inherent randomness of atmospheric turbulence. When present within the atmospheric channel, aerosols, the tiny solid/liquid particles suspended in the air, can cause reduction in received signal through absorption and scattering and by additional quasi-random modulations of Cn2; understanding of both being critical to designing of efficient and all-weather links. Due to its large spatio-temporal, and vertical variations and wide range of physical, optical and chemical properties, aerosols can introduce considerable inhomogeneity in the atmosphere.

Investigations from this thesis establish the hitherto unlooked dependence of Cn2 on the aerosol radiative effects. The roles of residence time, concentration and vertical distribution of aerosols are delineated. The influence of aerosols on Cn2 was found to be strong enough to induce a regime shift from weak to moderate turbulence. On the other hand, absorption of solar radiation by elevated aerosol black carbon (BC) layers increases the atmospheric stability and suppresses the refractive index fluctuations thereby favoring better channel properties. An increase in signal absorption is compensated by the large decrease in Cn2 and have strong implications for aerial FSO communication links. The possible use of elevated BC layers as proxies for identifying conducive altitudes for high-performance aerial FSO links is put forward. In order to characterize the seasonality in diurnal variations of Cn2 and aerosols, and their combined effects on FSO communication, concurrent and collocated observations of aerosols and the state of turbulence of atmospheric boundary layer (ABL) have been carried out at the climate research laboratory in the second campus of IISc at Challakere, for over one full year. These observational data are used to characterize the diurnal changes and its seasonality in optical link performances. Strong inverse dependency, controlled by the time of the day, exists between BC and Cn2. Merits of low nocturnal Cn2 are vitiated by the high attenuation caused by increased aerosol concentration resulting from confinement by the capping inversions. Fractional contribution of aerosols to optical attenuation is significant and shows large diurnal variations. Competing effects of Cn2 and aerosols on atmospheric links are estimated for different environmental conditions. Daytime attenuation is largely dominated by Cn2 while the nighttime values are dependent on the aerosol concentration as well. Finally, the effects of aerosols and ABL dynamics on the performance of ground-to-satellite FSO communication links are characterized, in terms of data rate, coherence length and scintillation index of the communication link. Existing Cn2 models do not account for the aerosol effects explicitly/realistically. Results from this thesis put forward the requirement of incorporating the effects of aerosols in Cn2 models and during the estimation of FSO communication link budget.

ALL ARE WELCOME

-----------------------------------------------------
S. K. Satheesh

Chair, Divecha Centre for Climate Change
Professor, Centre for Atmospheric and Oceanic Sciences
Indian Institute of Science
Bengaluru, India
Tel: 91-80-2293 3070; Fax: 91-80-2360 0865
Thanks & Regards

Sonali Pattanayak
Research Associate
Divecha Center for Climate Change
IISc,Bangalore-12