Free-Space Optical Communications: Attenuation in Clouds and … · 2013-12-12 · Free-Space...

Attenuation in Clouds and Downlink Availability > Florian Moll > 20.10.2009

Free-Space Optical Communications: Attenuation in Clouds and Downlink Availability

Joint Workshop of COST297 (HAPCOS, WG2) Florian

Moll, German Aerospace Center (DLR)

October 2009

Attenuation in Clouds and Downlink Availability > Florian Moll > 20.10.2009Slide 2

Table of Content

Motivation: Search for Cloud Propagating WavelengthsAttenuation in CloudsLink Availability Estimated from Earth Observation ProductsAvailability of Ground Station Diversity SystemsConclusion

Motivation: Search for Cloud Propagating Wavelengths

Wavelengths must lie inside an atmospheric windowAbove 20µm no feasible wavelength can be found due to absorption bands of H2

OIn the VIS and NIR clouds are totally opaque

Table of Content

Introduction: Search for Cloud Propagating WavelengthsAttenuation in CloudsLink Availability Estimated from Earth Observation ProductsAvailability of Ground Station Diversity SystemsConclusion

Attenuation in Clouds

Variable water contentParticle size and shapeVariable thickness

Cloud type Base height [km]

Vertical extent [km]

Water content [g/m3]

Stratus 0.1 –

0.7 0.2 –

0.8 0.29

Stratocumulus 0.6 –

1.5 0.2 –

0.8 0.15

Nimbostratus 0.1 –

1.0 2 -

3 0.65

Altostratus 2 –

6 0.2 -

2 0.41

Cumulus 0.5 –

1.0 0.5 –

5 1.00

Cirrus 6 –

10 1.0 –

2.5 0.064

Particle size distribution and complex index of refraction → cloud modelThe Mie calculation results in extinction coefficient of the cloud

High attenuation over the optical spectrumMinimization of cloud attenuation: Wavelengths between 10 and 12

Total attenuation is calculated with the vertical extent Stratus and Stratocumulus have an extent of 200-800m

Table of Content

Link Availability Estimated from Earth Observation Products

Different databases: MODIS, ECC, MSG, SYNOP, etcECC-data chosen for current workAssumption: all cloud occurrence blocks the optical link (vertical)Five years of data regarded: 1990/1995/2000/2004/2005

Color Composite Coverage

Location Availability [%] Std [%]

Kiruna(Sweden) 33 9

Neustrelitz(Germany) 37 4

Oberpfaffenhofen(Germany) 40 6

Aix-en-Provence(France) 68 5

Villafranca(Spain) 66 9

Calar Alto(Spain) 63 11

Mean cloud coverage over the year

Link Availability Estimated from Earth Observation ProductsHigh dynamics between and in the years

Table of Content

Multiple Stations increase availabilityDistance between stations must be sufficiently long (>900 km) to

negligible correlation between weather cellsIf stations are closer correlation must be taken into account

Fit of Correlation Coefficient

White area: over 60 % availability

Availability of Ground Station Diversity Systems

System in Europe System in Germany

Comparison of two ground station diversity systems

Almost 99 % availability

Big difference

Small difference

Availability of Ground Station Diversity Systems

Comparison of two ground station diversity systemsDecrease of variation → increase of reliability

Table of Content

Conclusion and things to remember

Wavelengths between 10 and 12 µm can be used for lower cloud attenuationOptical window VIII (7 –

15 µm) offers possibilities of use (CO2

-Laser, Quantum Cascade Lasers)ECC data makes it possible to estimate link availability in EuropeAvailabilities exceeding 65 % over the year can be reached with a single ground station in southern EuropeAn availability of almost 99 % can be reached with four ground stations in southern EuropeWhen ground stations have large distances (>900 km) the correlation of weather cells is very low → negligible

What remains

Statistics with larger databaseComparison of statistics derived from different databasesCalculation of ice cloud attenuation including realistic composition of non-spherical particles

Thank you very much for your attention…

Examples of Stratocumulus species

Examples of Stratus species

Examples of Cumulus species

Wavelength selection for better cloud transmissionHigh resolution calculation of molecular absorption is done with

GENLN2, so all absorption lines are taken into account

Cloud attenuation/ Mie scattering of spherical particles

Theory of Mie calculates the Extinction Efficiency Qe

of a single spherical water dropParameters: drop size r, wavelength λ, complex refractive index n

R. Martini (2005)

Particle density

Cloud attenuation/ Cloud models

Clouds are described with proper modelParticle size distribution (spherical shape)Complex refractive indexHomogenous, not polluted

Particle size distribution f(r) of DermendjianConstants a, b, α, γ

describe the cloud

modelSize distribution for Stratus and Stratocumulus cloud (right)

( ) exp( )f r a r b r

Free-Space Optical Communications: Attenuation in Clouds and … · 2013-12-12 · Free-Space...

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