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Document Title Free-space optical links for stratospheric platforms
Document Number CAP-0195-WP34-DLR-CON-P00
Author (s) Joachim Horwath
Date 19/07/2004
Participant (s) (short names) DLR
Workpackage(s) 3.4
Total number of slides (including title and this slide)
15
Security level (PUB, RES, CON)
CON
Description / Abstract
At first the advantages and applications of optical links for CAPANINA are introduced. Possible link distances and their geometric restrictions are explained. A short introduction about atmospheric effects is given.
1
Free-space optical links for stratospheric platform networks
German Aerospace Centre
(DLR)
2 Free-space optical links for stratospheric platform networks
Document Properties
Document Title Free-space optical links for stratospheric platforms
Document Number CAP-0195-WP34-DLR-CON-P00
Author (s) Joachim Horwath
Date 19/07/2004
Participant (s) (short names) DLR
Workpackage(s) 3.4
Total number of slides (including title and this slide)
15
Security level (PUB, RES, CON)
CON
Description / Abstract
At first the advantages and applications of optical links for CAPANINA are introduced. Possible link distances and their geometric restrictions are explained. A short introduction about atmospheric effects is given.
3 Free-space optical links for stratospheric platform networks
Outline
Content
Advantages
Applications of optical links for CAPANINA
Link distances
Atmospheric effects
Attenuation
Index of refraction
4 Free-space optical links for stratospheric platform networks
Advantages of optical links
High data-rate and available bandwidth
Small terminals and antenna dimensions
Low power consumption
No interference with radio waves
5 Free-space optical links for stratospheric platform networks
Application of optical links for HAPs
Downlink:
•diversity with microwave to save platform power
•optical downlink-spatial-diversity (cloud-cover diversity)
Main application: inter-platform links
6 Free-space optical links for stratospheric platform networks
graze
height
Inter-platform links: link distance
Geometric restriction Optical links blocked by cloudsMaximum link distance for 100% availability determined by maximum cloud ceiling
7 Free-space optical links for stratospheric platform networks
Inter platform links: link distance
Geometric restriction :Example of parabolically-shaped height profile due to the curvature of the Earth
8 Free-space optical links for stratospheric platform networks
Inter-platform links: link distance
Cloud ceilingMaximum cirrus altitude for temperate latitudes:
After: Goldfarb, L. et al, “A climatological study of cirrus clouds by lidar”
9 Free-space optical links for stratospheric platform networks
Inter-platform links: link distance
Maximum inter-platform link distance Two HAPs, same altitude
maximum link distance: 400 km
10 Free-space optical links for stratospheric platform networks
Atmospheric Effects: Attenuation
Atmospheric effects have to be taken into account for all optical CAPANINA links
11 Free-space optical links for stratospheric platform networks
Atmospheric Effects: Attenuation
HAP height: 22 kmA
tten
uati
on
[d
B]
Wavelength [nm]
30 Elevation45 Elevation
90 Elevation
Atmospheric attenuation for the downlink
Maximum cirrus altitude for temperate latitudes
12 Free-space optical links for stratospheric platform networks
Atmospheric Effects: Turbulence
Index-of-refraction effectsTurbulent air- fluctuations
Small-scale temperature and refractive-index inhomogenities (turbulence cells)
Acts like small lenses
• Wave front distortions• Beam broadening• Angle-of-arrival fluctuations• Intensity redistribution
13 Free-space optical links for stratospheric platform networks
Atmospheric Effects: Turbulence
DAperture > correlation length (intensity)
Decrease of power variance at the receiver
Aperture-Averaging (IM/DD):
( , )el opt
Aperture
I P Intensity x y dxdy
DAperture
Simulated intensity distribution at the receiver
14 Free-space optical links for stratospheric platform networks
Atmospheric Effects: Turbulence
Spatial Covariance Function (986 nm)
Correlation length [m]:
4.5×10-2 (BC) 3.2×10-2 (WC)
Speckle size
15 Free-space optical links for stratospheric platform networks
Summary
Conclusion
Main application:
HAP->HAP links, HAP->SAT links
Downlink with decreased availability
Link distances: 400 to ~780km are possible
Atmospheric effects have to be taken into account
Have been Simulated
Will be measured during the downlink experiment
