1. 1 Free-space optical links for stratospheric platform networks German Aerospace Centre (DLR)

2. 2 Free-space optical links for stratospheric platform networks Document Properties

3. 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. 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. 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. 6 Free-space optical links for stratospheric platform networks graze height Inter-platform links: link distance Geometric restriction Optical links blocked by clouds Maximum link distance for 100% availability determined by maximum cloud ceiling

7. 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. 8 Free-space optical links for stratospheric platform networks Inter-platform links: link distance Cloud ceiling Maximum cirrus altitude for temperate latitudes: After: Goldfarb, L. et al, “A climatological study of cirrus clouds by lidar”

9. 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. 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. 11 Free-space optical links for stratospheric platform networks Atmospheric Effects: Attenuation HAP height: 22 km Attenuation [dB] Wavelength [nm] 30  Elevation 45  Elevation 90  Elevation Atmospheric attenuation for the downlink Maximum cirrus altitude for temperate latitudes

12. 12 Free-space optical links for stratospheric platform networks Atmospheric Effects: Turbulence Index-of-refraction effects Turbulent 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. 13 Free-space optical links for stratospheric platform networks Atmospheric Effects: Turbulence D Aperture > correlation length (intensity) Decrease of power variance at the receiver Aperture- Averaging (IM/DD): D Aperture Simulated intensity distribution at the receiver

14. 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. 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