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Lunar Optical Data Links
Gregory Konesky
SGK Nanostructures, Inc.
Rutgers Symposium on Lunar Settlements
3-8 June 2007
Rutgers University
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Advantages of an Optical Data Link
High Bandwidth due to Intrinsically High Carrier Frequency
Reduced Component Size as compared toElectronic Counterparts
Ability to Concentrate Power in Narrow Beams
Very High Gain with relatively Small Apertures
Reduction in Transmitted Power Requirements
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Two Types of Lunar Optical Links
Pan-Lunar:
Between two points on the Moon
(or orbiting relay satellite)
Moon-to-Earth:
Direct transmission to the Earth’s surface
Satellite reception in Earth orbit
followed by retransmission to the surface
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Lunar Environment Considerations
Absence of significant atmosphere
-Path absorption losses minimal
-Spreading Loss dominant loss mechanism
-No Beam Wander, Scintillation, etc.
- No Weather (Clouds, Rain, Fog)
Other Environmental Considerations
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Top Four Sources of EnvironmentalConcern on the Moon
1. Dust
2. Dust
3. Dust
4. Everything Else
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1. DustElectrostatically attaches to surfaces
2. DustAtomically sharp, abrasive
3. DustWide range of particle distribution size
4. Everything ElseRadiation and Solar Flares, Temperature Swings
Micrometeorites (and not so “micro”)
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Lunar Line-of-Sight Range Limitation
Absent topological relief limitations,
curvature of the Lunar surface is limiting
For a given height, the Lunar Horizon
Will be approx. ¼ distance of Earth equivalent
Minimal Diffraction Effects (shadows)
at Optical wavelengths
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Link Budget Block DiagramMoon-to-Earth Optical Data Link
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Transmitter Power, 1 W @ 830 nm 0 dBW Transmitter Antenna Gain, 1 m Dia. 131.6 dBi Transmitter Optical Losses - 6.0 dB Space Propagation Losses -315.3 dB Losses in Vacuum 0 dB Spatial Pointing Losses - 1.0 dB Receiver Antenna Gain, 1 m Dia. 131.6 dBi Receiver Optical Losses - 6.0 dB Spatial Tracking Splitter Losses - 1.0 dB Receiver Sensitivity 84.0 dBW Link Margin 17.9 dB
Assume: 100 Mbps, 10-6 BER
Link Budget Calculation
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Aperture Gain 10 cm 111.6 dBi 0.5 m 125.5 dBi 1.0 m 131.6 dBi 1.5 m 135.1 dBi
Various Aperture Gains Expressed in dBi
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Laser Power dB Value 100 mW - 10 dBW 500 mW - 3 dBW 1.0 W 0 dBW 2.0 W 3 dBW
Various Laser Powers Expressed in dBW
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Moon - to - Earth Distancesand Associated Propagation Losses
Minimum: 364,800 km(Propagation Loss = - 314.8 dB)
Nominal: 384,00 km(Propagation Loss = - 315.3 dB)
Maximum: 403,200(Propagation Loss = - 315.7 dB)
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Associated Beam Diameters
Assume:1 meter Aperture on the Moon
830 nm Wavelength
Diffraction-Limited Beam Diameter Near Earth303 meters (364,800 km)335 meters (403,200 km)
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Earth Satellite vs Earth Surface-Based Reception
Satellite Optical Reception
Absence of Atmospheric Effects Absorption; Turbulence; Link Availability
Higher Cost Construction; Launch; Inaccessibility
Higher Tracking Slew RatesShort Reception Window; Higher Hand-Off Rates
Greater Point-Ahead Accuracy
Need to Re-Broadcast Down to Earth
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Enhanced Satellite Detection by Increasing Beam Divergence
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Link Margin is Maximized by Narrowing Beam Divergenceonce Lock-In Occurs
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Satellite vs Earth-Based Reception- continued
Earth-Based Optical Reception
Long Reception Window per Station
Atmospheric EffectsAbsorption; Turbulence; Availability; Pulse Spreading
CostRelatively Low Cost; Erecting; Maintaining; Repairing
Permits Building More Receiversas a Hedge Against Weather
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Increase Beam Divergence to Enhance Detection Probabilityby Earth-Based Receivers
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Narrowing Beam Divergence once Tracking Lock-In Occursto Maximize Link Margin
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Issues with Earth-based Reception
Atmospheric Absorption – Link Margin
Forward Scattering – Bandwidth
Turbulence – Adaptive Optics
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Hudson,1969
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Example of Multipath Forward Scattering
(Gagliardi and Karp, 1995)
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Pulse Stretching due to Multipath Scattering(Gagliardi and Karp, 1995)
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Issues Related to Atmospheric Turbulence
Naturally Occurring Temperature Variationsgive rise to Wind Eddies
Cause Index of Refraction Changeson the order of 10-6 and are Cumulative
Beam Wander – Eddies Larger than Beam DiameterBeam Spreading – Eddies Smaller than Beam Diameter
Departure from Wavefront Planarity reduces Bandwidth
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Example of relatively Clean Optical Pulse Trainand associated Detector Signal
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Example of a Distorted Optical Pulse Trainand associated Pulse-Stretched Detector Signal
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Adaptive Optics Basics
Cancels Wavefront Distortion through Phase Conjugation
Measures Wavefront Distortion through:Wavefront SensorArtificial Guide Star
Corrects Distortion through:Tilt Mirror
Segmented Mirror
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Typical Adaptive Optical System Block Diagram
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Shack – Hartmann Wavefront Sensing Technique(Tyson, 1998)
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Examples of Tilt Mirrors(Tyson, 1998)
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Example of a 19 Segment Deformable Mirror(Hardy, 1998)
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Example of a 512 Segment Deformable Mirror(Tyson, 1998)
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Example of a 941Segment Deformable Mirror(Tyson, 1998)
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Mt. Diablo to LLNL Optical Data Link Test w/wo Adaptive Optics28 km slant range and 2.5 Gbit/sec
(LFW May 2005)
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Conclusions
A Moon-to-Earth Optical Data Link can be establishedwith only modest Transmitter Power and Apertures
Satellite-based reception provides high availability dueto absence of Atmospheric Effects, but at a high cost
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Conclusions - continued
Earth-based reception prone to Atmospheric Distortions,Reduced Availability, but is a lower cost approach
Improve Availability through greater Site Redundancy
Correct Distortions with Adaptive Optics
Don’t Shoot through Clouds – Pulse Spreading