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Transcript of Optical Free Space Links for Satellite-Ground .Optical Free Space Links for Satellite-Ground...

  • Optical Free Space Links for Satellite-Ground Communications

    www.DLR.de

    Dirk Giggenbach German Aerospace Center (DLR), Tutorial held at ASMS/SPSC, Livorno, 2014 7th Advanced Satellite Multimedia Systems Conference 13th Signal Processing for Space Communications Workshop

  • Timeline of Laser-Comm. Space-Missions (selection)

    2016

    EDRS-C

    2013

    -Sat LLCD

    2005

    OICETS

    2007

    LCTSX NFIRE -LCTs

    1998

    SPOT-4

    2001

    Artemis SILEX GEOLITE

    Pictures: ESA, JAXA, NICT, NASA, MIT, DLR

    ETS-VI

    1994 2015

    EDRS-A OSIRIS

    2014

    Sentinel-1A SOTA OPALS

    2017

    LCRD

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 2 / 44

  • Content of this Tutorial

    Introduction to FSO in Space Applications

    Technologies and Subsystems

    Atmospheric Impact on Link Quality

    Mitigation of Atmospheric Effects

    Optical GEO Feeder-Links

    Summary

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 3 / 44

  • Application Scenarios of Mobile Optical Data Links

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 4 / 44

  • Space-Ground Scenarios Inter planetary

    GEO

    LEO

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 5 / 44

  • System Sensitivity of Optical vs. RF Point-to-Point

    these two laws result in total in a linear increase of photon-flux density at the receiver with laser frequency

    Diffraction limited divergence angle reduces linearly with wavelength increase of Rx-power with 1/2 :

    2Tx Rx

    Rx TxD DP P

    L

    Ideal optical receiver performance is limited by number of photons per bit, where required energy per photon increases with shorter wavelength:

    Photonh cE

    =

    with typical values (1m / 1cm , T=300K) ~60dB can be achieved

    Ideal optical systems are limited by photon-flux fluctuations, RF-systems by thermal noise:

    2

    62 4 10opt RF BRF opt opt

    SNR k TSNR hc

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 6 / 44

  • Properties of Point-to-Point Laser Links

    Linkbudget-Gain is invested in increase of datarate reduction of Tx-power reduction of antenna (telescope) size

    according mass-reduction

    Typical parameters laser-wavelengths in the near infrared

    (850nm / 1064nm / 1550nm)

    diffraction limited Tx-divergence: below 1/1000 degree x rad

    datarates from few 100Mbps up to several Gbps are implemented Challenges

    Link blocking by clouds and fog scenario-dependent

    Signal scintillation by index-of- refraction turbulence (IRT)

    Precise pointing and tracking; Link acquisition

    Other beneficial properties Inherent tap proof

    No mutual interference between links

    No spectrum regulatory limitations

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 7 / 44

  • Introduction to FSO in Space Applications

    Technologies and Subsystems

    Atmospheric Impact on Link Quality

    Mitigation of Atmospheric Effects

    Optical GEO Feeder-Links

    Summary

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 8 / 44

  • Components of full-duplex Space Laser Terminals

    Tx-Data

    Rx-Data

    A

    B

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 9 / 44

  • Coarse Pointing Assembly (CPA) Mechanisms

    Azimuth-Elevation Gimbal (OICETS)

    One-Mirror Periscope (Alphasat)

    Pictures: JAXA, ESA, ViaLight

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 10 / 44

  • Beam-Acquisition Strategies in Space-GND Links

    Orbital and Mechanical Position Uncertainties: ~ mrad

    OGS sends divergent Beacon towards Sat, covering uncertainty area, Sat stares with wide-FoV area-sensor in direction of OGS

    Diffraction-limited scanning towards the partner who is staring with a narrow-FoV no extra beacon required

    Mixed methods of above, might require extra beacon, or zoom-optics, and variable FoV-Optics

    Laser-spot

    Uncertainty area

    FoV: Field-of-View

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 11 / 44

  • high sensitivity (in Phot./bit) high data-rates requires plane Rx-wave atmosphere is a challenge long range space links (GEO)

    sensitivity limit (photon counting) high implementation effort limited data-rate long-range medium-rate (Exploration)

    Optical Intensity Detection (incoherent)

    Direct Detection (IM/DD) with On-Off Keying (OOK)

    PPM (Pulse Position Modulation)

    with local oscillator: hom./heterodyne-BPSK, QAM

    Selfhomodyne-DPSK with PreAmp

    Complex Field Detection (coherent)

    low to medium sensitivity high data-rates low implementation effort short range with atmosph. (LEO)

    Modulation Formats & their Application Areas

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 12 / 44

  • Optical Ground Stations: ESA-OGS, Izana, Tenerife, 2400m a.s.l. Built for SILEX (ARTEMIS) 1m Cassegrain-Telescope Coud-Room for experimental setups

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 13 / 44

  • OGS-OP, DLR (Experimental Downlinks and IRT measurement sensors)

    40cm Telescope inside Dome

    Control Room

    Control Room of OGS-OP

    More global OGSs: NICT (Tokyo, Kobe, ) JPL (Table Mountain, CA) NASA / MIT, for LLCD (White Sands, N.M) Modified SLR-stations

    40cm-class

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 14 / 44

  • Transportable Ground Stations

    60cm-class

    20cm-MOGS

    Pictures by DLR, ViaLight

    TOGS with transport van and control room

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 15 / 44

  • Introduction to FSO in Space Applications

    Technologies and Subsystems

    Atmospheric Impact on Link Quality

    Mitigation of Atmospheric Effects

    Optical GEO Feeder-Links

    Summary

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 16 / 44

  • Structure of Earth Atmosphere

    Relevant for optical

    propagation

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 17 / 44

  • Atmospheric Effects on Optical Signals

    Attenuation Index-of-Refraction-Turbulence (IRT)

    Scattering Absorption

    Mie-Scat. (Clouds)

    Rayleigh-Scat. (Molecules)

    Molecular Absorption Lines

    Absorption by Aerosols (dust,

    volcanic ash, sand)

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 18 / 44

  • Atmospheric Molecular Absorption

    Near Infrared FSO

    Thermal Infrared

    Radio Wave Visible

    Absorption by Water Vapour

    Atmos. Windows

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 19 / 44

  • Mie-Scattering Cloud attenuation (up to several 100dB/km)

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 20 / 44

  • What happens when a laser beam passes through turbulent air?

    Far-field intensity-speckles

    collimated laser beam at Tx

    turbulent volume with IRT-strength

    Cn2

    Wavefront distortion Interference

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 21 / 44

  • Atmospheric Effects on Optical Signals

    Attenuation Index-of-Refraction-

    Turbulence (IRT)

    Intensity-Scintillations

    Wavefront-Distortions

    Beam Tilt Beam Wander

    Rx Angle-of-Arrival Fluctuations

    Strengths of effects is scenario-dependent (beam-divergence, wavelength, distance, Cn2-profile)

    Beam-Broadening

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 22 / 44

  • Scintillation Pattern Structures < DRx-Antenna

    Low turbulence, large speckles: Strong turbulence, small speckles:

    (satellite downlink at high elevation) (long horizontal path)

    DRx-Telescope = 1m

    Examples of Intensitiy Scintillation Patterns at Receiver Telescope

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 23 / 44

  • Introduction to FSO in Space Applications

    Technologies and Subsystems

    Atmospheric Impact on Link Quality

    Mitigation of Atmospheric Effects

    Optical GEO Feeder-Links

    Summary

    Dirk Giggenbach > "Optical Free Space Links for Satellite-Ground Communications > 24 / 44

  • Downlink: Rx-Power - Aperture Averaging Effect

    Intensity Distribution at Receiver (Far-Field Speckle Pattern)

    DRx < I

    DRx > I

    ( ) ( ),Rx

    RxA

    P t I r t dA=

    Larger Rx-Aperture reduces variance of PRx in magnitude and spectrum

    Dirk Giggenbach > "Optic