Eye Safety Issues with Optical Communications Completed (Closed WGs)/Optical... · Eye Safety Issues with Optical Communications Free Space Optical Communications ... The Optical

© CNES 2012 1

SF32-39I

Eye Safety Issues with Optical

Communications

Free Space Optical Communications

Between “LEO and ground” or “GEO and ground”

SFCG-32

12-20 June, 2012

Darmstadt, Germany

© CNES 2012

Abstract

2

Several Space Agencies are working at future optical communications systems.

Types of links under consideration are Earth orbit to Earth orbit, Earth orbit to Earth, Earth to Earth orbit, that is the more frequent links, compared to links involving Moon, Mars, Deep Space, …

Contrary to radio frequencies, exposure of the human eye to optical laser beams may damage the eye cornea; exposure may occur with space-Earth and Earth-space links but also space-space if a human vessel is involved

Sensitivity of the eye cornea to optical beams is very much dependent upon power level but also wavelength

International norms exist defining the Maximum Permissible Exposure (MPE) versus power, wavelength and time; this paper is based upon the European norms

This paper addresses the eye safety criticality for the two optical wavelengths currently considered for future space communications: 1.55μm and 1.06μm

The paper concludes on 1.55μm being significantly safer than 1.06μm in term of Maximum Permited Exposure, for the considered links

© CNES 2012 3

Introduction (1)

Consultative Committee

for Space Data Systems Open international standards

for mission interoperability

Optical Link Study

Group Looking at requirements

for optical coms in

space

Interagency Operations

Advisory Group Interoperable mission support

infrastructure

Interoperability

Plenary Interagency

agreements on

space

interoperability

International coordination between space agencies is ensured through several organisations.

Regional organisations

© CNES 2012 4

Eye safety is a parameter to take into account in the design of future space optical telecommunications.

Earth observation and telecommunication services based on operational Free Space Optical Communications (FSOC) systems need to comply with eye safety regulations :

The European Comity for Standardisation (CEN) defined in its second version of the Norm EN 60825-1 the level of Maximum Permissible Exposure (MPE) to lasers. This level is based on the values of the International Commission on Non-Ionizing Radiation protection (ICNIRP). These values were derived from experimental studies and should not be considered as perfect limit. In any case, the CEN norm stresses the fact that exposure to laser has to be as low as possible.

Norme Européenne / Française NF EN 60825-/A11, Sécurité des appareils lasers, juin 2000.

This paper addresses the threats that 1.55μm and 1.06μm lasers represent for the human eye in order to derive the constraints on the FSOC system, e.g. in terms of emitted power and limit distance.

Introduction (2)

© CNES 2012 5

Eye safety study cases (1)

1) Observer close to the Optical Ground Station OGS

The observer is on Earth close to the OGS looking at the Laser Communication

Terminal (LCT) on board the GEO/LEO satellite. The distances between the

observer and the LCT are:

LEO: 700 km; GEO: 38450 km.

Only the downlink is considered with a duration of exposure of 10 sec and 0.1 sec.

2) Observer with binocular close to the OGS

Now, the observer uses binoculars. Binoculars increase the collecting aperture

size for photons and so the MPE is lower than for the previous case.

Even if the collecting aperture size increases, the scintillation effect does not

change because we are at saturation level.

© CNES 2012 6


3) Aircraft crossing the laser path

In this case, an airplane flying at 10km altitude and crossing the laser beam is

firstly considered. In this context, 10 sec and 0.1 sec time exposures are

taken into account (10 sec illumination is not realistic for an airplane, but

covers the case of aircraft in stationary flight (e.g. a helicopter)). Uplink and

downlink are analysed.

Secondly, a parameters analysis presents exclusion zones for stationary

aircrafts (e.g. helicopter) and for non-stationary airplanes.

The range of optical emitted power goes from 1W to very high power optical

links such as 300W.

© CNES 2012 7

4) Astronaut sighting the laser source with telescope

In this case, an astronaut on board a space vehicle on LEO orbit (e.g. Space

Station at 400km altitude) is considered. The minimum distance to the

laser emitter depends on the respective orbits:

~35000 km for GEO-astronaut (downlink);

300 km for LEO-astronaut (downlink);

400 km for ground-to-astronaut (uplinks to the GEO and the LEO

satellites).

As the telescope increases the collecting size for photons, the MPE is

lower than for the previous case. As for the binoculars cases, the

scintillation effect does not change because we are at saturation level.


© CNES 2012 8

The emitter parameters (pupil diameter size and emitted optical power) are considered as inputs.

The Optical Image Telemetry (OIT) link is highly asymmetrical, with image data transmission only on downlink (from LEO to ground).

The uplink is a laser beacon with increased divergence used for link acquisition and tracking.

A 0.8 µm laser beacon, compatible with Si detector as CMOS detector is also presented.

For completeness, uplink telecommunication link at 1.06 µm and 1.55 µm is considered in the analysis.

The feeder application involves symmetrical high data rate for up and down links, and 0.98 µm beacon (set closer to the telecom wavelength).

Both 0.8 µm and 0.98 µm wavelengths were included in the study but are not baselined today, contrary to the 1.06 µm and 1.55 µm wavelengths.

Emitter parameters

Optical Terminal Specifications

Beacon

Feeder TMI

Wavelengths 1,55 1,06 1,55 1,06 1,55 1,06 1,55 1,06 0,98 0,8 µm

Optical Power 100 100 10 10 100 100 1 1 120 120 W

Emitter diameter 250 250 60 60 200 200 200 200 NA NA mm

Divergence(-3 dB) 15,1 10,3 63,0 43,1 18,9 12,9 18,9 12,9 1000 1000 µrad

TMI

Uplink

TMI Unit

BaliseDownlink

FeederFeeder OIT OIT OIT

© CNES 2012 9

Emitted signal and Efficiency

The emitted signal is modulated at very high frequency (typically more than 10 GHz) to be consistent with the minimum 5 Gbps useful data rate required for OIT application.

For eye safety analysis, 57% emitter efficiency is considered for both uplink and downlink with the GEO and LEO-satellites. This efficiency includes all emission losses (telescope obscuration ratio, optical transmission factor, and wave front error).

For eye safety analysis, the considered receiver efficiency is:

■ For the eye only: 100%

■ For the system {eye + telescope}: 95%

■ For the system {eye + binoculars}: 90%

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Absorption and diffusion

– The transmission coefficients for the different wavelengths are estimated from MODTRAN

model. They are:

A transmission coefficient of 80-83% is considered for all the wavelengths except for

1.55 μm wavelength.

Lclouds = 0dB

The eye safety analysis considers cloud-free laser path as worst case of transmission.

Atmospheric Losses

Unit

800 980 1060 1550 nm

80% 83% 83% 90% No-Unit

Tranmission coefficient depending on Wavelenghts

© CNES 2012 11

Atmospheric losses Scintillations (1)

Scintillation

The wind is a turbulent flow of air generating random fluctuations of the optical index of air.

Thus, the optical wavefront propagating through the atmosphere is distorted, causing

scintillation at the receiver level.

Signal fluctuation probability

The scintillation leads to quick variation of the signal strength. It decreases the average signal intensity Io

but can also lead to temporary maximum by constructive interferences which may be dangerous for

the eye. This stochastic complex phenomenon depends on:

- The turbulence profile of the stratosphere (diurnal and time varying);

- The wavelength;

- The path distance in the stratosphere (depending on the elevation angle);

- The receiver aperture (averaging effect).

The normalized signal intensity variation due to scintillation follows a log-normal probability density

function with a mean value of 1 (because of normalization I/Io) and a variance (related to the

scintillation index SI) characteristic of the scintillation strength. Several models exist to compute the

scintillation variance for low scintillation turbulence (Rytov approximation). Equations also exist to

compute the scintillation variance for high turbulence from low turbulence results. Once the

scintillation variance is estimated, the log-normal probability density function of the signal intensity is

obtained.

More details in ANNEXE 1

© CNES 2012 12

Link parameters summary (Uplink)

link budget parameters depending on the transmission case

800 980 1060 1550

80% 83% 83% 90%

TMI 90° 90° 90° 90°

Feeder 40° 40° 40° 40°

TMI 0dB 0dB 0dB 0dB

Feeder 0dB 0dB 0dB 0dB

TMI 0 0 0 0

Feeder 0 0 0 0

TMI 57% 57% 57% 57%

Feeder 57% 57% 57% 57%

Eye only 100% 100% 100% 100%

Eye + binoculars 90% 90% 90% 90%

Eye + telescope 95% 95% 95% 95%

TMI 9,32 9,32 9,32 9,32

Feeder 9,32 9,32 9,32 9,32

Uplink

Clouds (dB)

Scintillation Margin (dB)

Depointing (dB)

Elevation angle

Eye Safety worst case

Receiver efficiency (%)

Emitter efficiency (%)

Transmission

Parameters

OIT

OIT

OIT

OIT

OIT

© CNES 2012 13

Link parameters summary (Downlink)

link budget parameters depending on the transmission case

800 980 1060 1550

80% 83% 83% 90%

TMI 90° 90° 90° 90°

Feeder 40° 40° 40° 40°

TMI 0dB 0dB 0dB 0dB

Feeder 0dB 0dB 0dB 0dBTMI 700 700 700 700TMI 0 0 0 0

Feeder 0 0 0 0

TMI 57% 57% 57% 57%

Feeder 57% 57% 57% 57%

Eye only 100% 100% 100% 100%

Eye + binoculars 90% 90% 90% 90%

Eye + telescope 95% 95% 95% 95%

TMI 6,91 6,91 6,91 6,91

Feeder 6,91 6,91 6,91 6,91Scintillation Margin (dB)

Clouds (dB)

Emitter efficiency (%)

Elevation angle

Transmission

Receiver efficiency (%)

Downlink

Depointing (dB)

ParametersEye Safety worst case

OIT

OIT

OIT

OIT

OIT

© CNES 2012 14

For frequency modulation higher than 55 kHz and view angle[1] smaller than 1.5mrad, the Norm

EN 60825-1 defines different level of MPE depending on the exposure time and the wavelengths:

Analysis of eye safety regulations

[1] The view angle is the angle delimited by the eye and the extremities of the laser source (telescope). With our telescopes size, the

minimum view angle of 1.5mrad corresponds to a distance observer-telescope around 166 meters. Even for a 1 meter telescope, the

minimum distance would be 400m.

MPE at the cornea as a function of the exposure time

Dimensioning parameter: Maximum Permissible Exposure (MPE)

Wave

length

nm

© CNES 2012 15

Unit

nm

C4 10^ (0.002 x ( -700)) = 1,58 10^ (0.002 x ( -700)) = 3,63 5 No-Unit

C7 1 1 1 No-Unit

Coefficients depending on Wavelenghts

Coefficients 800 980 1060 1550

The coefficient C4 and C7 are derived from the Norm NF EN 60825

Coefficient for MPE calculation at 1.06 and 1.55

Two different exposure times are considered :

-An exposure time of 10 seconds as it is representative of any exposure time longer than 10 seconds because

for the wavelengths of interest, the MPE is constant. This is clearly a worst case approach because it is

unlikely that an observer keeps sighting during such long duration. Over 10 sec or more, scintillation is

averaged; so its contribution is set to 0dB.

- An exposure time of 0.1 second to cover the case of short-term signal amplification by scintillation. 0.1

second is selected. The amplification gains are applied to the received signal.

MPE at the different laser

wavelengths and for the two

cases of exposure time

Unit

nm

EMP for 10s

exposure time15,8 36,3 50 1000 W/m2

EMP for 0,1s

exposure time 50,6 116 160 100000 W/m2

980 1060 1550

EMP depending on Wavelenghts

800

1.55 μm lasers are significantly safer than 1.06 μm lasers in terms of MPE

Dimensioning parameter: Maximal Permissible Exposure (MPE)

© CNES 2012 16

Observer’s collecting aperture

We will consider observers with bare eyes, but also observers with 50 mm

binoculars and with 500 mm telescope. These optical magnifying devices

increase the collecting size of the eye. From the eye pupil size, specified to 7

mm in EN 60825-1, the following K parameter is defined:

eyeDiameter

toolsopticalDiameterK

_

__

K Ratio vs observer tool

Because the collecting aperture increases, the MPE has to be

decreased by a factor equal to the square of κ.

Name 800 nm 980 nm 1 060 nm 1 550 nm

Binoculars 7 7 7 7

Telescope 71 71 71 71

Wavelenghts

K ratio depending on Wavelenghts

50 mm

500 mm

Size

Tools

© CNES 2012 17

Synthesis of EMP vs. observer, exposure time and wavelengths

Knowing the ratio κ, the EMP (or MPE) is obtained thanks to:

2

_)(

eyeonlyEMPEMP

Unit

800 980 1060 1550 nm

Eye only 12,0 15,6 17,0 30,0 dBW/m2

50mm Binocular -5,1 -1,5 -0,1 12,9 dBW/m2

500mm Telescope -25,1 -21,5 -20,1 -7,1 dBW/m2

EMP for 10 sec exposure time depending on Wavelenghts

EMP for 10 seconds exposure time depending on wavelengths and observer tools

Unit

800 980 1060 1550 nm

Eye only 17,0 20,7 22,0 50,0 dBW/m2

50mm Binocular 0,0 3,6 5,0 32,9 dBW/m2

500mm Telescope -20,0 -16,4 -15,0 12,9 dBW/m2

EMP for 0,1 sec exposure time depending on Wavelenghts

EMP for 0.1 seconds exposure time depending on wavelengths and observer tools

The eye safety study is based on these EMP tables.

The different cases of study are considered as the most relevant worst cases

© CNES 2012 18

METHODOLOGY

Optical link budget

LlosrGrAtmosGeePlasererGrGeAtmosDEPlaser ),,,,,,,,Pr(

Plaser: Total amount of output optical power (Laser + HPOA + DWDM).

: Emitter efficiency (telescope obscuration, transmission and optical quality)

: Receiver efficiency (transmission of bare eye or {eye + optical magnifier})

Ge: Emitter gain defined by telescope pupil size, depends on laser wavelength.

Gr: Receiver gain: depends only on laser wavelength because the receiver pupil diameter is set to 1.128 m.

We get a surface of 1m² and directly Pr in dBW/m². It eases comparison with EMP requirements.

Llos: Free space losses. It depends on the wavelength, on the distance between the satellite and the

observer and on the elevation angle E°.

Aatmos: Losses due to atmospheric propagation: clouds attenuation, absorption & diffusion, scintillation

effect.

e

r

Then, the received power Pr and the EMP depending on the exposure time are compared. When

Pr is lower than the EMP, the system is eye safe. Conversely, the constraints on the link

parameters (e.g. emitted power) and/or on the distance of the observer are derived for the most

interesting cases (e.g. aircrafts).

© CNES 2012 19

Parameters depending on the case of study

The dependents on the case of study are Plaser, , Ge, Aatmos, Llos. r

Receiving surface diameter(mm) 1128

Altitude observer(km) 0

Elevation Angle(°) 40

Minimal Distance(km) 38450

Emitter efficiency: ne 57%

Plaser (dBW) 17,56

Emitter Gain: Ge(dB) 114,1

Free Space Loses: Llos(dB) -289,88

Atmospheric Loses: Atmos(dB) -0,64

Reception Gain: Gr 127,1

Reception Loses: nr (dB) 0,0

Power received (dBW/m2) -31,81

Eye safety: source - observer

Example of eye safety link budget

© CNES 2012 20

EYE SAFETY ANALYSIS RESULTS (1)

Observer close to the OGS -10 sec and 0,1 s exposure times -The downlinks from LEO or GEO are considered. No clouds

-Contribution of the scintillation set to 0dB (10s) and 6.91 dB (0.1s)

-OIT is the acronym of Optical Image Telemetry

Results for observer on Earth, 0.1 s

Results for observer on Earth, 10 s

Downlink is not critical for observer with bare eyes close to OGS

Wavelengths 1,55 1,06 1,55 1,06

Power received (dBW/m2) -31,81 -29,13 -19,18 -16,28

EMP over 10 secs (dBW/m2) 30 17 30 17

Margin (dBW/m2) 62 46 49 33

Downlink

Feeder link OIT

(dB)

Wavelengths 1,55 1,06 1,55 1,06


EMP over 0,1 sec (dBW/m2) 50 22 50 22

Margin (dBW/m2) 75 44 62 31

Downlink

Feeder link OIT

(dB)

© CNES 2012 21


The EMP is reduced because the collecting aperture size is increased.

The receiver {eye + binocular} efficiency is less than for the previous part. It is set to 90%. 0,1 and 10 s exp times

The downlinks from LEO or GEO are considered. There are no clouds.

The contribution of the scintillation is set to 0dB (10s) and 6.91 dB (0.1 s).

Results observer with binoculars on Earth, 10 sec

Results observer with binoculars on Earth, 0.1 sec

Downlink is not critical for observer with binoculars close to OGS.

Observer with binocular close to the OGS

Wavelengths 1,55 1,06 1,55 1,06


EMP over 10 secs (dBW/m2) 12,9 -0,1 12,9 -0,1

Margin (dBW/m2) 45 29 33 17

Downlink

Feeder link OIT

(dB)

Wavelengths 1,55 1,06 1,55 1,06


EMP over 0,1 secs (dBW/m2) 32,9 5,0 32,9 5,0

Margin (dBW/m2) 58 28 46 15

Downlink

Feeder link OIT

(dB)

© CNES 2012 22

Aircraft crossing the laser path

For the uplink, the aircraft flying above the most turbulent atmospheric layers,

the scintillation gain is kept to 9.32dB for the 0.1 second duration case.

Following a worst case approach for the downlink, the attenuation due to molecule absorption is removed and the scintillation gain for the 0.1 second duration case is kept to 6.91dB.

Losses due to the cockpit are not taken into account. The analyzed configurations are the downlinks and the uplinks from LEO or GEO to an aircraft at 10km altitude.

For the OIT case (elevation angle 90°), the distances are: ■ OGS to aircraft (uplink): 10km;

■ LEO to aircraft (downlink: 690km.

For the feeder link case (elevation angle 40°), the distances are: ■ OGS to aircraft (uplink): 16km;

■ LEO to aircraft (downlink: 36 834km.


© CNES 2012 23

Aircraft crossing the laser path 10 s exposure time (1)

The downlinks and the uplinks with a LEO or GEO satellite are considered. These

FSOCs links cross the path of an aircraft at 10km altitude. Because of the time

averaging (10 sec) the contribution of the scintillation is set to 0dB for both

links. There are no clouds.

Results observer on-board an airplane, 10 sec

The considered exposure time is conservative for an airplane: At 10 km distance

from emitter, the width of the laser beam (10 µrad typical divergence) is about

20 cm. So, an exposure time of 10 sec is relevant only for helicopters in near

stationary flight (although helicopters do no fly that high).

Feeder OIT

Wavelengths 1,55 1,06 1,55 1,06 1,55 1,06 1,55 1,06 0,98 0,8

Power received (dBW/m2) -31,2 -27,9 -18,6 -15,3 34,1 36,8 18,2 21,1 -6,9 -2,6

EMP over 10 secs (dBW/m2) 30,0 17,0 30,0 17,0 30,0 17,0 30,0 17,0 15,6 12,0

Margin (dBW/m2) 61,2 44,9 48,6 32,3 -4,1 -19,8 11,8 -4,1 22,5 14,6

BaliseDownlink Uplink

Feeder link OIT Feeder link OIT

(dB)

© CNES 2012 24

Aircraft crossing the laser path 10 s exposure time (2)

■ Feeder link

- For 10 s exposure time, the feeder uplink does not meet eye safety constraints, marginally for 1.55 µm and largely for 1.06 µm wavelength.

- The 1.06 µm case results are far away from the eye safety limit. A complete exclusion zone providing margins and aircraft navigation errors of any aircraft from the vicinity of the laser beam is mandatory. The laser beam direction is fixed for a GEO satellite, but the elevation of the beam need to be considered in the definition of the exclusion zone.

- For the 1.55 µm feeder uplink, the EMP is also exceeded but by 15dB less than for 1.06 µm case. So for the 1.55 µm feeder uplink eye safety sets coupled constraints on the flight altitude (distance to the emitter) and on the flight velocity (exposure time) related to the total amount of output optical power emitted Plaser.

■ Optical Image Telemetry

- The 1.55 µm OIT uplink meets eye safety constraints with large margin, while they are marginally violated for 1.06 µm wavelength.

- For 1.06 µm OIT system, the aircraft exclusion zone can be very wide (cone with 70 deg half angle) because of the variable direction of the laser beam to track the LEO satellite.

- Downlink and beacon offer very large margins vs. eye safety constraints.

© CNES 2012 25

The downlinks and the uplinks with a LEO or GEO satellite are considered. The contribution of the scintillation is set to:

- A positive gain of 6.91dB for the downlink (conservative).

- A positive gain of 9.32dB for the uplink.

There are no clouds.

Results observer on-board an airplane, 0.1 sec

The 1.55 µm uplink (OIT and feeder) meets eye safety constraints with large margin, while they are still violated for 1.06 µm wavelength.

1.06 µm uplinks are not eye safety compliant.

Downlink and beacon offer very large margins eye safety constraints.

Aircraft crossing the laser path 0.1 s exposure time

Feeder OIT

Wavelengths 1,55 1,06 1,55 1,06 1,55 1,06 1,55 1,06 0,98 0,8

Power received (dBW/m2) -24,3 -21,0 -11,7 -8,4 43,4 46,1 27,5 30,4 2,4 6,7

EMP over 0,1 secs (dBW/m2) 50,0 22,0 50,0 22,0 50,0 22,0 50,0 22,0 20,7 17,0

Margin (dBW/m2) 74,3 43,0 61,7 30,4 6,6 -24,1 22,5 -8,4 18,2 10,3

BaliseDownlink Uplink


(dB)

© CNES 2012 26

Aircraft exclusion zone (1)

Systems at 1.06 µm impose an airplane exclusion

zone even at low power of emission (cf.

previous results for OIT with 1 W optical power

at 1.06 µm). For systems at 1.55 µm, exclusion

zone is only required for high power uplinks,

with minimum distance of aircraft to emitter

depending on emitted power and aircraft

velocity. The following specifically addresses

high power 1.55 µm uplinks in order to delimit

an airplane exclusion zone. For the uplink, the

scintillation gain is kept to 9.32dB.

-80

-70

-60

-50

-40

-30

-20

-10

0

0

1,9

2,9

3,9

4,9

5,9

6,9

7,9

8,9

9,9

10

,9

11

,9

12

,9

13

,9

14

,9

15

,9

16

,9

17

,9

18

,9

19

,9

20

,9

21

,9

22

,9

θ (μrad)

I(θ)/

Io (

dB

)

Emitted optical pattern at 1.55 with D=200mm

The attenuation of side lobes is very large because of the high

directivity of optical system. The pink curve represents the

maximum envelope of the emission pattern.

-140

-120

-100

-80

-60

-40

-20

0 θ

66,8

132,8

198,8

264,8

330,8

396,8

462,8

528,8

594,8

660,8

726,8

792,8

858,8

924,8

990,8

1056

,8

1122

,8

1188

,8

1254

,8

1320

,8

θ (μrad)

I(θ)/Io

(dB)

Discrepancy (dB)

I(θ)/Io (dB)

optical side lobes power attenuation

More details in ANNEXE 2

© CNES 2012 27

Results for aircraft : Conclusions

Aircrafts exclusion zone above the OGS

© CNES 2012 28

Astronaut with telescope on the laser trajectory (1)

■ The Astronaut on-board a LEO vehicle at 400km altitude is looking in all directions with a 500mm telescope. The EMP is reduced because the collecting aperture size increases. The receiver {eye + telescope} efficiency is set to 95%. The distances are:

~35000 km for GEO-astronaut (downlink);

300 km for LEO-spationaute (downlink);

400 km for ground-to-astronaut (uplinks for the GEO and the LEO satellites), consistent with Space Station orbit

■ The different uplinks and the downlinks are considered.

For the downlink:

- The scintillation gain for the 0.1 second duration case is removed because the astronaut is at 400km altitude.

- The attenuation due to molecule absorption is removed because the spacecraft is located above the troposphere.

© CNES 2012 29


10 sec exposure time The downlinks and the uplinks from LEO or GEO to a spatial vehicle like the ISS at 400km

altitude with a spationaute on-board are considered. Over 10 seconds, the contribution of the scintillation is set to 0dB for both links. There are no clouds.

Results observer with telescope on-board a LEO spatial vehicle, 10 sec

OIT at 1.55 µm meet the eye safety constraints with comfortable margin.

Eye safety constraints are violated for 1.06 µm wavelength for OIT.

Feeder uplink is problematic at 1.06 µm and 1.55 µm because of the size of the great collecting aperture and the small distance between the ground and the LEO space vehicle.

Systems at 1.55 µm offer around 20dB margin over system at 1.06 µm.

Feeder OIT

Wavelengths 1,55 1,06 1,55 1,06 1,55 1,06 1,55 1,06 0,98 0,8

Power received (dBW/m2) -30,6 -27,3 -11,6 -8,3 5,7 8,4 -14,1 -11,2 -35,3 -34,5

EMP over 10 secs (dBW/m2) -7,1 -20,1 -7,1 -20,1 -7,1 -20,1 -7,1 -20,1 -21,5 -25,1

Margin (dBW/m2) 23,5 7,2 4,6 -11,8 -12,8 -28,5 7,0 -8,9 13,8 9,4

Downlink Uplink Balise


(dB)

© CNES 2012 30


0.1 sec exposure time ■ The contribution of the scintillation is set to:

- A positive gain of 0 dB for the downlink (because the spationaute is at 400km).

- A positive gain of 9.32dB for the uplink.

- There are no clouds.

Results observer with telescope on-board a LEO spatial vehicle, 0.1 sec

■ OIT at 1.55 µm meet the eye safety constraints with comfortable margin.

Eye safety constraints are violated for 1.06 µm wavelength for OIT.

Feeder uplink is problematic at 1.06 µm and 1.55 µm because of the size of the collecting aperture and the small distance between the ground and the LEO spatial vehicle. Systems at 1.55 µm offer around 20dB margin over system at 1.06 µm.

(dB)

Feeder OIT

Wavelengths 1,55 1,06 1,55 1,06 1,55 1,06 1,55 1,06 0,98 0,8

Power received (dBW/m2) -30,6 -27,3 -11,6 -8,3 15,0 17,7 -4,8 -1,9 -26,0 -25,1

EMP over 0,1 secs (dBW/m2) 12,9 -15,0 12,9 -15,0 12,9 -15,0 12,9 -15,0 -16,4 -20,0

Margin (dBW/m2) 43,5 12,2 24,6 -6,7 -2,1 -32,7 17,7 -13,2 9,6 5,1

Downlink Uplink Balise


(dB)

© CNES 2012 31

General conclusions (1)

Eye Safety Constraints Study Results

1,55 µm 1,06 µm 1,55 µm 1,06 µm 1,55 µm 1,06 µm 1,55 µm 1,06 µm

Observer 49 31 62 44

Binocular 33 15 45 28

Airplane 48,6 30,4 11,8 -8,4 61,2 43 -4,1 -24,1

Spationaute 4,6 -11 7 -13,2 23,5 7,2 -12,8 -32,7

Lowest Margin (dBW/m2) 10 second exposure time and 0,1 second exposure time are compliant

Lowest Margin (dBW/m2) 10 second exposure time is not compliant

Lowest Margin (dBW/m2) 10 second exposure time and 0,1 second exposure time are not compliant

TMI Feeder

Downlink Uplink Downlink Uplink

Astronaut

(dB)

(dB)

(dB)

OIT

© CNES 2012 32


■ Optical Image Telemetry (OIT)

1.55 μm wavelengths is eye safe with good margins in all cases.

The 1.06 μm wavelength presents eye safety risk for uplink. If such link is used (not the case in the current OIT link definition), a wide aircraft exclusion area around the ground station and a coordination with manned space missions would be needed. Moreover, downlink is also not safe for a astronaut observer. So coordination with manned space missions would also be needed.

■ Feeder link

- Downlink is safe for 1.55 μm and 1.06 μm wavelengths

- The 1.55 µm uplink marginally violates eye safety constraints for stationary aircraft like helicopters and astronaut cases. For these cases, restriction zone with diameter less than 100m along the laser path would be sufficient to protect from eye damage.

- The Uplinks at 1.55 µm with optical emitted power up to 300W is eye safe above 400m for airplanes faster than 100 m.s-1.

- The 1.06 µm uplink large violates eye safety constraints, requiring aircraft exclusion area around the ground station and coordination with manned space missions.

© CNES 2012 33


■OIT and Feeder applications do not represents risk at 1.55μm and

1.06μm wavelengths for observer on the earth with or without

binocular.

■Systems at 1.55μm give in average 20dB margin vs. 1.06μm system

for eye safety constraints.

■The 1.55 µm wavelength is therefore clearly preferable

for links between space and ground.

Eye safety sets limited constraints for low altitude airplanes and

stationary aircrafts (see Figure Aircrafts exclusion zone above the

OGS) only in the feeder uplink case, where emitted optical power is

very large (up to 300 W assumed in the analysis).

© CNES 2012 34

ANNEXE 1

© CNES 2012 35

From the log-normal probability density function, two parameters can be derived:

For Eye Safety: a scintillation gain worst case, it deals with the constructive interferences (Figure right).

I > I threshold_eye safety: Y%

Lognormal distribution of the signal intensity

For the worst case eye safety, the focus will be on the scintillation gain.

Note: Values for integration are given for illustration.


© CNES 2012 36


The scintillation variance is a bounded random value. This means that the signal intensity fluctuation is limited to a saturation level. The limit scintillation variance is far higher for the uplink than for the downlink because the turbulence layers are close to the OGS (stratosphere). At saturation level, the following changes have to be noticed:

- No more dependency on the wavelengths,

- For an average turbulence profile, when the entire stratosphere is crossed, the dependency to receiver aperture size disappears[1].

Hence, for the uplink and the downlink, two worst case scintillation index σsat_uplink and σsat_downlink can be defined for all cases (observer, observer + binocular, observer + telescope because of independency on aperture receiver). This scintillation index is valid for the different wavelengths.

Previous ground-to-satellite optical link experiments) give homogeneous values for the scintillation indices between:

- 0.05 to 1.5 for the uplink;

- 0.03 to 0.09 for the downlink.

[1] For a more turbulent atmosphere, the independency on receiver aperture requires less distance through the stratosphere.

© CNES 2012 37

A worst case level of saturation is considered:

σsa_uplink = 4 σsat_downlink = 1

It is prefered to stick to these very conservative values, since the one reported in the literature may be linked to specific conditions and thus may not be relevant worst cases for the eye safety analysis.

Using the log-normal probability density function of the normalized signal intensity variation with variance equal to 4 for the uplink and 1 for the downlink, results in the following worst case amplification of the received signal (at 99% probability):

9.3 dB for the uplink; 6.9 dB for the downlink.

The time scale of the scintillation phenomenon is also a key parameter for eye safety analysis. Indeed, as scintillation is quick, the longest time duration (i.e. minimum frequency of the scintillation spectrum) during which the optical power received will be stable needs to be determined. This is analysed for GEO & LEO cases.


© CNES 2012 38

Scintillation dynamics for GEO-ground link (1)

As explained earlier, the scintillation leads to quick variation of the signal intensity.

Figures A, B and C in the next slides are related to optical up and down links

experiment between the OGS located in Canary Islands and OPALE instrument on

board ARTEMIS.

The figure below presents time series of received beacon intensity variation at the OGS:

Figure A : Downlink signal from GEO-OPALE to OGS

- Raw data (1 kHz sampling frequency) in blue;

- Moving average and standard deviation over 1000 samples in red;

- Moving average and standard deviation over 1000 samples in green.

© CNES 2012 39

During stationary case of transmission, the scintillation leads to quick signal intensity variation and fading peaks do not exceed 1ms.

The frequency content of the received signal depends on the relative motion of the turbulence layers along the path. The atmospheric turbulence can be modelled with a gradient index profile. The selected profiles were obtained through measurement campaign performed at the Teide Observatory by Instituto de Astrofisica de Canarias in 2003. Two profiles have been defined, one for night conditions corresponding to the Best Case of turbulences (BC) and one for day corresponding for the Worst Case (WC). Turbulences during day are higher than during night.

Between 5km and 20km, the relative motion of the turbulence layers is defined by the root mean square value of wind velocity between 10m.s-1 and 30 m.s-1. It characterizes the strength of the turbulence.


© CNES 2012 40

The Power Spectral Density (PSD) of experimental received beacon is

compared to PSD models based on the atmospheric turbulence profiles.


Figure B : Typical clear sky condition downlink signal PSD

The PSD shows a cut-off frequency of a few tens of Hz, showing there are no important signal fluctuations with

frequencies higher than 100 Hz for a Geo-satellite to ground downlink.

© CNES 2012 41

At low frequency (typically below 1 Hz), scintillation is mixed to changes of

atmospheric transmission induced by light clouds or dust, or to measurement

errors. This is particularly visible on measurements reported in Figure C:


Dust, clouds

events have low

frequencies

variation

Figure C : Clouds event Downlink signal PSD

Low frequencies in the PSD of the signal are due to events like clouds or measurement disturbances. So, one question

remains, what is the typical lowest frequency of scintillation? At this point, following its worst case way of thinking, we

assumes preliminarily that all the others frequencies contribution in the PSD could be due to scintillation. Consequently:

HzdownGeoscinf 10__min_

© CNES 2012 42

For the uplink, there is a lack of data. [REF] deals with optical up and down links

experiment between the OGS located in Canary Islands and OPALE instrument

on board ARTEMIS. From [REF], the variation of the scintillation index versus

time can be extracted.

REF : Morio Toyoshima, Shiro Yamakawa, "Long-Term Statistics of Laser Beam Propagation in an Optical Ground-

To-Geostationary Satellite Communications Links," IEEE Transactions on antennas and propagation (53), 2005.

Scintillation index versus time for the uplink


© CNES 2012 43


Overall, even in clear sky condition with low atmospheric turbulences,

the instability of the scintillation index between the 6 samples per

second yields to conclude that lower frequency component than

10Hz are not relevant for uplink scintillation.

Moreover, when the scintillation increases, the low frequency

components of the PSD of the received signal have smaller

contribution. Hence, for saturated scintillation level, it is clear that a

. of 10 Hz is a worst case value that may be too low.

upGeoscinf __min_

upGeoscinf __min_

HzupGeoscinf 10__min_

© CNES 2012 44

For LEO-to-ground link, [REF] gives the PSD of

such downlinks signal for different turbulences

regimes. The measured data were taken from the

laser communication experiments between the

Optical Inter-orbit Communications Engineering

Test Satellite (OICETS) and the National Institute

of Information and Communications Technology

(NICT) OGS in 2012. Data rate up to 150-200 Mbps

(optionally 300 Mbps, towards Ka-band) with 10 W

RF output power

Frequency components lower than 10Hz are

not present in the signal spectrum. For the

uplink, the spectrum is less accurate in

[REF]. We consider here the same minimum

frequency for the uplink and the downlink.

Scintillation dynamics for LEO-ground link (1)

LEO Measured and simulated downlink PSD

for different turbulence strengths

HzdownLEOscinfupLEOscinf 10__min___min_ HzdownLEOscinfupLEOscinf 10__min___min_ HzdownLEOscinfupLEOscinf 10__min___min_ HzdownLEOscinfupLEOscinf 10__min___min_ HzdownLEOscinfupLEOscinf 10__min___min_

HzdownLEOscinfupLEOscinf 10__min___min_

[REF] : Morio Toyoshima, Takashi Sasaki, "Scintillation model of

laser beam propagation in satellite-to-ground atmospheric links,"

62nd International Astronautical Congress, 2010.

© CNES 2012 45

ANNEXE 2

© CNES 2012 46


EMP: EMP(T, h) with T (duration of exposure) dependant on h [100m;30km] and

Vairplane [5;500 m.s-1]

Flight speeds between 0m.s-1 to 500m.s-1 are considered. Helicopters go from

stationary flight to 100 m.s-1 at altitude between 0 to 10km. Airplanes flight at 10km

altitude at an average speed of 250 m.s-1.

As 90% of the total amount of optical power is concentrated in the main lobe (20*10-

6 rad divergence), the time duration of exposure to the laser is computed depending

on the altitude (h) and the speed (V) of the fighting vehicle with the following formula:

Exposure time varies from 4x10-6 to 3.6x10-2 seconds when altitude varies from

100m to 30km and speed from 5 m.s-1 to 500 m.s-1.

Then, from Table 2.1-1 we get the related EMP(T,h).

][*10*20

6

sV

hT

airplane

© CNES 2012 47


Received optical power: Precv(h,Plaser) with h [100m;30km] and Plaser [0.1W; 300W]

The received optical power Precv(h,Plaser) at a given altitude h between (100m and

30km) is computed with a scintillation gain of 9.32dB and Plaser between 0.1W and 300W.

Depointing angle and exclusion zone:

The exclusion zone is defined by:

The minimum distance from the emitter (h) where the difference between the EMP(T,h)

and the received optical power Precv is positive.

The angular offset from the emission direction where the EMP is lower or equal to the

power in the emission pattern. The offset angle q is computed from the pink curve in

Figure slide 29 that smoothes the pattern of radiation by keeping only the maximums. This

offset angle multiplied by the distance h gives the radius of the conical exclusion zone for

the distance h.

][*),( mhhTdh

© CNES 2012 48

Results for stationary aircraft (1)

■ For stationary flights, the maximum exposure time of 10 seconds is considered.

For this duration, scintillation gain is removed. Helicopters fly below 10 km.

■ The minimal altitude to be eye safe increases with the emitted optical power Plaser

as depicted in Figure 5.3-3. Below 20 W, exclusion zone is required between ground

and a given value h < 15 km. Above 20 W, an exclusion zone is required up to 15

km.

0 km

5 km

10 km

15 km

20 km

25 km

1 W 5 W 10 W 20 W 50 W 100 W 150 W 200 W 300 W

Minimum distance to laser source for stationary aircraft

© CNES 2012 49

Results for stationary aircraft (2)

Because of the large attenuation of side

lobes power level, this radius of the

exclusion zone radius dh never exceeds

20 cm (computed at 10 km distance for the

worst case emitted power of 300W).

Considering aircraft navigation

uncertainty, the exclusion zone radius is

increased to 100 m, independently from

altitude (i.e. cylindrical zone as illustrated

here).

h (km)

d10km< 20cm

E

d5km

Exclusion zone

D ~ 100m

d2km

30

0

Exclusion zone for stationary flight around laser path

© CNES 2012 50

Results for moving aircraft (1)

■ Low speed flights correspond to the worst case of eye safety because of greater duration of exposition to the laser. Airplanes flight usually at an altitude between 0.1km and 15km at a speed between 100 m.s-1 and 500 m.s-1. Considering also helicopters, the minimum velocity can be lower, typically down to 10 m. s-1. The duration of exposition (time to cross the laser beam) is between 4x10-6 to 3.6x10-1 seconds, far below 10 seconds assumed in the initial analysis.

The radius of the required exclusion zone is defined in the following table for the worst case emission power of 300 W as a function of aircraft velocity and distance to the laser emitter. The cases where the exclusion zone exists (non zero value of dh) are highlighted in red.

■ High altitude commercial airplane in cruise: h > 10km,

For high altitude aircraft (> 10 km in the worst case of zenith sighting), corresponding to commercial aircraft in cruise, there is no eye safety constraint, whatever the velocity.

■ Low altitude airplane: 0.1km < h < 10km

For lower distance, the exclusion zone depends on the velocity. Distances lower than 100m from the OGS are forbidden to any aircraft.

The minimal altitude to be safe depends on the emitted optical power. For optical power below 100W there are no restrictions for low altitude airplanes above 100m. Above 400m there are no eye safety restrictions for airplane faster than 100 m.s-1 and emitted optical power lower than 300W.

Total optical

power

Safe altitude

Vaircraft >

100m/s

100,00 W 100m

150,00 W 100m

200,00 W 200m

300,00 W 400m

© CNES 2012 51

Optical Power 300 W

92,3 dB 78,3 dB 72,3 dB 66,3 dB 60,3 dB 56,7 dB 54,2 dB 52,3 dB

v d-> 0,10 km 0,50 km 1 km 2 km 4 km 6 km 8 km 10 km

0 0,021 m 0,036 m 0,045 m 0,057 m 0,072 m 0,079 m 0,090 m 0,097 m

5 m/s 0,001 m 0,004 m 0,007 m 0,008 m 0,013 m 0,017 m 0,000 m 0,000 m

10 m/s 0,001 m 0,003 m 0,004 m 0,007 m 0,000 m 0,000 m 0,000 m 0,000 m

20 m/s 0,001 m 0,002 m 0,003 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

30 m/s 0,001 m 0,002 m 0,003 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

40 m/s 0,001 m 0,002 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

50 m/s 0,001 m 0,002 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

60 m/s 0,000 m 0,001 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

70 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

80 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

90 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

100 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

150 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

200 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

250 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

300 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

350 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

400 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

450 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

500 m/s 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m 0,000 m

Eye safety constraints vs. aircraft velocity & distance to laser source (Plaser = 300W)

Results for moving aircraft (2)

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