OLSG Report_Final_06_05_12 - Interagency Operations Advisory ...

Optical Link Study Group (OLSG) Final Report
IOAG.T.OLSG.2012.V1
falls below the MPE. In line 9 of the table, the NOHD is calculated based on the method
specified by the ICAO. This calculation, however, makes a far-field assumption that is not
accurate for all of the scenarios of interest. A more refined calculation including near-field
effects (following methods outlined in the ANSI standard Z136.1) is illustrated in line 10 and
shows the LEO and GEO Relay uplinks are eye-safe using 1550 nm. The Lunar scenario could
also be designed to be eye-safe with minor modifications to the scenario assumptions, thus
avoiding a requirement for a laser safety system. All other scenarios (under the current
assumptions) will require a laser safety system.
Laser uplinks from Earth are traditionally protected by local onsite occupational health and
safety standards, coordination with air traffic control authorities, and automated airspace
monitoring. While onsite occupational health and safety measures vary by region, control
measures to ensure coordination with air traffic control authorities have been identified in
the Manual on Laser Emitters and Flight Safety published by ICAO. To protect the safety of
aircraft against the hazardous effects of laser emitters, protected zones should be
established around the affected airspace within the laser-beam free flight zone (up to 600
meters above ground), critical flight zone (up to 3,050 meters), and sensitive flight zone
(above 3,050 meters). Within the laser-beam free flight zone, the intensity of laser light is
restricted to a level that is unlikely to cause any visual disruption, where irradiance is not to
exceed 50nW/cm 2 unless some form of mitigation is applied. Within the critical flight zone,
irradiance is not to exceed 5 µW/cm 2 and within the sensitive flight zone, it is not to exceed
100 µW/cm 2 . According to ICAO, these restrictions refer to visible laser beams only. But in
all navigable airspace, the irradiance level of any laser beam, visible or invisible, is expected
to be less than or equal to the MPE, unless the prior permission has been obtained by the
proper authority. Physical, procedural, and automated control measures established to
ensure that aircraft operations will not be exposed to levels of illumination greater than the
maximum acceptable irradiance level should meet one or more of the operator control
measures:
Ability to physically block the laser beam to prevent light from being directed into
protected airspace
Ability to adjust the laser beam divergence and output power or pulse energy
emitted through the system aperture to meet exposure levels
Redirection of beam in a specific area
Manual operation of a shutter or beam-termination system, used in conjunction with
airspace observers
Scanning the laser beam to reduce the level of illumination
Automated system designed to detect aircraft and terminate or redirect the beam or
shutter the system
2.3.2.3 Interference and Backscattering
A further concern to be dealt with by optical ground communications terminals located in
close proximity to astronomical telescopes is the issue of optical interference. The uplink
beacon always suffers some losses from Mie scattering and scattering by dust and water
droplets or ice crystals suspended in the air. An astronomical telescope pointing in the
direction of the beacon will image the beacon as a line emanating from the ground terminal
and extending to the position of the space terminal (see Figure 5).
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