OLSG Report_Final_06_05_12 - Interagency Operations Advisory ...

Optical Link Study Group (OLSG) Final Report
IOAG.T.OLSG.2012.V1
Executive Summary
In response to an action assigned at Interagency Operations Advisory Group-14 (IOAG-14),
the Optical Link Study Group (OLSG) was established to assess if there is a “business case”
for cross support in the space communication domain for optical space communication. The
application of optical communication for payload data return has the highest potential for
missions with high data rate requirements, with the understanding that the tracking,
telemetry, and command (TTC) is to be conducted by a radio frequency (RF) communication
system. The motivation for optical space communication systems stems from the
expectation that substantially higher (10 times) data rates than RF-based solutions might be
feasible with similar onboard terminal burden (mass, volume and power). The OLSG
assessed this expectation by defining mission scenarios and corresponding space
communication system designs, examining actual or potential onboard terminal realizations,
and analyzing associated ground terminal solutions. In the course of this study, the OLSG
converged on investigating two wavelengths in its analysis—1550 nm and 1064 nm. The
maturity of onboard space terminals that are already realized (e.g., for Earth relay intersatellite
links) or are in preparation as demonstrations (e.g., for Moon-to-Earth links through
the Earth atmosphere) now requires that economical ground segment solutions be
identified for potential future operational implementations.
The OLSG found that cross support will allow sharing of the cost and usage of the global
optical terminal infrastructure needed to serve future missions and will boost missions’
scientific return. Each of the scenarios (Low Earth Orbit [LEO], Moon, Lagrange, Mars Spaceto-Earth,
and Earth relay) was analyzed to determine ground segment solutions that
maximize the data return for the mission. The OLSG found that it is always possible to
develop a technical solution for the ground segment; however, the number of ground
stations involved would be a substantial cost burden for a single agency. Special attention
was given to studying the effects of potential disruptions of optical communications due to
weather (clouds, optical turbulence and other atmospherics) and aviation interference.
Specifically, the ground segment of the optical space communication system has the
following inherent difficulties:
1. An uplink beacon is needed to facilitate the space terminal pointing. Such a beacon
has to penetrate navigable airspace and can only be operated with permission from
a national civil aviation authority, which might lead to usage constraints. The safety
assessment of the laser uplinks follows the definitions of the International Civil
Aviation Organization (ICAO), which allow one to compute the Nominal Ocular
Hazard Distance (NOHD) beyond which the laser beam is considered eye safe. The
1550 nm wavelength for uplink is favored over 1064 nm, since the maximum
permitted exposure level to the human eye is twenty times higher for 1550 nm. It
can be shown that due to the narrow beam widths of the beacon uplink, exposure
risk to aviation is very small. The technical means to cope with potential air traffic
requirements are available and practiced regularly by laser ranging stations and
astronomical observatories (laser guide stars). Our analysis shows that systems for
the LEO and Earth Relay scenarios can be designed in an eye-safe manner using 1550
nm, but that is not possible for the other scenarios using the assumptions made in
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