OLSG Report_Final_06_05_12 - Interagency Operations Advisory ...

Optical Link Study Group (OLSG) Final Report
IOAG.T.OLSG.2012.V1
of the Sun; however, with the right modulation and coding, it is possible to maintain
communications with a previously-acquired terminal passing in front of the Sun.
This process can be particularly difficult for a deep space optical communications system,
because of the very low photon flux, the relatively large apertures (which are harder to
protect from heating), and the modulation generally proposed. Such circumstances are
easily envisioned when considering the outer planets; for example, from Pluto, the Earth is
always very close to the Sun. The OLSG evaluated this problem for a Mars scenario, where
there is an optical communications terminal in orbit around Mars and an optical
communications terminal at Earth. Figure 6 illustrates what is happening during solar
conjunction in this scenario. SEP is the Sun-Earth-Probe angle, while SPE is the Sun-Probe-
Earth angle. Small SEP angles interfere with the Earth terminal’s ability to acquire and track
the laser communication signal. Small SPE angles interfere with the Mars terminal’s ability
to acquire and track the uplink beacon laser from Earth. During solar opposition, small SPE
angles also affect the Mars terminal, as the Earth is again very close to the Sun.
SPE Angle
(3)
SEP Angle
(5)
SPE Angle
(3)
Figure 6: Sun-Earth-Probe and Sun-Probe-Earth angles.
Each day there is a time of sunrise, Mars rise, sunset, and Mars set. Most of the time, both
the Sun and Mars will be in the sky simultaneously. From an optical communication systems
engineering perspective, a critical design driver is the fact that Mars is simultaneously at its
farthest distance and at its smallest SEP angle. The communication outages that arise when
either the Earth or Mars is too close to the Sun have been evaluated for various SPE angles
(and the corresponding SEP angles during solar conjunction) and are shown in Table 5. The
objective of ground terminal design will be to minimize the number of outage days. For
example, the Lunar Laser Communications Demonstration (LLCD) will operate with an SPE of
two degrees.
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