OLSG Report_Final_06_05_12 - Interagency Operations Advisory ...

3.2 Highly Elliptical Earth Orbit (HEO) Scenario
Optical Link Study Group (OLSG) Final Report
IOAG.T.OLSG.2012.V1
The HEO Scenario is not elaborated as it is considered similar to the Lunar Scenario with
shorter distances.
3.3 Geostationary Earth Orbit (GEO) Scenario
This scenario is discussed in section 4.1.1.2.3 Earth Relay Optical Feeder Link and section 4.2
Telecom Mission Optical Feeder Uplink.
3.4 Lunar Scenario
3.4.1 Concept of Operations
3.4.1.1 Basic Concept of Operations
The Lunar Scenario refers to a system providing optical communications from a lunar
orbiting satellite to a ground station on the Earth’s surface. This scenario is of particular
interest since the NASA LLCD is currently in preparation for a 2013 launch. Much of this
scenario description is drawn from LLCD experience, as well as extrapolation to potential
future lunar missions. The space terminal of LLCD is called the Lunar Lasercomm Space
Terminal (LLST) and the dedicated ground terminal is called the Lunar Lasercomm Ground
Terminal (LLGT).
The first criterion for a free space optical link is geometric line of site from the spacecraft to
a ground terminal. As with other scenarios, the optical link quality is affected by the ground
station elevation angle, with lower elevations reducing the link capabilities. For all lunar
orbiting spacecraft, the first obvious requirement is line of sight to the Moon itself; thus,
lunar elevation angles will be consistent for all missions in this scenario analysis. Orbitspecific
information further refines the scenario, though this information will vary from
mission to mission.
LLCD will be launched on LADEE. This science mission will fly a relatively low altitude (250
km) retrograde lunar orbit. Due to the specific orbit, the satellite regularly passes behind
the Moon and loses contact with the ground station. For LADEE, the passes with geometric
line of sight vary between 40 and 80 minutes in duration. Lunar polar orbits that do not
pass behind the Moon (from perspective of the ground site) would have more continuous
geometric access when the Moon is in the sky.
Under current plans, LLCD will be able to communicate with two ground stations. One—the
LLGT—is a transportable terminal currently slated to be located on Mount Haleakala, on
Maui, Hawaii. The second ground site is the OCTL, located on Table Mountain, California;
this site is referred to as the Lunar Lasercomm OCTL Terminal (LLOT).
The geometry from LADEE to the two ground sites illustrates several general features of
lunar missions. These features are particularly relevant for interoperability, as multiple
ground sites would be a major benefit of interoperability. While California and Hawaii are
not at the same longitude, the fact that they are in the same hemisphere means that the
Moon is visible simultaneously to both sites much of the time. Consider Figure 29, which
shows the access times to the LLST from the two ground sites over a one-month period
beginning July 1, 2013. Access is plotted at the given elevation angle. From the figure, one
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