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Abstract - NASA is presently overseeing a project to
create the world’s first free-space laser
communications system that can be operated over
a range ten times larger than the near-earth ranges
that have been demonstrated to date. To be flown
on the Lunar Atmosphere and Dust Environment
Explorer (LADEE), which is planned for launch by
NASA in 2012, it will demonstrate high-rate laser
communications from Lunar orbit to a
transportable ground terminal on the Earth. To
support up to 622 Mbps over the approximately
400 thousand kilometer link, the system will make
use of a high peak-power doped-fiber transmitter,
a hybrid pointing and tracking system, high
efficiency modulation and coding techniques,
superconducting photon-counting detectors, and a
scalable optical collector architecture. It also will
support up to 20 Mbps on the optical uplink, plus a
highly accurate continuous two-way time-of-flight
measurement capability with the potential to
perform ranging with sub-centimeter accuracy to
the moving spacecraft. The project is being
undertaken by MIT Lincoln Laboratory (MIT/LL)
and the NASA Goddard Space Flight Center
(GSFC.)
INTRODUCTION
In the near future the National
Aeronautics and Space Administration
anticipates a significant increase in demand for
long-haul communications services from deep
space to Earth. Distances will range up to 40
AU (1 AU = 149.6 million kilometers,) with data
rate requirements in the 1’s to 1000’s of
Mbits/second. It has long been known that
optical communications has several great
advantages over RF systems: extremely wide
bandwidth transmitters and receivers available
for carrying high rate data, extremely high gain
from small terminals, and unregulated spectrum.
Although the use of optical communications
techniques in space have been just out of reach for many years, in the past decade several
international successes have shown it to be a real
technology that could be considered for NASA’s
needs. In the U.S., the GOLD demonstration and
GeoLITE; internationally, the SILEX system;
and collaboratively internationally, OICETS and
TESat, have all demonstrated long-distance and
space-based lasercom.
There have also been several recent
NASA studies, investigating the potential for
lasercom. “RF and Optical Communications: A
Comparison of High Data Rate Returns From
Deep Space in the 2020 Timeframe,” a 2007
report from one study ([1]), showed that
lasercom has the potential to deliver any data
rate possible by RF systems, but with less mass
and using much less power. Some high data
rates, especially from very long distances, are not
even feasible using RF, but are within sight of
today’s lasercom technology. (See, e.g. [2])
In 2003, NASA kicked off its Mars
Laser Communications Demonstration, which
was to have been the world’s first truly deep
space laser communications mission. This
program’s goal was to demonstrate 3-50 Mbps
(depending on the distance) from a NASA
satellite, the Mars Telecom Orbiter. The
program, with the space terminal developed by
MIT Lincoln Laboratory, one ground terminal
based on the Hale Telescope developed by the
Jet Propulsion Laboratory, and one ground
terminal based on a novel telescope receive array
developed by Lincoln Lab, would have
demonstrated many firsts ([3], [4]): a lasercom
duplex link with up to a 40 minute round trip,
point-ahead angles of up to 100 beamwidths, a
space terminal with high-bandwidth beam
The LLCD System
The main goal of LLCD is to transmit
up to 622 Mbps on the optical downlink, and up
to 20 Mbps on the optical uplink. To do this, the
LLST, shown in Figure 2, comprises 3 modules:
an Optical Module, a Modem Module, and a
Controller Electronics Module. The fullygimballed
Optical Module resides on an outer
face of the small spacecraft, and the two other
modules reside inside. Connections are made via
electrical and optical fiber harnesses. There are
3 wavelengths between 1550nm and 1570 nm.
The Optical Module comprises a 2-axis
gimbal, based on COTS parts ([6]), a 10-
centimeter reflector telescope, a spatial
acquisition detector, and fiber-coupling optics.
The acquisition detector is a simple quadrant
detector, with a field of view approximately 2
mrad. It is used both for detection of a scanned
uplink signal, and as a tracking sensor for initial
pull-in of that signal. The receiver is a fibercoupled
detector with an optical preamplifier.
Low-frequency tracking error signals are
generated by physically nutating the fiber using
piezo-electric transducers. The transmitter is
also fiber coupled, and the two beams are
combined/separated in small optics using a
dichroic beamsplitter. The transmitter and
receiver fiber can achieve Point-Ahead via a
piezo-electric transducer holding the transmit
fiber.
The LLCD Mission
The LADEE mission has been designed
to be of short duration – only 4 months total.
Thus, the LLCD goal will be to achieve nearly as
much lasercom time as possible in the first
month, with the vagaries of weather at the single
ground station being the biggest variable. It is
expected that there will be nearly 20 hours of
operations, including pointing, acquisition, and
communications experiments in conditions
including daytime, nighttime, full moon, new
moon, high elevation, low elevation, and all sorts
of atmospheric conditions.
A final feature of the LLCD system is
its time-of-flight measurement system. With
such short pulses on both the uplink and
downlink it was reasoned that a careful
coordination of clocks could allow measurement
of two-way time-of-flight to a small fraction of
the pulse times. Since the uplink pulses are
about 300 psec, LLCD is expecting to make
instantaneous two-way measurements with much
less than 150 psec total error. Averaged over
even a short pass, such a capability could provide
sub-centimeter ranging even while the system’s
range is varying at rates up to several Kilometers
per second.
V. Conclusion
We have attempted to give a snapshot
of the design of the upcoming LLCD system, to
be launched in mid-2012. It is hoped that a
successful mission will open up the possibility of
achieving both extremely high data rates and
very small space terminals for future NASA
mission. Starting at the Moon, and then scaling
up to Lagrange point missions, and ultimately,
planetary missions, lasercom could revolutionize data collection and mission strategies across the
Solar System.