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Microelectromechanical Systems for Spacecraft Communications 169

disposed 1208 apart around the periphery. Each leg is 0.9 mm long and is actuated
by a linear electrostatic comb drive. To elevate the mirror in a piston motion to a
nominal elevation of 50 mm for subsequent tip or tilt actuation, a voltage of 30 V
must be applied to each leg. To cause a tip deflection, legs 2 and 3 can be held
stationary while leg 1 is actuated with an appropriate (e.g., sine wave) drive signal
with nominal peak amplitude of 50 V. To cause a tilt deflection, legs 2 and 3 can be
driven 1808 out-of-phase with each other while leg 1 is held stationary. Greater
dynamic range can be achieved for the tip-case by actuating opposing legs as in the
tilt case. Several test articles of this type were evaluated.
The MEMX micromirrors assessed were manufactured using polysilicon sur-
face machining technology developed at Sandia National Laboratory; however, this
technology has since been transferred to Fairchild Semiconductor in Portland,
Maine.

8.7 APPLICATIONS OF MEMS TO SPACECRAFT
OPTICAL COMMUNICATIONS
Optical communication links offer many advantages over microwave links. In
particular, free-space laser systems can provide narrow beam widths and high
gains with much smaller hardware. High gains allow for much higher data rates,
on the order of Gbps for sufficiently close link ranges, for example, near terrestrial
space. 4
The Jet Propulsion Laboratory (JPL) in association with NASA is building an
Optical Communications Telescope Laboratory (OCTL) transceiver station at its
Table Mountain Facility, and they have explored laser communications links for
deep space communications (the Galileo Optical Experiment) and near-terrestrial
3
communications (Ground Orbit Lasercom Demonstration). More recently the Mars
Laser Communications Demonstrator (MLCD) program has begun to develop an
optical telecomm terminal for the Mars Telecommunications Orbiter (MTO),
scheduled for launch in 2009. 110 Data rates ranging from 1 to 2.5 Gbps are planned
for future near-terrestrial space demonstrations and up to 30 Mbps for deep space
3
links such as MLCD. Laser downlinks have also been explored for communication
with submarines via satellite. 111

8.7.1 OPTICAL BEAM STEERING
Recent collaborative work between MEMX Corporation and JHU/APL 107 was
based on previously developed MEMX optical switches. These special test units
were evaluated for applications in laboratory tests as beamsteerers using a digital
pointing and tracking system. Highly accurate and stabilized body-mounted track-
ing systems are essential to the implementation of long-haul optical communication
channels and could be operated potentially from geosynchronous earth orbit (GEO)
to ground-based or air-platform optical receiver terminals. For such spacecraft
applications, moderate to high-powered laser diodes are likely to be required.
Coupled with their potential operation at partial atmospheric pressures, MEMS

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